rfc1765.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

         When all routers have identical values for ospfExtLsdbLimit (as
         required), the above flooding modification will only be invoked
         during a short period of convergence. During convergence, there
         will be retransmissions of LSAs. However, after convergence the
         retransmissions will cease, as the routers settle on a database
         having less than ospfExtLsdbLimit non-default As-external-LSAs.

         In OverflowState, non-default AS-external-LSAs ARE still
         accepted in the following conditions:

            (1) If the LSA updates an LSA that currently exists in the
                router's link-state database.

            (2) LSAs having LS age of MaxAge are always accepted. The
                processing of these LSAs follows the procedures
                described in Sections 13 and 14 of [1].

            (3) If adding the LSA to the router's database would keep
                the number of non-default AS-external-LSAs less than or
                equal to ospfExtLsdbLimit, the LSA is accepted.







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RFC 1765                 OSPF Database Overflow               March 1995


      2.3.2.  Originating AS-external-LSAs

         Originating AS-external-LSAs is described in Section 12.4.5 of
         [1].  When a router is in OverflowState, it does not originate
         non-default AS-external-LSAs. In other words, the only AS-
         external-LSAs originated by a router in OverflowState have Link
         State ID 0.0.0.0.

      2.3.3.  Receiving self-originated LSAs

         Receiving self-originated LSAs is described in Section 13.4 of
         [1].  When in OverflowState, a router receiving a self-
         originated non-default AS-external-LSA responds by flushing it
         from the routing domain using the premature aging scheme
         described in Section 14.1 of [1].

   2.4.  Leaving OverflowState

      If ospfExitOverflowInterval is non-zero, then as soon as a router
      enters OverflowState, it sets a timer equal to the value of
      ospfExitOverflowInterval (plus or minus a random value in the
      range of 10 percent). When this timer fires, the router leaves
      OverflowState and begins originating non-default AS-external-LSAs
      again.

      This allows a router to automatically recover from transient
      overflow conditions. For example, an AS boundary router that
      imports a great many AS-external-LSAs may crash. Other routers may
      then start importing the routes, but until the crashed AS boundary
      router is either a) restarted or b) its AS-external-LSAs age out,
      there will be a much larger database than usual.  Since such an
      overflow is guaranteed to go away in MaxAge seconds (1 hour),
      automatic recovery may be appropriate (and fast enough) if the
      overflow happens off-hours.

      As soon as the router leaves OverflowState, it is again eligible
      to reenter OverflowState according to the text of Section 2.2.

3.  An example

   As an example, suppose that a router implements the database overflow
   logic, and that its ospfExtLsdbLimit is 10,000 and its
   ospfExitOverflowInterval is set to 600 seconds. Suppose further that
   the router itself is originating 400 non-default AS-external-LSAs,
   and that the current number of non-default AS-external-LSAs in the
   router's database is equal to 9,997.





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RFC 1765                 OSPF Database Overflow               March 1995


   Next, it receives a Link State Update packet from a neighbor,
   containing 6 non-default AS-external-LSAs, none of which have current
   database copies.  The first two LSAs are then installed in the
   database. The third LSA is also installed in the database, but causes
   the router to go into OverflowState.  Going into OverflowState causes
   the router to flush (via premature aging) its 400 self-originated
   non-default LSAs. However, these 400 LSAs are still considered to be
   part of the link-state database until their re-flooding (with age set
   to MaxAge) is acknowledged (see Section 14 of [1]); for this reason,
   the last three LSAs in the received update are discarded without
   being acknowledged.

   After some small period of time all routers will converge on a common
   database, having less than 10,000 non-default AS-external-LSAs.
   During this convergence period there may be some link-state
   retransmissions; for example, the sender of the above Link State
   Update packet may retransmit the three LSAs that were discarded. If
   this retransmission happens after the flushing of the 400 self-
   originated LSAs is acknowledged, the 3 LSAs will then be accepted.

   Going into OverflowState also causes the router to set a timer that
   will fire some time between 540 and 660 seconds later. When this
   timer fires, the router will leave OverflowState and re-originate its
   400 non-default AS-external-LSAs, provided that the current database
   has less than 9600 (10,000 - 400) non-default AS-external-LSAs. If
   there are more than 9600, the timer is simply restarted.

4.  Administrative response to database overflow

   Once the link-state database has overflowed, it may take intervention
   by network management before all routing is restored.  (If the
   overflow condition is transient, routing may be restored
   automatically; see Section 2.4 for details.) An overflow condition is
   indicated by SNMP traps (see Appendix B). Possible responses by a
   network manager may include:

    o   Increasing the value of ospfExtLsdbLimit. Perhaps it had been
        set too conservatively, and the routers are able to support
        larger databases than they are currently configured for.

    o   Isolating routers having limited resources within OSPF stub
        areas or NSSAs.  This would allow increasing the value of
        ospfExtLsdbLimit in the remaining routers.

    o   Reevaluating the need to import certain external routes. If
        ospfExtLsdbLimit cannot be increased, the network manager will
        want to make sure that the more important routes continue to be
        imported; this is accomplished by turning off the importing of



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RFC 1765                 OSPF Database Overflow               March 1995


        less important routes.

5.  Operational experience

   The database overflow scheme described in this memo has been
   implemented in the Proteon router for a number of years, with the
   following differences. First, the router did not leave OverflowState
   until it was restarted (i.e., ospfExitOverflowInterval was always 0).
   Second, default AS-external-LSAs were not separated from non-default
   AS-external-LSAs. Operationally the scheme performed as expected:
   during overflow conditions, the routers converged on a common
   database having less than a configured number of AS-external-LSAs.

6.  Possible enhancements

   Possible enhancements to the overflow scheme include the following:

    o   Other LSA types, with the exception of the transit LSAs
        (router-LSAs and network-LSAs), could be limited in a similar
        fashion. For example, one could limit the number of summary-
        LSAs, or group-membership-LSAs (see [6]).

    o   Rather than flushing all of its non-default AS-external-LSAs
        when entering OverflowState, a router could flush a fixed number
        whenever the database size hits ospfExtLsdbLimit. This would
        allow the router to prioritize its AS-external-LSAs, flushing
        the least important ones first.

A. Related MIB parameters

   The following OSPF MIB variables have been defined to support the
   database overflow procedure described in this memo (see [4] for more
   information):

    ospfExtLsdbLimit
        As in Section 2.1 of this memo, the maximum number of non-
        default AS-external-LSAs that can be stored within the database.
        If set to -1, there is no limit.

    ospfExitOverflowInterval
        As in Section 2.1 of this memo, the number of seconds that,
        after entering OverflowState, a router will attempt to leave
        OverflowState. This allows the router to again originate non-
        default AS-external-LSAs.  When set to 0, the router will not
        leave OverflowState until restarted.






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RFC 1765                 OSPF Database Overflow               March 1995


    ospfLsdbOverflow
        A trap indicating that the number of non-default AS-external-
        LSAs has exceeded or equaled ospfExtLsdbLimit. In other words,
        this trap indicates that the router is entering OverflowState.

    ospfLsdbApproachingOverflow
        A trap indicating that the number of non-default AS-external-
        LSAs has exceeded ninety percent of "ospfExtLsdbLimit".

References

   [1] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.

   [2] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC 1587,
       RainbowBridge Communications, Stanford University, March 1994.

   [3] Moy, J., Editor, "OSPF Protocol Analysis", RFC 1245, Proteon,
       Inc., July 1991.

   [4] Baker F., and R. Coltun, "OSPF Version 2 Management Information
       Base", Work in Progress.

   [5] Moy, J., Editor, "Experience with the OSPF Protocol", RFC 1246,
       Proteon, Inc., July 1991.

   [6] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon, Inc.,
       March 1994.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   John Moy
   Cascade Communications Corp.
   5 Carlisle Road
   Westford, MA 01886

   Phone: [phone] Ext. 394
   Fax:   [phone]
   EMail: [e-mail]









Moy                                                             [Page 9]