rfc1587.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

         for the most part must contain a forwarding address.
         The execption to this is if the translation to a type-5
         LSA is the result of an address range match, in which
         case the type-5 LSA will not contain a forwarding address
         (see section 4.1 for details).
         The forwarding address contained in type-5 LSAs will
         result in more efficient routing to the AS external
         networks when there are multiple NSSA area
         border routers. Having the forwarding address in the
         type-7 LSAs will ease the translation of type-7 into
         type-5 LSAs as the NSSA area border router will
         not be required to compute the forwarding address.

         If the network between the NSSA AS boundary router and the
         adjacent AS is advertised into OSPF as an internal OSPF



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RFC 1587                    OSPF NSSA Option                  March 1994


         route, the forwarding address should be the next hop
         address as is currently done in type-5 LSAs, but unlike
         type-5 LSAs if the intervening network is not advertised
         into OSPF as an internal OSPF route, the forwarding
         address should be any one of the router's active OSPF
         interface addresses.

   Type-5 and type-7 metrics and path types are directly comparable.

3.4  Originating Type-7 LSAs

   NSSA AS boundary routers may originate type-7 LSAs.  All NSSA area
   border routers must also be AS boundary routers since they all must
   have the capability of translating a type-7 LSAs into a type-5 LSAs
   (see section 3.6 routes for the translation algorithm).  NSSA area
   border routers must set the E-bit (external bit) as well as the B-bit
   (border bit) in its router (type-1) LSAs (both in the backbone and in
   the NSSA area).

   When an NSSA internal AS boundary router originates a type-7 LSA that
   it wants to be translated into a type-5 LSA by the NSSA area border
   router (and subsequently flooded into the backbone), it must set the
   P-bit in the LS header's option field and add a valid forwarding
   address in the type-7 LSA.

   If a router is attached to another AS and is also an NSSA area border
   router, it may originate a both a type-5 and a type-7 LSA for the
   same network.  The type-5 LSA will be flooded to the backbone (and
   all attached type-5 capable areas) and the type-7 will be flooded
   into the NSSA.  If this is the case, the P-bit must be reset in the
   type-7 NSSA so the type-7 LSA isn't again translated into a type-5
   LSA by another NSSA area border router.

   A type-7 default route (network 0.0.0.0) may be originated into the
   NSSA by an NSSA area border router or by an NSSA AS boundary router
   which is internal to the NSSA.  The type-7 default route originated
   by the NSSA area border router must have the P-bit reset so that the
   default route originated by the NSSA area border router will not find
   its way out of the NSSA into the rest of the AS system via another
   NSSA area border router.  The type-7 default route originated by an
   NSSA AS boundary router which is not an NSSA area border router may
   have the P-bit set.  Type-7 routes which are originated by the NSSA
   area border router will not get added to other NSSA area border
   router's routing table.

   A default route must not be injected into the NSSA as a summary
   (type-3) LSA as in the stub area case.  The reason for this is that
   the preferred summary default route would be chosen over all more



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   specific type-7 routes.  Because summary routes are preferred to
   external routes and to ensure that summary routes are chosen over
   external within the NSSA, all summary routes (unlike stub areas in
   which this is optional) must be imported into an NSSA.

3.5 Calculating Type-7 AS External Routes

   This section is very similar to section 16.4 (Calculating AS external
   routes) in the OSPF specification.  An NSSA area border router should
   examine both type-5 LSAs and type-7 LSAs if either type-5 or type-7
   routes need to be updated.  Type-7 LSAs should be examined after
   type-5 LSAs.  An NSSA internal router should examine type-7 LSAs when
   type-7 routes need to be recalculated.

   In relation to the steps to calculate the routing table as presented
   in the OSPF specification (chapter 16, "Calculation of the Routing
   Table"), type-7 LSAs should be examined after step 5 where the routes
   to external destinations are calculated.

   Type-7 routes are calculated by examining type-7 LSAs.  Each of LSAs
   are considered in turn. Most type-7 LSAs describe routes to specific
   IP destinations.  A type-7 LSA can also describe a default route for
   the NSSA (destination = DefaultDestination).  For each type-7 LSA:

      1. If the metric specified by the LSA is LSInfinity, the
         age of the LSA equals MaxAge or the advertising router
         field is equal to this router's router ID, examine the
         next advertisement.

      2. Call the destination described by the LSA N. Look up the
         routing table entry for the AS boundary router (ASBR) that
         originated the LSA. If no entry exists for the ASBR
         (i.e., ASBR is unreachable), do nothing with this LSA and
         consider the next in the list.

         If the destination is the default route (destination =
         DefaultDestination) and if the originator of the LSA and
         the calculating router are both NSSA area border routers
         do nothing with this LSA and consider the next in the list.

         Else, this LSA describes an AS external path to destination
         N. If the forwarding address (as specified in the forwarding
         address field of the LSA) is 0.0.0.0, the packets routed
         to the external destination N will be routed to the
         originating ASBR. If the forwarding address is not 0.0.0.0,
         look up the forwarding address in the routing table. Packets
         routed to the external destination N will be routed within
         the NSSA to this forwarding address. An intra-area path



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         must therefore exist to the forwarding address. If no such
         path exists, do nothing with the LSA and consider the next
         in the list.

         Call the routing table distance to the forwarding address
         (or the distance to the originating ASBR if the forwarding
         address is 0.0.0.0) X, and the cost specified in the type-7
         LSA Y. X is in terms of the link-state metric, and Y is a
         Type-1 or Type-2 external metric.

      3. Now, look up the routing table entry for the destination
         N. If no entries exist for N, install the AS external path
         to N, with the next hop equal to the list of next hops to
         the forwarding address/ASBR, and the advertising router equal
         to 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.

      4. Else, if the paths present in the table are not Type-1 or
         Type-2 external paths, do nothing (AS external paths have
         the lowest priority).

      5. Otherwise, compare the cost of this new AS external path
         to the ones present in the table. Note that type-5 and
         type-7 routes are directly comparable. Type-1 external
         paths are always shorter than Type-2 external paths.
         Type-1 external paths are compared by looking at the sum
         of the distance to the forwarding address/ASBR and the
         advertised Type-1 paths (X+Y). Type-2 external paths are
         compared by looking at the advertised Type-2 metrics,
         and then if necessary, the distance to the forwarding
         address/ASBR.

         When a type-5 LSA and a type-7 LSA are found to have the
         same type and an equal distance, the following priorities
         apply (listed from highest to lowest) for breaking the tie.

                 a. Any type 5 LSA.
                 b. A type-7 LSA with the P-bit set and the forwarding
                    address non-zero.
                 c. Any other type-7 LSA.

         If the new path is shorter, it replaces the present paths
         in the routing table entry. If the new path is the same
         cost, it is added to the routing table entry's list of
         paths.



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RFC 1587                    OSPF NSSA Option                  March 1994


3.6 Incremental Updates

   Incremental updates for type-7 LSAs should be treated the same as
   incremental updates for type-5 LSAs (see section 16.6 of the OSPF
   specification).  That is, if a new instance of a type-7 LSA is
   received it is not necessary to recalculate the entire routing table.
   If there is already an OSPF internal route to the destination
   represented by the type-7 LSA, no recalculation is necessary.
   Otherwise, the procedure in the proceeding section will have to be
   performed but only for the external routes (type-5 and type-7) whose
   networks describe the same networks as the newly received LSA.

4.0 Originating Type-5 LSAs

4.1 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 not an area border router this translation
   algorithm should be skipped.  All reachable area border routers in
   the NSSA should now be examined noting the one with the highest
   router ID.  If this router has the highest router ID, it will be the
   one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise
   the translation algorithm should not be performed.

   All type-7 routes that have been added to the routing table should be
   examined.  If the type-7 LSA (associated with the route being
   examined) has the P-bit set and a non-zero forwarding address, the
   following steps should be taken.

      The translation procedure must first check for a configured type-7
      address range.  Recall that an type-7 address range consists of an
      [address,mask] pair and a status indication of either Advertise or
      DoNotAdvertise.  At most a single type-5 LSA is made for each
      range.  If the route being examined falls within the type-7
      address range, (the [address,mask] pair of the route equal to or a
      more specific instance of the [address,mask] pair of the type-7
      address range), one of following three actions may take place.

         1. When the range's status indicates Advertise and the
            route's address and mask are equal to the address
            and mask of the type-7 range a type-5 LSA should be
            originated if:

            o there currently is no type-5 LSA originated from
              this router corresponding to the type-7 LSA,





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RFC 1587                    OSPF NSSA Option                  March 1994


            o the path type or the metric in the corresponding
              type-5 LSA is different from the type-7 LSA or

            o The forwarding address in the corresponding
              type-5 LSA is different from the type-7 LSA.

              The newly originated type-5 LSA will describe
              the same network and have the same network mask,
              metrics, forwarding address, external route tag
              and path type as the type-7 LSA, however, the
              advertising router field will be the router ID
              of this area border router.

         2. When the range's status indicates Advertise and the
            route's address or mask indicates a more specific
            route (i.e., the route's address is subsumed by the
            range or the route has a longer mask), a type-5 LSA
            is generated with link-state ID equal to the range's
            address (if necessary, the link-state ID can also have
            one or more of the range's "host" bits set; see
            Appendix F of the OSPF specification for details),
            the network mask, external route tag and
            path type will be set to the configured type-7 range
            values. The advertising router field will be the
            router ID of this area border router.
            The forwarding address will not be set.
            The path type should always be set to the highest
            path type that is subsumed by the net range.
            The metric for the type-5 LSA will be set as follows:

            o if the path type is externl type 2, the type-5
              metric should be set to the largest type-7 metric
              subsumed by this net range + 1.

            o if the path type is external type 1, the type-5
              metric should be set to the largest metric.

            For example, given a net range of [10.0.0.0, 255.0.0.0]
            for an area that has type-7 routes of:

                    10.1.0.0 path type 1, metric 10
                    10.2.0.0 path type 1, metric 11
                    10.3.0.0 path type 2, metric 5

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





Coltun & Fuller                                                [Page 11]

RFC 1587                    OSPF NSSA Option                  March 1994


             As another example, given a net range of
             [10.0.0.0, 255.0.0.0] for an area that has
             type-7 routes of:

                    10.1.0.0 path type 1, metric 10
                    10.2.0.0 path type 1, metric 11
                    10.3.0.0 path type 1, metric 5

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

             These metric and path type rules will avoid routing
             loops in the event that path type 1 and 2 are both
             used within the area.

         3. When the range's status indicates DoNotAdvertise,