Demand OSPF and Demand RIP 

                              Gerry Meyer
                             IETF San Jose


1. Demand RIP and Demand OSPF Differences

   o  RIP and OSPF both learn routes and presume them subsequently.

   o  RIP and OSPF both propagate changed routing information only.

   o  RIP has a topological restriction.  Each WAN destination must be
      directly accessible.

      No routing information is re-propagated on the WAN, so no routes
      exist through third parties.

   o  OSPF allows WAN destinations to be reachable through a third
      party.

      But only if configured to have no direct route.

   o  RIP requires and makes extensive use of a link level circuit manager.

   o  RIP uses information from the circuit manager to time out routes
      when the link is reported down.

      This allows dynamic reconfiguration via alternative routes.

   o  OSPF makes little or no use of a link level circuit manager.

   o  OSPF has no dynamic reconfiguration of demand links.

      So can't make effective use of alternative routes.


2. Illustration for Effect of Split Horizon

    +------+       +------+          +------+       +------+
    | Sub- |       | Sub- |          | Sub  |       | Sub- |
    | Area |o o o o| Area |          | Area |o o o o| Area |
    |  A   |       |  B   |          |  A   |       |  B   | 
    +------+       +------+          +------+       +------+
            o                                o         o
             o                                o        o
              o                                o       o 
               o                                o      o
                o                                o     o
                 o                                o    o
                  o                                o   o
                   +------+                         +------+
                   | Sub- |                         | Sub- |
                   | Area |                         | Area |
                   |  C   |                         |  C   |
                   +------+                         +------+

         Figure: 1a                       Figure: 1b


3. Typical Usage - Star Network

               +---+       +---+       +---+
               |RT5|       |RT4|o o o o|RT3|
               +---+       +---+       +---+
                    .        .        .
                     .       .       .
                      .      .      .
                       .     .     .
                        .    .    .
                         .   .   .
                          .  .  .
               +---+       +---+       +---+
           	   |RT6|. . . .|RT1|. . . .|RT2|
               +---+       +---+       +---+
                          .  o  o        .
                         .   o   o       .
                        .    o    o      .
                       .     o     o     .
                      .      o      o    .
                     .       o       o   .
                    .        o        o  .
               +---+       +---+       +---+
               |RT5|       |RT4|       |RT3|
               +---+       +---+       +---+

              Figure 2:  Loose Star Topology

The golden rule:

   o  There must be enough channels to satisfy the required
      simultaneous use.

      If there is a reasonable probability that more than 2 simultaneous
      channels will be in use carrying data to the central site use 
      grouped ISDN interfaces to provide the required level of channel use.


4. Typical Usage - Resilience

                 +--+  +                     +  +--+
                 |H1|--|  +---+       +---+  |--|H1|
                 +--+  |--|RT1|       |RT2|--|  +--+
                       |  +---+       +---+  |
                       |      o  ..  o       |
                       |       .    .        |
                       |      .      .       |
                       |      . ISDN .       |
                       |      .      .       |
                       |       .    .        |
                       |         ..          |
                       |         o    |      |
                       |       +---+  |      |
                       |       |RT5|--|      |
                       |       +---+  |      |
                       |         o    |      |
                       |         ..    LAN   |
                       |       .    .        |
                       |      .      .       |
                       |      . X.25 .       |
                       |      .      .       |  
                       |       .    .        |
                       |      o  ..  o       |
                 +--+  |  +---+       +---+  |  +--+
                 |Hn|--|--|RT4|       |RT3|--|--|Hn|
                 +--+  |  +---+       +---+  |  +--+
                       +                     +

                      LAN                   LAN

          Figure 3: Sites with ISDN BRI routers and X.25 routers