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Chapter 8 - PIM-DVMRP Networks

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Chapter 8: PIM-DVMRP Networks
Overview
Consider these facts. Approximately 80 percent of the Internet routers are Cisco routers, and the Multicast Backbone (MBONE) runs on top of the Internet. The multicast protocol that is used on the MBONE is DVMRP and Cisco does not support a full implementation of DVMRP. So how do we get MBONE multicast traffic into a Cisco network? Very easily. Cisco routers interoperate with DVMRP routers for route exchange.
At the outset of this chapter, it is important to clarify the distinction between a routing protocol and a routed protocol. OSPF, for example, is a routing protocol. Routing protocols are used to determine a path to the destination for a routed protocol. Routed protocols include IP, IPX, AppleTalk and DECNet. Routed protocols carry their data inside of specific packets. If we are using OSPF, then we are routing IP packets, which do not travel inside of OSPF packets; they travel inside of IP packets. The same argument can be made for IP multicast data, which travels inside of IP packets. The packet does not care how it gets routed to the destination as long as it gets there. It makes no difference if the network is running DVMRP, PIM-DM, or PIM-SM. Therefore, if a mechanism exists so that PIM and DVMRP can exchange routes, then MBONE packets can be delivered to non-DVMRP networks.
No configuration commands can enable PIM-DVMRP interoperability; thus, no commands are needed because PIM-DVMRP interaction on a Cisco router is automatic. In the network of Figure 8-1, we have a Cisco router connected to an MBONE router running mrouted. When the DVMRP router sends a periodic neighbor probe message on the common interface between the two routers, the Cisco router realizes that a DVMRP router is out there and PIM-DVMRP interoperability will be automatically enabled.
Figure 8-1: PIM router discovery of a DVMRP neighbor
The interaction between the two domains depends on the type of connection between them. In a tunnel connection, the PIM router does not respond to the neighbor probe, but other information is exchanged. When the PIM router receives a DVMRP route report, the DVMRP routes are installed in a separate DVMRP routing table on the PIM router. The PIM router then poison-reverses the appropriate routes learned from the DVMRP router and sends a route report to the DVMRP neighbor. Selected routes from the unicast routing table are also advertised in the route report, while DVMRP probes and grafts are exchanged between the PIM and DVMRP routers over the DVMRP tunnel (see Figure 8-2).
Figure 8-2: DVMRP-PIM exchanges through a DVMRP tunnel.
For a non-tunnel connection, such as ethernet, the information exchange is modified slightly from the tunnel case (see Figure 8-3). Again, DVMRP probes are not sent by the PIM router. If the PIM routers in Figure 8-3 send a DVMRP neighbor probe onto the ethernet network, then the other PIM neighbor would receive them and think that the other PIM router is a DVMRP router.
Figure 8-3: DVMRP-PIM exchanges over a regular interface.
The route report only contains selected routes from the unicast routing table and does not contain poison-reversed DVRMP routes, as in the tunnel case. Received DVRMP route reports are actually ignored by the PIM routers. Although Prunes, Grafts, and Graft Acknowledgments are also exchanged, Prunes from the DVMRP neighbor are also ignored. The PIM routers sends IGMP joins for any group that has IGMP state on the PIM routers. This makes the DVMRP router think that hosts on the ethernet have joined the group, causing the DVMRP router to forward traffic for these groups onto the ethernet. Obviously, the PIM routers do not act like a true DVMRP router. An interface command that you can use to instruct the PIM routers to behave more like a DVMRP router on a multi-access network is
ip dvmrp unicast-routing
The interface command causes routes received in DVMRP Report messages to be cached in the DVMRP routing table; these routes will have preference over routes in the unicast routing table. Also, IGMP Joins for groups that have state on the PIM router will no longer be sent (see Figure 8-4). This command is not used to enable DVMRP between Cisco routers but to force the router to act more like a DVMRP router when there is a non-Cisco DVMRP neighbor. IGMP Group Joins no longer need to be sent to the DVMRP neighbor because the PIM router sends poison-reversed routes in the route report that inform the DVMRP neighbor which traffic needs to be forwarded to the PIM neighbor. The Cisco router now functions more like a true DVMRP router, except that DVMRP neighbor probes are not being sent and received Prunes are still ignored.
Figure 8-4: PIM routers configured to exchange DVMRP route reports

 


 
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