RFC 1793 (rfc1793) - Page 2 of 32

Extending OSPF to Support Demand Circuits

RFC 1793               OSPF over Demand Circuits              April 1995


   to-MultiPoint Interface).

   This memo provides functionality similar to that specified for RIP in
   [2], with the main difference being the way the two proposals handle
   oversubscription (see Sections 4.3 and 7 below).  However, because
   OSPF employs link-state routing technology as opposed to RIP's
   Distance Vector base, the mechanisms used to achieve the demand
   circuit functionality are quite different.

   Please send comments to .

Acknowledgments

   The author would like to acknowledge the helpful comments of Fred
   Baker, Rob Coltun, Dawn Li, Gerry Meyer, Tom Pusateri and Zhaohui
   Zhang. This memo is a product of the OSPF Working Group.

Table of Contents

    1.      Model for demand circuits .............................. 3
    2.      Modifications to all OSPF routers ...................... 4
    2.1     The OSPF Options field ................................. 4
    2.2     The LS age field ....................................... 5
    2.3     Removing stale DoNotAge LSAs ........................... 6
    2.4     A change to the flooding algorithm ..................... 6
    2.5     Interoperability with unmodified OSPF routers .......... 7
    2.5.1   Indicating across area boundaries ...................... 8
    2.5.1.1 Limiting indication-LSA origination .................... 9
    3.      Modifications to demand circuit endpoints ............. 10
    3.1     Interface State machine modifications ................. 10
    3.2     Sending and Receiving OSPF Hellos ..................... 11
    3.2.1   Negotiating Hello suppression ......................... 11
    3.2.2   Neighbor state machine modifications .................. 12
    3.3     Flooding over demand circuits ......................... 12
    3.4     Virtual link support .................................. 13
    3.5     Point-to-MultiPoint Interface support ................. 14
    4.      Examples .............................................. 15
    4.1     Example 1: Sole connectivity through demand circuits .. 15
    4.2     Example 2: Demand and non-demand circuits in parallel . 19
    4.3     Example 3: Operation when oversubscribed .............. 23
    5.      Topology recommendations .............................. 25
    6.      Lost functionality .................................... 25
    7.      Future work: Oversubscription ......................... 26
    8.      Unsupported capabilities .............................. 28
    A.      Format of the OSPF Options field ...................... 30
    B.      Configurable Parameters ............................... 31
    C.      Architectural Constants ............................... 31
            References ............................................ 32



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