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RFC 3101 - The OSPF Not-So-Stubby Area (NSSA) Option


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Network Working Group                                          P. Murphy
Request for Comments: 3101                          US Geological Survey
Obsoletes: 1587                                             January 2003
Category: Standards Track

               The OSPF Not-So-Stubby Area (NSSA) Option

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This memo documents an optional type of Open Shortest Path First
   (OSPF) area that is somewhat humorously referred to as a "not-so-
   stubby" area (or NSSA).  NSSAs are similar to the existing OSPF stub
   area configuration option but have the additional capability of
   importing AS external routes in a limited fashion.

   The OSPF NSSA Option was originally defined in RFC 1587.  The
   functional differences between this memo and RFC 1587 are explained
   in Appendix F.  All differences, while expanding capability, are
   backward-compatible in nature.  Implementations of this memo and of
   RFC 1587 will interoperate.

Table Of Contents

   1.0 Overview .................................................  2
      1.1 Motivation - Transit Networks .........................  2
      1.2 Motivation - Corporate Networks .......................  4
      1.3 Proposed Solution .....................................  5
   2.0 NSSA Intra-Area Implementation Details ...................  7
      2.1 The N-bit .............................................  7
      2.2 Type-7 Address Ranges .................................  7
      2.3 Type-7 LSAs ...........................................  8
      2.4 Originating Type-7 LSAs ...............................  9
      2.5 Calculating Type-7 AS External Routes ................. 10
      2.6 Incremental Updates ................................... 14
      2.7 Optionally Importing Summary Routes ................... 14
   3.0 Intra-AS Implementation Details .......................... 15
      3.1 Type-7 Translator Election ............................ 15
      3.2 Translating Type-7 LSAs into Type-5 LSAs .............. 16
      3.3 Flushing Translated Type-7 LSAs ....................... 19
   4.0 Security Considerations .................................. 20
   5.0 Acknowledgements ......................................... 21
   6.0 Contributors ............................................. 22
   7.0 References ............................................... 22
   Appendix A: The Options Field ................................ 23
   Appendix B: Router-LSAs ...................................... 24
   Appendix C: Type-7 LSA Packet Format ......................... 26
   Appendix D: Configuration Parameters ......................... 27
   Appendix E: The P-bit Policy Paradox ......................... 28
   Appendix F: Differences from RFC 1587 ........................ 30
   Author's Addresses ........................................... 32
   Full Copyright Statement ..................................... 33

1.0  Overview

1.1  Motivation - Transit Networks

   Wide-area transit networks often have connections to moderately
   complex "leaf" sites.  A leaf site may have multiple IP network
   numbers assigned to it.  Typically, one of the leaf site's networks
   is directly connected to a router provided and administered by the
   transit network while the others are distributed throughout and
   administered by the site.  From the transit network's perspective,
   all of the network numbers associated with the site make up a single
   "stub" entity.  For example, BBN Planet has one site composed of a
   class-B network, 130.57.0.0, and a class-C network, 192.31.114.0.
   From BBN Planet's perspective, this configuration looks something
   like the diagram on the next page, where the "cloud" consists of the
   subnets of 130.57 and network 192.31.114, all of which are learned by
   RIP on router BR18.

                  192.31.114
                      |
                    (cloud)
                -------------- 130.57.4
                      |
                      |
                   ------ 131.119.13 ------
                   |BR18|------------|BR10|
                   ------            ------
                                        |
                                        V
                                to BBN Planet "core" OSPF system

   Topologically, this cloud looks very much like an OSPF stub area.
   The advantages of running the cloud as an OSPF stub area are:

      1. External routes learned from OSPF's Type-5 AS-external-LSAs are
         not advertised beyond the router labeled "BR10".  This is
         advantageous because the link between BR10 and BR18 may be a
         low-speed link or the router BR18 may have limited resources.

      2. The transit network is abstracted to the "leaf" router BR18 by
         advertising only a default route across the link between BR10
         and BR18.

      3. The cloud becomes a single, manageable "leaf" with respect to
         the transit network.

      4. The cloud can become, logically, a part of the transit
         network's OSPF routing system.

   However, the original definition of the OSPF protocol (See [OSPF])
   imposes topological limitations that restrict simple cloud topologies
   from becoming OSPF stub areas.  In particular, it is illegal for a
   stub area to import routes external to OSPF; thus it is not possible
   for routers BR18 and BR10 to both be members of the stub area and to
   import into OSPF as Type-5 AS-external-LSAs routes learned from RIP
   or other IP routing protocols.  In order to run OSPF out to BR18,
   BR18 must be a member of a non-stub area or the OSPF backbone before
   it can import routes other than its directly connected network(s).
   Since it is not acceptable for BR18 to maintain all of BBN Planet's
   Type-5 AS external routes, BBN Planet is forced by OSPF's topological
   limitations to only run OSPF out to BR10 and to run RIP between BR18
   and BR10.

1.2  Motivation - Corporate Networks

   In a corporate network that supports a large corporate infrastructure
   it is not uncommon for its OSPF domain to have a complex non-zero
   area infrastructure that injects large routing tables into its Area 0
   backbone.  Organizations within the corporate infrastructure may
   routinely multi-home their non-zero OSPF areas to strategically
   located Area 0 backbone routers, either to provide backbone
   redundancy or to increase backbone connectivity or both.  Because of
   these large routing tables, OSPF aggregation via summarization is
   routinely used and recommended.  Stub areas are also recommended to
   keep the size of the routing tables of non-backbone routers small.
   Organizations within the corporation are administratively autonomous
   and compete for corporate backbone resources.  They also want
   isolation from each other in order to protect their own network
   resources within the organization.

   Consider the typical example configuration shown below where routers
   A1, B1 and A2, B2 are connected to Area 1 and Area 2 respectively,
   and where routers A0 and B0 are Area 0 border routers that connect to
   both Area 1 and Area 2.  Assume the 192.168.192/20 and 192.168.208/22
   clouds are subnetted with a protocol different from the corporate
   OSPF instance.  These other protocols could be RIP, IGRP, or second
   and third OSPF instances separate from the corporate OSPF backbone
   instance.

   Area 1 and Area 2 would like to be stub areas to minimize the size of
   their link state databases.  It is also desirable to originate two
   aggregated external advertisements for the subnets of 192.168.192/20
   and 192.168.208/22 in such a way that the preferred path to
   192.168.192/20 is through A0 and the preferred path to 192.168.208/22
   is through B0.

                  +---A0------Area 0 cloud------B0---+
                  |   |                          |   |
                  |   |                          |   |
                  |   |T1                   56kbs|   |
             56kbs|   |                          |   |T1
                  |   |                          |   |
                  |   |       Area 1 cloud       |   |
                  |   A1-----192.168.192/20-----B1   |
                  |                                  |
                  +---A2                        B2---+
                       |                         |
                       |      Area 2 cloud       |
                       +-----192.168.208/22------+

   The current standard OSPF stub area has no mechanism to support the
   redistribution of routes for the subnets of 192.168.192/20 and
   192.168.208/22 within a stub area or the ability to aggregate a range
   of external routes in any OSPF area.  Any solution to this dilemma
   must also honor Area 1's path of choice to 192.168.192/20 through A0
   with redundancy through B0 while at the same time honoring Area 2's
   path of choice to 192.168.208/20 through B0 with redundancy through
   A0.

1.3 Proposed Solution

   This document describes a new optional type of OSPF area, somewhat
   humorously referred to as a "not-so-stubby" area (or NSSA), which has
   the capability of importing external routes in a limited fashion.

   The OSPF specification defines two general classes of area
   configuration.  The first allows Type-5 LSAs to be flooded throughout
   the area.  In this configuration, Type-5 LSAs may be originated by
   routers internal to the area or flooded into the area by area border
   routers.  These areas, referred to herein as Type-5 capable areas (or
   just plain areas in the OSPF specification), are distinguished by the
   fact that they can carry transit traffic.  The backbone is always a
   Type-5 capable area.  The second type of area configuration, called
   stub, does not allow Type-5 LSAs to be propagated into/throughout the
   area and instead depends on default routing to external destinations.

   NSSAs are defined in much the same manner as existing stub areas.  To
   support NSSAs, a new option bit (the "N" bit) and a new type of LSA
   (Type-7) are defined.  The "N" bit ensures that routers belonging to
   an NSSA agree on its configuration.  Similar to the stub area's use
   of the "E" bit, both NSSA neighbors must agree on the setting of the
   "N" bit or the OSPF neighbor adjacency will not form.

   Type-7 LSAs provide for carrying external route information within an
   NSSA. Type-7 LSAs have virtually the same syntax as Type-5 LSAs with
   the obvious exception of the link-state type.  (See section 2.3 for
   more details.)   Both LSAs are considered a type of OSPF AS-
   external-LSA.  There are two major semantic differences between
   Type-5 LSAs and Type-7 LSAs.

      o  Type-7 LSAs may be originated by and advertised throughout an
         NSSA; as with stub areas, Type-5 LSAs are not flooded into
         NSSAs and do not originate there.

      o  Type-7 LSAs are advertised only within a single NSSA; they are
         not flooded into the backbone area or any other area by border
         routers, though the information that they contain may be
         propagated into the backbone area.  (See Section 3.2.)

   In order to allow limited exchange of external information across an
   NSSA border, NSSA border routers will translate selected Type-7 LSAs
   received from the NSSA into Type-5 LSAs.  These Type-5 LSAs will be
   flooded to all Type-5 capable areas.  NSSA border routers may be
   configured with address ranges so that multiple Type-7 LSAs may be
   aggregated into a single Type-5 LSA.  The NSSA border routers that
   perform translation are configurable.  In the absence of a configured
   translator one is elected.

   In addition, an NSSA border router should originate a default LSA (IP
   network is 0.0.0.0/0) into the NSSA.  Default routes are necessary
   because NSSAs do not receive full routing information and must have a
   default route in order to route to AS-external destinations.  Like
   stub areas, NSSAs may be connected to the Area 0 backbone at more
   than one NSSA border router, but may not be used as a transit area.
   Note that a Type-7 default LSA originated by an NSSA border router is
   never translated into a Type-5 LSA, however, a Type-7 default LSA
   originated by an NSSA internal AS boundary router (one that is not an
   NSSA border router) may be translated into a Type-5 LSA.

   Like stub areas, NSSA border routers optionally import summary routes
   into their NSSAs as Type-3 summary-LSAs.  If the import is disabled,
   particular care should be taken to ensure that summary routing is not
   obscured by an NSSA's Type-7 AS-external-LSAs.  This may happen when
   the AS's other IGPs, like RIP and ISIS, leak routing information into
   the NSSA.  In these cases all summary routes should be imported into
   the NSSA.  The recommended default behavior is to import summary
   routes into NSSAs.  Since Type-5 AS-external-LSAs are not flooded
   into NSSAs, NSSA border routers should not originate Type-4 summary-
   LSAs into their NSSAs.  Also an NSSA's border routers never originate
   Type-4 summary-LSAs for the NSSA's AS boundary routers, since Type-7
   AS-external-LSAs are never flooded beyond the NSSA's border.

   When summary routes are not imported into an NSSA, the default LSA
   originated into it by its border routers must be a Type-3 summary-
   LSA.  This default summary-LSA insures intra-AS connectivity to the
   rest of the OSPF domain, as its default summary route is preferred
   over the default route of a Type-7 default LSA.  Without a default
   summary route the OSPF domain's inter-area traffic, which is normally
   forwarded by summary routes, might exit the AS via the default route
   of a Type-7 default LSA originated by an NSSA internal router.  The
   Type-7 default LSAs originated by NSSA internal routers and the no-
   summary option are mutually exclusive features. When summary routes
   are imported into the NSSA, the default LSA originated by a NSSA
   border router into the NSSA should be a Type-7 LSA.

   In our transit topology example the subnets of 130.57 and network
   192.31.114 will still be learned by RIP on router BR18, but now both

   BR10 and BR18 can be in an NSSA and all of BBN Planet's external
   routes are hidden from BR18; BR10 becomes an NSSA border router and
   BR18 becomes an AS boundary router internal to the NSSA.  BR18 will
   import the subnets of 130.57 and network 192.31.114 as Type-7 LSAs
   into the NSSA.  BR10 then translates these routes into Type-5 LSAs
   and floods them into BBN Planet's backbone.

   In our corporate topology example if Area 1 and Area 2 are NSSAs the
   external paths to the subnets of the address ranges 192.168.192/20
   and 192.168.208/22 can be redistributed as Type-7 LSAs throughout
   Area 1 and Area 2 respectively, and then aggregated during the
   translation process into separate Type-5 LSAs that are flooded into
   Area 0.  A0 may be configured as Area 1's translator even though B0
   is the translator of Area 2.

2.0  NSSA Intra-Area Implementation Details

2.1  The N-bit

   The N-bit ensures that all members of an NSSA agree on the area's
   configuration.  Together, the N-bit and E-bit reflect an interface's
   (and consequently the interface's associated area) external LSA
   flooding capability.  As explained in [OSPF] Section 10.5, if Type-5
   LSAs are not flooded into/throughout the area, the E-bit must be
   clear in the option field of the received Hello packets.  Interfaces
   associated with an NSSA will not send or receive Type-5 LSAs on that
   interface but may send and receive Type-7 LSAs.  Therefore, if the
   N-bit is set in the options field, the E-bit must be clear.

   To support the NSSA option an additional check must be made in the
   function that handles the receiving of the Hello packet to verify
   that both the N-bit and the E-bit found in the Hello packet's option
   field match the area type and ExternalRoutingCapability of the area
   of the receiving interface.  A mismatch in the options causes
   processing of the received Hello packet to stop and the packet to be
   dropped.

2.2  Type-7 Address Ranges

   NSSA border routers may be configured with Type-7 address ranges.
   Each Type-7 address range is defined as an [address,mask] pair.  Many
   separate Type-7 networks may fall into a single Type-7 address range,
   just as a subnetted network is composed of many separate subnets.
   NSSA border routers may aggregate Type-7 routes by advertising a
   single Type-5 LSA for each Type-7 address range.  The Type-5 LSA
   resulting from a Type-7 address range match will be distributed to
   all Type-5 capable areas.  Section 3.2 details how Type-5 LSAs are
   generated from Type-7 address ranges.

   A Type-7 address range includes the following configurable items.

      o  An [address,mask] pair.

      o  A status indication of either Advertise or DoNotAdvertise.

      o  An external route tag.

2.3  Type-7 LSAs

   External routes are imported into NSSAs as Type-7 LSAs by NSSA AS
   boundary routers.  An NSSA AS boundary router (ASBR) is a router that
   has an interface associated with the NSSA and is exchanging routing
   information with routers belonging to another AS.  Like OSPF ASBRs,
   an NSSA router indicates it is an NSSA ASBR by setting the E-bit in
   its router-LSA.  As with Type-5 LSAs a separate Type-7 LSA is
   originated for each destination network.  To support NSSAs the link-
   state database must therefore be expanded to contain Type-7 LSAs.

   Type-7 LSAs are identical to Type-5 LSAs except for the following
   (see [OSPF] Section 12.4.4, "AS external links").

      1. The Type field in the LSA header is 7.

      2. Type-7 LSAs are only flooded within the originating NSSA.  The
         flooding of Type-7 LSAs follows the same rules as the flooding
         of Type-1 and Type-2 LSAs.

      3. Type-7 LSAs are only listed within the OSPF area data
         structures of their respective NSSAs, making them area
         specific.  Type-5 LSAs, which are flooded to all Type-5 capable
         areas, have global scope and are listed in the OSPF protocol
         data structure.

      4. NSSA border routers select which Type-7 LSAs are translated
         into Type-5 LSAs and flooded into the OSPF domain's transit
         topology.

      5. Type-7 LSAs have a propagate (P) bit that, when set, tells an
         NSSA border router to translate a Type-7 LSA into a Type-5 LSA.

      6. Those Type-7 LSAs that are to be translated into Type-5 LSAs
         must have their forwarding address set.

   Type-5 LSAs that are translations of Type-7 LSAs copy the Type-7
   LSAs' non-zero forwarding addresses.  Only those Type-5 LSAs that are
   aggregations of Type-7 LSAs may have 0.0.0.0 as a forwarding address.
   (See Section 3.2 for details.)  Non-zero forwarding addresses produce
   efficient inter-area routing to an NSSA's AS external destinations
   when it has multiple border routers.  Also the non-zero forwarding
   addresses of Type-7 LSAs ease the process of their translation into
   Type-5 LSAs, as NSSA border routers are not required to compute them.

   Normally the next hop address of an installed AS external route
   learned by an NSSA ASBR from an adjacent AS points at one of the
   adjacent AS's gateway routers.  If this address belongs to a network
   connected to the NSSA ASBR via one of its NSSAs' active interfaces,
   then the NSSA ASBR copies this next hop address into the forwarding
   address field of the route's Type-7 LSA that is originated into this
   NSSA, as is currently done with Type-5 LSAs. (See [OSPF] Section
   12.4.4.1.)  For an NSSA with no such network the forwarding address
   field may only be filled with an address from one of the its active
   interfaces or 0.0.0.0.  If the P-bit is set, the forwarding address
   must be non-zero; otherwise it may be 0.0.0.0.  If an NSSA requires
   the P-bit be set and a non-zero forwarding address is unavailable,
   then the route's Type-7 LSA is not originated into this NSSA.

   When a router is forced to pick a forwarding address for a Type-7
   LSA, preference should be given first to the router's internal
   addresses (provided internal addressing is supported).  If internal
   addresses are not available, preference should be given to the
   router's active OSPF stub network addresses.  These choices avoid the
   possible extra hop that may happen when a transit network's address
   is used.  When the interface whose IP address is the LSA's forwarding
   address transitions to a Down state (see [OSPF] Section 9.3), the
   router must select a new forwarding address for the LSA and then re-
   originate it.  If one is not available the LSA should be flushed.

   The metrics and path types of Type-5 LSAs are directly comparable
   with the metrics and path types of Type-7 LSAs.

2.4 Originating Type-7 LSAs

   NSSA AS boundary routers may only originate Type-7 LSAs into NSSAs.
   An NSSA internal AS boundary router must set the P-bit in the LSA
   header's option field of any Type-7 LSA whose network it wants
   advertised into the OSPF domain's full transit topology.  The LSAs of
   these networks must have a valid non-zero forwarding address.  If the
   P-bit is clear the LSA is not translated into a Type-5 LSA by NSSA
   border routers.

   When an NSSA border router originates both a Type-5 LSA and a Type-7
   LSA for the same network, then the P-bit must be clear in the Type-7
   LSA so that it isn't translated into a Type-5 LSA by another NSSA
   border router.  If the border router only originates a Type-7 LSA, it
   may set the P-bit so that the network may be aggregated/propagated
   during Type-7 translation.  If an NSSA's border router originates a
   Type-5 LSA with a forwarding address from the NSSA, it should also
   originate a Type-7 LSA into the NSSA.  If two NSSA routers, both
   reachable from one another over the NSSA, originate functionally
   equivalent Type-7 LSAs (i.e., same destination, cost and non-zero
   forwarding address), then the router having the least preferred LSA
   should flush its LSA.  (See [OSPF] Section 12.4.4.1.)  Preference
   between two Type-7 LSAs is determined by the following tie breaker
   rules:

      1. An LSA with the P-bit set is preferred over one with the P-bit
         clear.

      2. If the P-bit settings are the same, the LSA with the higher
         router ID is preferred.

   A Type-7 default LSA for the network 0.0.0.0/0 may be originated into
   the NSSA by any NSSA router.  The Type-7 default LSA originated by an
   NSSA border router must have the P-bit clear.  An NSSA ASBR that is
   not an NSSA border router may originate a Type-7 default LSA with the
   P-bit set.  A Type-7 default LSA may be installed by NSSA border
   routers if and only if its P-bit is set.  (See Appendix E.)

   NSSA border routers must originate an LSA for the default destination
   into all their directly attached NSSAs in order to support intra-AS
   routing and inter-AS routing.  This default destination is advertised
   in either a Type-3 LSA or a Type-7 LSA, as described in Section 2.7.
   The default LSA's metric should be configurable. For Type-7 default
   LSAs, the metric type (1 or 2) should also be configurable.

2.5 Calculating Type-7 AS External Routes

   This calculation must be run when Type-7 LSAs are processed during
   the AS external route calculation.  This calculation may process
   Type-5 LSAs, but it is written that way only for comparison sake.

   Non-default Type-7 LSAs with the P-bit clear may be installed in the
   OSPF routing table of NSSA border routers.  Since these LSAs are not
   propagated throughout the OSPF domain, traffic that originates
   external to an NSSA and that passes through one of the NSSA's border
   routers may be unexpectedly diverted into the NSSA.  (See Appendix
   E.)

   An NSSA border router should examine both Type-5 LSAs and Type-7 LSAs
   if either Type-5 or Type-7 routes need to be updated or recalculated.
   This is done as part of the AS external route calculation.  An NSSA
   internal router should examine Type-7 LSAs when Type-7 routes need to
   be recalculated.

   What follows is only a modest modification of [OSPF] Section 16.4.
   Original paragraphs are tagged with [OSPF].  Paragraphs with minor
   changes are tagged with ~[OSPF].  Paragraphs specific to NSSA are
   tagged with [NSSA].

   AS external routes are calculated by examining AS-external-LSAs, be
   they Type-5 or Type-7.  Each of the AS-external-LSAs is considered in
   turn.  Most AS-external-LSAs describe routes to specific IP
   destinations.  An AS-external-LSA can also describe a default route
   for the Autonomous System (Destination ID = DefaultDestination,
   network/subnet mask = 0x00000000).  For each AS-external-LSA:
   ~[OSPF]

      (1) If the metric specified by the LSA is LSInfinity, or if the
          age of the LSA equals MaxAge, then examine the next LSA.
          [OSPF]

      (2) If the LSA was originated by the calculating router itself,
          examine the next LSA.
          [OSPF]

      (3) Call the destination described by the LSA N.  N's address is
          obtained by masking the LSA's Link State ID with the
          network/subnet mask contained in the body of the LSA.  Look up
          the routing table entries that match the LSA's type for the AS
          boundary router (ASBR) that originated the LSA.  For a Type-5
          LSA, routing table entries may only be selected from each
          attached Type-5 capable area.  Since the flooding scope of a
          Type-7 LSA is restricted to the originating NSSA, the routing
          table entry of its ASBR must be found in the originating NSSA.
          If no entries exist for the ASBR (i.e. the ASBR is unreachable
          over the transit topology for a Type-5 LSA, or, for a Type-7
          LSA, it is unreachable over the LSA's originating NSSA), do
          nothing with this LSA and consider the next in the list.
          [NSSA]

          Else if the destination is a Type-7 default route (destination
          ID = DefaultDestination) and one of the following is true,
          then do nothing with this LSA and consider the next in the
          list:

            o  The calculating router is a border router and the LSA has
               its P-bit clear.  Appendix E describes a technique
               whereby an NSSA border router installs a Type-7 default
               LSA without propagating it.

            o  The calculating router is a border router and is
               suppressing the import of summary routes as Type-3
               summary-LSAs.
            [NSSA]

          Else, this LSA describes an AS external path to destination N.
          Examine the forwarding address specified in the AS-external-
          LSA.  This indicates the IP address to which packets for the
          destination should be forwarded.
          [OSPF]

          If the forwarding address is set to 0.0.0.0 then packets
          should be sent to the ASBR itself.  If the LSA is Type-5, from
          among the multiple non-NSSA routing table entries for the ASBR
          (both NSSA and non-NSSA ASBR entries might exists on an NSSA
          border router), select the preferred entry as follows:
          ~[OSPF]

            If RFC1583Compatibility is set to "disabled", prune the set
            of routing table entries for the ASBR as described in OSPF
            Section 16.4.1.  In any case, among the remaining routing
            table entries, select the routing table entry with the least
            cost; when there are multiple least cost routing table
            entries the entry whose associated area has the largest OSPF
            Area ID (when considered as an unsigned 32-bit integer) is
            chosen.
            [OSPF]

          Since a Type-7 LSA only has area-wide flooding scope, when its
          forwarding address is set to 0.0.0.0, its ASBR's routing table
          entry must be chosen from the originating NSSA.  Here no
          pruning is necessary since this entry always contains non-
          backbone intra-area paths.
          [NSSA]

          If the forwarding address is non-zero look up the forwarding
          address in the routing table.  For a Type-5 LSA the matching
          routing table entry must specify an intra-area or inter-area
          path through a Type-5 capable area.  For a Type-7 LSA the
          matching routing table entry must specify an intra-area path
          through the LSA's originating NSSA.  If no such path exists

          then do nothing with this LSA and consider the next in the
          list.
          [NSSA]

      (4) Let X be the cost specified by the preferred routing table
          entry for the ASBR/forwarding address, and Y the cost
          specified in the LSA.  X is in terms of the link state metric,
          and Y is a type 1 or 2 external metric.
          [OSPF]

      (5) Now, look up the routing table entry for the destination N.
          If no entry exists for N, install the AS external path to N,
          with the next hop equal to the list of next hops to the
          ASBR/forwarding address, and advertising router equal to the
          ASBR.  If the external metric type is 1, then the path-type is
          set to Type-1 external and the cost is equal to X + Y.  If the
          external metric type is 2, the path-type is set to Type-2
          external, the link-state component of the route's cost is X,
          and the type 2 cost is Y.
          [OSPF]

      (6) Otherwise compare the AS external path described by the LSA
          with the existing paths in N's routing table entry.  If the
          new path is preferred, it replaces the present paths in N's
          routing table entry.  If the new path is of equal preference,
          it is added to the present paths in N's routing table entry.
          [OSPF]

          Preference is defined as follows:

          (a) Intra-area and inter-area paths are always preferred over
              AS external paths.
              [OSPF]

          (b) Type 1 external paths are always preferred over type 2
              external paths.  When all paths are type 2 external paths,
              the paths with the smallest advertised type 2 metric are
              always preferred.
              [OSPF]

          (c) If the new AS external path is still indistinguishable
              from the current paths in N's routing table entry, and
              RFC1583Compatibility is set to "disabled", select the
              preferred paths based on the intra-AS paths to the
              ASBR/forwarding addresses, as specified in Section 16.4.1.
              Here intra-NSSA paths are equivalent to the intra-area
              paths of non-backbone regular OSPF areas.
              [NSSA]

          (d) If the new AS external path is still indistinguishable
              from the current paths in N's routing table entry, select
              the preferred path based on a least cost comparison.  Type
              1 external paths are compared by looking at the sum of the
              distance to the ASBR/forwarding addresses and the
              advertised type 1 metric (X+Y).  Type 2 external paths
              advertising equal type 2 metrics are compared by looking
              at the distance to the ASBR/forwarding addresses.
              ~[OSPF]

          (e) If the current LSA is functionally the same as an
              installed LSA (i.e., same destination, cost and non-zero
              forwarding address) then apply the following priorities in
              deciding which LSA is preferred:

                 1. A Type-7 LSA with the P-bit set.

                 2. A Type-5 LSA.

                 3. The LSA with the higher router ID.

              [NSSA]

2.6 Incremental Updates

   Incremental updates for Type-7 LSAs should be treated the same as
   incremental updates for Type-5 LSAs (see [OSPF] Section 16.6).  When
   a new instance of a Type-7 LSA is received it is not necessary to
   recalculate the entire routing table.  Call the destination described
   by the Type-7 LSA N.  N's address is obtained by masking the LSA's
   Link State ID with the network/subnet mask contained in the body of
   the LSA.  If there is already an intra-area or inter-area route to
   the destination, no recalculation is necessary (internal routes take
   precedence).

   Otherwise, the procedure in the preceding section will have to be
   performed but only for the external routes (Type-5 and Type-7) whose
   destination is N.  Before this procedure is performed, the present
   routing table entry for N should be invalidated.

2.7 Optionally Importing Summary Routes

   In order for OSPF's summary routing to not be obscured by an NSSA's
   Type-7 AS-external-LSAs, all NSSA border router implementations must
   support the optional import of summary routes into NSSAs as Type-3
   summary-LSAs.  The default behavior is to import summary routes.  A
   new area configuration parameter, ImportSummaries, is defined in
   Appendix D.  When ImportSummaries is set to enabled, summary routes

   are imported.  When it is set to disabled, summary routes are not
   imported.  The default setting is enabled.

   When OSPF's summary routes are not imported, the default LSA
   originated by an NSSA border router into the NSSA should be a Type-3
   summary-LSA. This protects the NSSA from routing intra-AS traffic out
   the AS via the default route of a Type-7 default LSA originating from
   one of the NSSA's internal routers.  When summary routes are imported
   into the NSSA, the default LSA originated by an NSSA border router
   must not be a Type-3 summary-LSA; otherwise its default route would
   be chosen over the potentially more preferred default routes of
   Type-7 default LSAs.

3.0 Intra-AS Implementation Details

3.1 Type-7 Translator Election

   It is not recommended that multiple NSSA border routers perform
   Type-7 to Type-5 translation unless it is required to route packets
   efficiently through Area 0 to an NSSA partitioned by Type-7 address
   ranges.  It is normally sufficient to have only one NSSA border
   router perform the translation.  Excessive numbers of Type-7
   translators unnecessarily increase the size of the OSPF link state
   data base.

   A new area configuration parameter, NSSATranslatorRole, is defined in
   Appendix D.  It specifies whether or not an NSSA router will
   unconditionally translate Type-7 LSAs to Type-5 LSAs when acting as
   an NSSA border router. Configuring the identity of the translator can
   be used to bias the routing to aggregated destinations. When
   NSSATranslatorRole is set to Always, Type-7 LSAs are always
   translated regardless of the translator state of other NSSA border
   routers.  When NSSATranslatorRole is set to Candidate an NSSA border
   router will participate in the translator election process described
   below.

   A new area parameter, NSSATranslatorState, is maintained in an NSSA's
   OSPF area data structure.  By default all NSSA routers initialize
   NSSATranslatorState to disabled.  When an NSSA border router's
   NSSATranslatorState changes from disabled to either enabled or
   elected, it begins translating the NSSA's Type-7 LSAs into Type-5
   LSAs.  When its NSSATranslatorState changes from either enabled or
   elected to disabled, it ceases translating the NSSA's Type-7 LSAs
   into Type-5 LSAs. (See paragraphs below.)

   A new bit, Nt, is defined for the router-LSAs of NSSAs.  (See
   Appendix B.)  By default routers clear bit Nt when originating
   router-LSAs.  However, when an NSSA border router has its

   NSSATranslatorState enabled, it sets bit Nt in the router-LSA it
   originates into the NSSA.  An NSSA router whose NSSATranslatorRole is
   set to Always should re-originate a router-LSA into the NSSA whenever
   its NSSATranslatorState changes.

   When an NSSA router with the NSSA's NSSATranslatorRole set to Always
   attains border router status, it should change NSSATranslatorState
   from disabled to enabled.  When it loses border router status, it
   should change NSSATranslatorState from enabled to disabled.

   All NSSA border routers must set the E-bit in the Type-1 router-LSAs
   of their directly attached non-stub areas, even when they are not
   translating.  This allows other NSSA border routers to see their ASBR
   status across the AS's transit topology.  Failure to do so may cause
   the election algorithm to elect unnecessary translators.  Every NSSA
   border router is a potential translator.

   An NSSA border router whose NSSA's NSSATranslatorRole is set to
   Candidate must maintain a list of the NSSA's border routers that are
   reachable both over the NSSA and as ASBRs over the AS's transit
   topology.  Any change in this list, or to the Nt bit settings of
   members of this list, causes the NSSA border router to reevaluate its
   NSSATranslatorState.  If there exists another border router in this
   list whose router-LSA has bit Nt set or who has a higher router ID,
   then its NSSATranslatorState is disabled.  Otherwise its
   NSSATranslatorState is elected.

   An elected translator will continue to perform translation duties
   until supplanted by a reachable NSSA border router whose Nt bit is
   set or whose router ID is greater.  Such an event may happen when an
   NSSA router with NSSATranslatorRole set to Always regains border
   router status, or when a partitioned NSSA becomes whole.  If an
   elected translator determines its services are no longer required, it
   continues to perform its translation duties for the additional time
   interval defined by a new area configuration parameter,
   TranslatorStabilityInterval.  This minimizes excessive flushing of
   translated Type-7 LSAs and provides for a more stable translator
   transition.  The default value for the TranslatorStabilityInterval
   parameter has been defined as 40 seconds. (See Appendix D.)

3.2 Translating Type-7 LSAs into Type-5 LSAs

   This step is performed as part of the NSSA's Dijkstra calculation
   after Type-5 and Type-7 routes have been calculated.  If the
   calculating router is currently not an NSSA border router translator,
   then this translation algorithm should be skipped.  Only installed

   Type-7 LSAs and those non-default Type-7 LSAs originated by the
   router itself should be examined.  Locally sourced Type-7 LSAs should
   be processed first.

   Note that it is possible for a Type-5 LSA generated by translation to
   supplant a Type-5 LSA originating from a local OSPF external source.
   Future reoriginations of the locally sourced Type-5 LSA should be
   suppressed until the Type-5 LSA generated by translation is flushed.

   A Type-7 LSA and a Type-7 address range best match one another if
   there does not exist a more specific Type-7 address range that
   contains the LSA's network.  For each eligible Type-7 LSA perform the
   following:

      (1) If the Type-7 LSA has the P-bit clear, or its forwarding
          address is set to 0.0.0.0, or the most specific Type-7 address
          range that subsumes the LSA's network has DoNotAdvertise
          status, then do nothing with this Type-7 LSA and consider the
          next one in the list.  Otherwise term the LSA as translatable
          and proceed with step (2).

      (2) If the Type-7 LSA is not contained in any explicitly
          configured Type-7 address range and the calculating router has
          the highest router ID amongst NSSA translators that have
          originated a functionally equivalent Type-5 LSA (i.e. same
          destination, cost and non-zero forwarding address) and that
          are reachable over area 0 and the NSSA, then a Type-5 LSA
          should be generated if there is currently no Type-5 LSA
          originating from this router corresponding to the Type-7 LSA's
          network, or there is an existing Type-5 LSA and either it
          corresponds to a local OSPF external source whose path type
          and metric is less preferred (see Section 2.5 step (6), parts
          (b) and (d)), or it doesn't and the Type-5 LSA's path type or
          cost(s) have changed (See Section 2.5 step (5)) or the
          forwarding address no longer maps to a translatable Type-7
          LSA.

          The newly originated Type-5 LSA will describe the same network
          and have the same network mask, path type, metric, forwarding
          address and external route tag as the Type-7 LSA.  The
          advertising router field will be the router ID of this NSSA
          border router.  The link-state ID is equal to the LSA's
          network address (in the case of multiple originations of
          Type-5 LSAs with the same network address but different mask,
          the link-state ID can also have one or more of the network's
          "host" bits set).

      (3) Else the Type-7 LSA must be aggregated by the most specific
          Type-7 address range that subsumes it.  If this Type-7 address
          range has the same [address,mask] pair as the LSA's network
          and no other translatable Type-7 LSA with a different network
          best matches this range, then flag the LSA as not contained in
          any explicitly configured Type-7 address range and process the
          LSA as described in step (2).  Otherwise compute the path type
          and metric for this Type-7 address range as described below.

          The path type and metric of the Type-7 address range is
          determined from the path types and metrics of those
          translatable Type-7 LSAs that best match the range plus any
          locally sourced Type-5 LSAs whose network has the same
          [address,mask] pair.  If any of these LSAs have a path type of
          2, the range's path type is 2, otherwise it is 1.  If the
          range's path type is 1 its metric is the highest cost amongst
          these LSAs; if the range's path type is 2 its metric is the
          highest Type-2 cost + 1 amongst these LSAs.  (See Section 2.5
          step (5).)  1 is added to the Type-2 cost to ensure that the
          translated Type-5 LSA does not appear closer on the NSSA
          border than a translatable Type-7 LSA whose network has the
          same [address,mask] pair and Type-2 cost.

          A Type-5 LSA is generated from the Type-7 address range when
          there is currently no Type-5 LSA originated by this router
          whose network has the same [address,mask] pair as the range or
          there is but either its path type or metric has changed or its
          forwarding address is non-zero.

          The newly generated Type-5 LSA will have a link-state ID equal
          to the Type-7 address range's address (in the case of multiple
          originations of Type-5 LSAs with the same network address but
          different mask, the link-state ID can also have one or more of
          the range's "host" bits set).  The advertising router field
          will be the router ID of this area border router.  The network
          mask and the external route tag are set to the range's
          configured values.  The forwarding address is set to 0.0.0.0.
          The path type and metric are set to the range's path type and
          metric as defined and computed above.

          The pending processing of other translation eligible Type-7
          LSAs that best match this Type-7 address range is suppressed.
          Thus at most a single Type-5 LSA is originated for each Type-7
          address range.

   For example, given a Type-7 address range of [10.0.0.0, 255.0.0.0]
   that subsumes the following Type-7 routes:

                 10.1.0.0/24 path type 1, cost 10
                 10.2.0.0/24 path type 1, cost 11
                 10.3.0.0/24 path type 2, type 2 cost 5

   a Type-5 LSA would be generated with a path type of 2 and a metric 6.

   Given a Type-7 address range of [10.0.0.0, 255.0.0.0] that subsumes
   the following Type-7 routes:

                 10.1.0.0/24 path type 1, cost 10
                 10.2.0.0/24 path type 1, cost 11
                 10.3.0.0/24 path type 1, cost 5

   a Type-5 LSA will be generated with a path type of 1 and a metric 11.

   These Type-7 address range metric and path type rules will avoid
   routing loops in the event that path types 1 and 2 are both used
   within the area.

   As with all newly originated Type-5 LSAs, a Type-5 LSA that is the
   result of a Type-7 LSA translation or aggregation is flooded to all
   attached Type-5 capable areas.

   Like Type-3 address ranges, a Type-7 address range performs the dual
   function of setting propagation policy via its
   Advertise/DoNotAdvertise parameter and aggregation via its network
   address and mask pair. Unlike the origination of Type-3 summary-LSAs,
   the translation of a Type-7 LSA into a Type-5 LSA may result in more
   efficient routing when the forwarding address is set, as is done
   during step (2) of the translation procedure.

   Another important feature of this translation process is that it
   allows a Type-7 address range to apply different properties
   (aggregation, forwarding address, and Advertise/DoNotAdvertise
   status) for the Type-7 routes it subsumes, versus those Type-7 routes
   subsumed by other more specific Type-7 address ranges contained in
   the Type-7 address range.

3.3 Flushing Translated Type-7 LSAs

   If an NSSA border router has either translated or aggregated an
   installed Type-7 LSA into a Type-5 LSA that should no longer be
   translated or aggregated, then the Type-5 LSA should either be
   flushed or reoriginated as a translation or aggregation of other
   Type-7 LSAs.

   If an NSSA border router is translating Type-7 LSA's into Type-5
   LSA's with NSSATranslatorState set to elected and the NSSA border
   router has determined that its translator election status has been
   deposed by another NSSA border router (see Section 3.1), then, as
   soon as the TranslatorStabilityInterval has expired without the
   router reelecting itself as a translator, Type-5 LSAs that are
   generated by aggregating Type-7 LSAs into their best matched Type-7
   address ranges (see Section 3.2, Step (3)) are flushed.  Conversely
   Type-5 LSAs generated by translating Type-7 LSAs are not immediately
   flushed, but are allowed to remain in the OSPF routing domain as if
   the originator is still an elected translator.  This minimizes the
   flushing and flooding impact on the transit topology of an NSSA that
   changes its translators frequently.

4.0 Security Considerations

   There are two types of issues that need be addressed when looking at
   protecting routing protocols from misconfigurations and malicious
   attacks.  The first is authentication and certification of routing
   protocol information.  The second is denial of service attacks
   resulting from repetitive origination of the same router
   advertisement or origination of a large number of distinct
   advertisements resulting in database overflow.  Note that both of
   these concerns exist independently of a router's support for the NSSA
   option.

   The OSPF protocol addresses authentication concerns by authenticating
   OSPF protocol exchanges.  OSPF supports multiple types of
   authentication; the type of authentication in use can be configured
   on a per network segment basis.  One of OSPF's authentication types,
   namely the Cryptographic authentication option, is believed to be
   secure against passive attacks and provides significant protection
   against active attacks.  When using the Cryptographic authentication
   option, each router appends a "message digest" to its transmitted
   OSPF packets.  Receivers then use the shared secret key and the
   received digest to verify that each received OSPF packet is
   authentic.

   The quality of the security provided by the Cryptographic
   authentication option depends completely on the strength of the
   message digest algorithm (MD5 is currently the only message digest
   algorithm specified), the strength of the key being used, and the
   correct implementation of the security mechanism in all communicating
   OSPF implementations.  It also requires that all parties maintain the
   secrecy of the shared secret key.  None of the standard OSPF
   authentication types provide confidentiality, nor do they protect
   against traffic analysis.  For more information on the standard OSPF
   security mechanisms, see Sections 8.1, 8.2, and Appendix D of [OSPF].

   [DIGI] describes the extensions to OSPF required to add digital
   signature authentication to Link State data and to provide a
   certification mechanism for router data.  [DIGI] also describes the
   added LSA processing and key management as well as a method for
   migration from or co-existence with standard OSPF V2.

   OSPF addresses repetitive origination of advertisements by mandating
   a limit on how frequent new instances of any particular LSA can be
   originated and accepted during the flooding procedure.  The frequency
   at which new LSA instances may be originated is set to once every
   MinLSInterval seconds, whose value is 5 seconds.  (See [OSPF] Section
   12.4.)  The frequency at which new LSA instances are accepted during
   flooding is once every MinLSArrival seconds, whose value is set to 1
   second.  (See [OSPF] Section 13, Appendix B, and G.1.)

   Proper operation of the OSPF protocol requires that all OSPF routers
   maintain an identical copy of the OSPF link state database.  However,
   when the size of the link state database becomes very large, some
   routers may be unable to keep the entire database due to resource
   shortages; this is termed "database overflow".  When database
   overflow is anticipated, the routers with limited resources can be
   accommodated by configuring OSPF stub areas and NSSAs.  [OVERFLOW]
   details a way of gracefully handling unanticipated database
   overflows.

5.0 Acknowledgements

   This document was produced by the OSPF Working Group, chaired by John
   Moy.

   In addition, the comments of the following individuals are also
   acknowledged:

      Antoni Przygienda  Redback Networks, Inc
      Alex Zinin         cisco

   It is also noted that comments from

      Phani Jajjarvarpu  cisco
      Dino Farinacci     cisco
      Jeff Honig         Cornell University
      Doug Williams      IBM

   were acknowledged in the predecessor of this document, RFC 1587.

6.0 Contributors

   This document, RFC 3101, adds new sections, features, edits, and
   revisions to its predecessor, RFC 1587, "The OSPF NSSA Option",
   authored by Rob Coltun, Movaz Networks, and Vince Fuller.  Content
   from RFC 1587 is used throughout RFC 3101.  In addition to adding new
   features, this document makes the NSSA specification consistent with
   the OSPFv2 specification, RFC 2328, authored by John Moy, Sycamore
   Networks, Inc.  Section 2.5, Calculating Type-7 AS External Routes,
   and Section 2.6,  Incremental Updates, rely heavily on text in RFC
   2328's Section 16.4 and Section 16.6 respectively.  Section 4.0,
   Security Considerations, is an edit of similar content in Rob
   Coltun's RFC 2370, "The OSPF Opaque LSA option", Section 6.0.

   Acee Lindem, Redback Networks, Inc, is also recognized for the first
   full known implementation of this specification. Acee's
   implementation resulted in substantive content change.

7.0 References

   [DIGI]     Murphy, S., Badger, M. and B. Wellington, "OSPF with
              Digital Signatures", RFC 2154, June 1997.

   [MUEX]     Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
              1994.

   [OSPF]     Moy, J., "OSPF Version 2", RFC 2328, April 1998.

   [OPAQUE]   Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July
              1998.

   [OVERFLOW] Moy, J., "OSPF Database Overflow", RFC 1765, March 1995.

Appendix A: The Options Field

   The OSPF options field is present in OSPF Hello packets, Database
   Description packets and all link state advertisements.  See [OSPF]
   Appendix A.2 and [OPAQUE] Appendix A.1 for a description of the
   options field.  Six bits are assigned but only two (the E-bit and the
   N/P bit) are described completely in this section.

                  --------------------------------------
                  | * | O | DC | EA | N/P | MC | E | * |
                  --------------------------------------

                      The Type-7 LSA options field

      E-bit:  Type-5 AS-external-LSAs are not flooded into/through OSPF
              stub areas and NSSAs.  The E-bit ensures that all members
              of a stub area or NSSA agree on that area configuration.
              The E-bit is meaningful only in OSPF Hello and Database
              Description packets.  When the E-bit is clear in the Hello
              packet sent out a particular interface, it means that the
              router will neither send nor receive Type-5 AS-external-
              LSAs on that interface (in other words, the interface
              connects to a stub area or NSSA).  Two routers will not
              become neighbors unless they agree on the state of the E-
              bit.

      N-bit:  The N-bit describes the router's NSSA capability.  The N-
              bit is used only in Hello packets and ensures that all
              members of an NSSA agree on that area's configuration.
              When the N-bit is set in the Hello packet that is sent out
              a particular interface, it means that the router will send
              and receive Type-7 LSAs on that interface.  Two routers
              will not form an adjacency unless they agree on the state
              of the N-bit.  If the N-bit is set in the options field,
              the E-bit must be clear.

      P-bit:  The P-bit is used only in the Type-7 LSA header.  It flags
              the NSSA border router to translate the Type-7 LSA into a
              Type-5 LSA.  The default setting for the P-bit is clear.

Appendix B: Router-LSAs

   Router-LSAs are the Type-1 LSAs.  Each router in an area originates a
   router-LSA.  The LSA describes the state and cost of the router's
   links (i.e., interfaces) to the area.  All of the router's links to
   the area must be described in a single router-LSA.  For details
   concerning the construction of router-LSAs, see [OSPF] Section
   12.4.1.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   |       1       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Advertising Router                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     LS sequence number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |             length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  0  Nt|W|V|E|B|        0      |            # links            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Link ID                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Link Data                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     # TOS     |        TOS 0 metric           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      TOS      |        0      |            metric             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      TOS      |        0      |            metric             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Link ID                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Link Data                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |

   In router-LSAs, the Link State ID field is set to the router's OSPF
   Router ID.  Router-LSAs are flooded throughout a single area only.

      bit V
          When set, the router is an endpoint of one or more fully
          adjacent virtual links having the described area as their
          transit area (V is for virtual link endpoint).

      bit E
          When set, the router is an AS boundary router (E is for
          external).  ALL NSSA border routers set bit E in those
          router-LSAs originated into directly attached Type-5 capable
          areas.  An NSSA's AS boundary routers also set bit E in their
          router-LSAs originated into the NSSA.  (See Section 3.1 for
          details.)

      bit B
          When set, the router is an area border router (B is for
          border).

      bit W
          When set, the router is a wild-card multicast receiver (W is
          for wild).

      bit Nt
          When set, the router is an NSSA border router that is
          unconditionally translating Type-7 LSAs into Type-5 LSAs (Nt
          stands for NSSA translation).  Note that such routers have
          their NSSATranslatorRole area configuration parameter set to
          Always.  (See Appendix D and Section 3.1.)

   The remainder of the router-LSAs specification is defined in [OSPF]
   Section A.4.2.

Appendix C: Type-7 LSA Packet Format

        0                                 32
        ------------------------------------
        |                | Options |   7   |
        |                -------------------
        |        Link-State Header         |
        |                                  |
        ------------------------------------
        | Network Mask                     |
        ------------------------------------  ______
        |E| TOS  |        metric           |  .
        ------------------------------------  .  repeated for each TOS
        | Forwarding Address               |  .
        ------------------------------------  .
        | External Route Tag               |  ______
        ------------------------------------

   The definitions of the link-state ID, network mask, metrics and
   external route tag are the same as the definitions for Type-5 LSAs
   (See [OSPF] Appendix A.4.5), except for the forwarding address and
   the N/P-bit.  The Options field must have the N/P bit set as
   described in Appendix A when the originating router desires that the
   external route be propagated throughout the OSPF domain.

   Forwarding address
      Data traffic for the advertised destination will be forwarded to
      this address.  If the forwarding address is set to 0.0.0.0, data
      traffic will be forwarded to the LSA's originator (i.e., the
      responsible NSSA AS boundary router).  Normally the next hop
      address of an installed AS external route learned by an NSSA ASBR
      from an adjacent AS points at one of the adjacent AS's gateway
      routers.  If this address belongs to a network connected to the
      NSSA ASBR via one of its NSSAs' active interfaces, then it is the
      forwarding address for the route's Type-7 LSA originated into this
      NSSA.  For an NSSA with no such network the forwarding address is
      either an address from one of its active interfaces or 0.0.0.0.
      If the P-bit is set, the forwarding address must be non-zero,
      otherwise it may be 0.0.0.0. (See Section 2.3 for details.)

Appendix D:  Configuration Parameters

   [OSPF] Appendix C.2 lists the area configuration parameters.  The
   area ID and the list of address ranges for Type-3 summary routes
   remain unchanged.  Section 2.2 of this document lists the
   configuration parameters for Type-7 address ranges.  The following
   area configuration parameters have been added:

      NSSATranslatorRole

         Specifies whether or not an NSSA border router will
         unconditionally translate Type-7 LSAs into Type-5 LSAs.  When
         it is set to Always, an NSSA border router always translates
         Type-7 LSAs into Type-5 LSAs regardless of the translator state
         of other NSSA border routers.  When it is set to Candidate, an
         NSSA border router participates in the translator election
         process described in Section 3.1.  The default setting is
         Candidate.

      TranslatorStabilityInterval

         Defines the length of time an elected Type-7 translator will
         continue to perform its translator duties once it has
         determined that its translator status has been deposed by
         another NSSA border router translator as described in Section
         3.1 and 3.3.  The default setting is 40 seconds.

      ImportSummaries

         When set to enabled, OSPF's summary routes are imported into
         the NSSA as Type-3 summary-LSAs.  When set to disabled, summary
         routes are not imported into the NSSA.  The default setting is
         enabled.

   Implementations must provide a vehicle for setting the P-bit when
   external routes are imported into the NSSA as Type-7 LSAs.  Without
   configuration, the default setting of the P-bit is clear.  (See
   Section 2.3 and Appendix B.)

   For NSSAs the ExternalRoutingCapability area configuration parameter
   must be set to accept Type-7 external routes.  Additionally there
   must be a way of configuring the metric of the default LSA that a
   border router advertises into its directly attached NSSAs. If a
   Type-7 default LSA is advertised, its metric type (1 or 2) should
   also be configurable.

Appendix E: The P-bit Policy Paradox.

   Non-default Type-7 LSAs with the P-bit clear may be installed in the
   OSPF routing table of NSSA border routers.  (See Section 2.5.)  These
   LSAs are not propagated throughout the OSPF domain as translated
   Type-5 LSAs.  (See Section 3.2.)  Thus, traffic that is external to
   an NSSA and that passes through one of the NSSA's border routers may
   be hijacked into the NSSA by a route installed from a Type-7 LSA with
   the P-bit clear.  This may be contrary to the expected path at the
   source of the traffic.  It may also violate the routing policy
   intended by the Type-7 LSA's clear P-bit.  A Type-7 address range
   that is configured with DoNotAdvertise exhibits the same paradox for
   any installed Type-7 LSAs it subsumes, regardless of the P-bit
   setting.

   This paradox is best illustrated by the following example.  Consider
   an OSPF domain (AS 1842) with connections for default Internet
   routing and to external AS 4156.  NSSA 1 and OSPF Area 2 are
   partially defined in the following diagram:

                              AS 4156
                                |
            Area 2              |
                                |
              A2                A0   Area 0      C0-----Internet
              |                 |                |      Default
              |                 |                |
              |                 |                |
              +-----------------B0---------------+
                                /\
                               /  \
                              /    \
         Internet------------A1    B1------AS 4156 (P-bit clear)
         Default (P-bit set)
                                 NSSA 1

   Here A0, B0, and C0 are Area 0 routers, A1 and B1 are NSSA 1 routers,
   and A2 is an Area 2 router.  B0 is a border router for both NSSA 1
   and Area 2.

   If the Type-7 external routes imported by B1 for AS 4156 are
   installed on B0 so that the NSSA 1 tree below A1 can take advantage
   of them, then A2's traffic to AS 4156 is hijacked through B0 by B1,
   rather than its computed path through A0.

   An NSSA border router's installed Type-7 default LSAs will exhibit
   this paradox when it possesses a Type-7 address range [0,0]
   configured with DoNotAdvertise, as these LSAs are not propagated even

   though their P-bit is set.  In the example above, if A1's default is
   installed on B0, which has a configured Type-7 address range [0,0]
   with DoNotAdvertise set, then A2's Internet traffic is hijacked
   through B0 by A1 rather than the computed path through C0.

Appendix F: Differences from RFC 1587

   This section documents the differences between this memo and RFC
   1587.  All differences are backward-compatible.  Implementations of
   this memo and of RFC 1587 will interoperate.

F.1 Enhancements to the import of OSPF's summary routes.

   The import of OSPF's summary routes into an NSSA as Type-3 summary-
   LSAs is now optional.  In RFC 1587 the import of summary routes was
   mandated in order to guarantee that inter-area summary routing was
   not obscured by an NSSA's Type-7 AS-external-LSAs. The current
   recommended default behavior is to import summary routes.  When
   summary routes are not imported into an NSSA, the default LSA
   originated by its border routers must be a Type-3 summary-LSA.

   See Sections 1.3 and 2.7 for details.

F.2 Changes to Type-7 LSAs.

   The setting of the forwarding address in Type-7 LSAs has been further
   refined.

   See Section 2.3 for details.

F.3 Changes to the Type-7 AS external routing calculation.

   The Type-7 external route calculation has been revised.  Most
   notably:

      o  The path preference defined in [OSPF] Section 16.4.1 has been
         included.

      o  A Type-7 default route with the P-bit clear will not be
         installed on an NSSA border router.  This protects the default
         routing of other OSPF Areas.  (See Appendix E.)

      o  The LSA type of two AS-external-LSAs plays no role in
         determining path preference except when the LSAs are
         functionally the same (i.e., same destination, cost and non-
         zero forwarding address).

   See Section 2.5 for details.

F.4 Changes to translating Type-7 LSAs into Type-5 LSAs

   The translator election algorithm of RFC 1587 has been updated to
   close a bug that results when the translator with the highest router
   ID loses connectivity to the AS's transit topology.  The default
   translator election process occurs only in the absence of an existing
   translator.

   The identity of the translator is optionally configurable, with more
   than one allowed.  This allows the network designer to choose the
   most cost effective intra-AS route for NSSA originated Type-5 LSA
   aggregations of Type-7 LSAs.

   Self-originated non-default Type-7 LSAs are now included in the
   translation process.

   The translation process has been strengthened to close some of the
   weak points of RFC 1587.

   See Sections 3.1 and 3.2 for details.

F.5 Changes to flushing translated Type-7 LSAs

   An NSSA border router, which was elected by the augmented RFC 1587
   translator selection process defined in Section 3.1 and which has
   been deposed from its translation duties by another NSSA border
   router, flushes its self-originated Type-5 LSAs that resulted from
   the aggregation of Type-7 LSAs.  This prevents these obsolete
   aggregations from short circuiting the preferred path through the new
   translator(s).  A deposed translator continues to maintain its self-
   originated Type-5 LSAs resulting from translation until they age out
   normally.

   See Section 3.3 for details.

F.6 P-bit additions

   The P-bit default has been defined as clear.  RFC 1587 had no default
   setting. (See Appendix C.)

   A discussion on the packet forwarding impact of installing Type-7
   LSAs with the P-bit clear on NSSA border routers has been added as
   Appendix E.

Author's Addresses

   Pat Murphy
   US Geological Survey
   345 Middlefield Road
   Menlo Park, California 94560

   Phone: (650) 329-4044
   EMail: pmurphy@noc.usgs.net

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