Connecting to DVMRP Networks

You might, on occasion, have to connect your PIM router to a DVMRP router. This is not necessarily a large-scale multicast issue ”routers that can speak only DVMRP can be encountered in an internetwork of any size . However, the most likely circumstance is when you are connecting to the MBone.

When you configure an interface on a Cisco router to run PIM, it listens for DVMRP Probe messages. When Probes are heard , as demonstrated in the output in Example 7-4, Cisco IOS Software automatically enables DVMRP on the interface. No special configuration is required. PIM routes are advertised to the DVMRP neighbor in DVMRP Report messages. DVMRP Report messages learned from the neighbor are kept in a separate DVMRP routing table shown in Example 7-5, but it is still PIM on the Cisco router that makes the multicast forwarding decisions. DVMRP Graft messages are sent and received normally, but it is the handling of Prunes and Probes that makes the Cisco IOS Software implementation of DVMRP different from a full implementation.

Example 7-4 This Router Is Receiving DVMRP Probe Messages on Interface E0 from Neighbor 10.224.1.1

 Godzilla#  debug ip dvmrp detail  DVMRP debugging is on Godzilla# DVMRP: Received Probe on Ethernet0 from 10.224.1.1 DVMRP: Aging routes, 0 entries expired DVMRP: Received Probe on Ethernet0 from 10.224.1.1 DVMRP: Aging routes, 0 entries expired DVMRP: Received Probe on Ethernet0 from 10.224.1.1 DVMRP: Aging routes, 0 entries expired 

Example 7-5 The show ip dvmrp route Command Displays DVMRPSpecific Route Information

 Godzilla#  show ip dvmrp route  DVMRP Routing Table - 7 entries 10.224.2.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 10.224.3.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 10.224.4.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 10.224.5.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 10.224.6.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 172.16.70.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 192.168.50.0/24 [0/1] uptime 00:04:21, expires 00:02:38     via 10.224.1.1, Ethernet0, [version mrouted 3.255] [flags: GPM] 

The first difference between a full implementation of DVMRP and a Cisco IOS Software-based implementation of DVMRP is the handling of Probes. As already mentioned, the detection of Probe messages is how a Cisco router discovers DVMRP neighbors. Suppose, however, that the DVMRP neighbor is on a multiaccess network, and more than one Cisco router is attached to the same network. If one of the Cisco routers were to originate a Probe, the neighboring Cisco routers would mistakenly assume the originator is a DVMRP router rather than a PIM router, as illustrated by Figure 7-2. Therefore, Cisco routers listen for Probe messages but do not originate them.

Figure 7-2. If the Top Cisco Router Were to Generate a DVMRP Probe Message, the Bottom Cisco Router Would Mistakenly Record the Originator as a DVMRP Neighbor; Therefore, Cisco Routers Do Not Generate DVMRP Probes

The second difference is the handling of Prune messages. Recall from the DVMRP discussion in Chapter 5 that a DVMRP router is required to maintain state for each downstream neighbor. If a downstream neighbor sends a Prune message, only that neighbor's state is pruned. Traffic is still forwarded on the interface unless all DVMRP neighbors send a Prune. This addresses the situation in which there are multiple downstream neighbors on a multiaccess network, and it prevents a Prune from one neighbor causing an unwanted Prune from another neighbor.

NOTE

Also recall from Chapter 5 that PIM-DM uses a Prune override mechanism to address this problem, instead of requiring the maintenance of neighbor states.

However, Cisco routers do not maintain DVMRP neighbor state. Therefore, to avoid the problem of one downstream neighbor's Prunes pruning traffic needed by another downstream neighbor, Cisco routers ignore DVMRP Prune messages received on multiaccess interfaces. On point-to-point interfaces, Prunes are received and processed normally, because by definition there can be only one downstream neighbor. Cisco routers send Prune messages normally on both multiaccess and point-to-point interfaces on which there are DVMRP neighbors.

The difficulty, as this approach stands, should be apparent to you. If DVMRP routers connected across a multiaccess network to upstream Cisco routers cannot prune themselves , the Cisco routers forward unwanted multicast traffic into the DVMRP domain. The solution to the difficulty is, once again, tunnels.

In Figure 7-3, a Cisco router is connected to two DVMRP routers across a multiaccess network. By creating tunnels to each of the DVMRP routers, Cisco IOS Software sees the DVMRP neighbors as connected via point-to-point links rather than a multiaccess link. The Cisco router then accepts prunes.

Figure 7-3. Tunnels Are Used to Create Point-to-Point Connections to the DVMRP Routers Across the Multiaccess Network, so DVMRP Pruning Works Correctly

Example 7-6 shows the configuration for the Cisco router in Figure 7-3.

Example 7-6 Configuring the Cisco Router in Figure 7-3 to Accept Prunes via Point-to-Point Links

  interface Tunnel0   ip unnumbered Ethernet0   ip pim sparse-dense-mode   tunnel source Ethernet0   tunnel destination 10.1.1.2   tunnel mode dvmrp   !   interface Tunnel1   no ip address   ip pim sparse-dense-mode   tunnel source Ethernet0   tunnel destination 10.1.1.4   tunnel mode dvmrp   !   interface Ethernet0   ip address 10.1.1.1 255.255.255.0  

The only significant difference from the earlier tunnel configurations you have seen is that the tunnel mode is set to DVMRP rather than the default GRE. As with the earlier tunnel configurations, PIM is configured on the tunnels but not on the physical interface. If there were also Cisco PIM routers on the multiaccess network, just configure PIM on the Ethernet interface so that the DVMRP routers connect over the tunnels and the PIM routers connect over the Ethernet.

NOTE

Remember that if multicast sources are reachable via the DVMRP routers, you must configure static mroutes to avoid RPF failures.