Distance Vector Multicast Routing Protocol (DVMRP)

Distance Vector Multicast Routing Protocol (DVMRP)
Distance Vector Multicast Routing Protocol (DVMRP) has achieved widespread use in the multicast
world. A few years ago, you might have often heard the term “DVMRP tunnel” used
when discussing the implementation of multicast feeds from an ISP or a feed from the multicast
backbone (MBONE). As the name indicates, this protocol uses a distance-vector algorithm. It
uses several of the features that other distance-vector protocols (such as Routing Information
Protocol, RIP) implement. Some of these features are a 32 max hop count, poison reverse, and
60-second route updates. It also allows for IP classless masking of addresses.
Just as with other routing protocols, DVMRP-enabled routers must establish adjacencies in
order to share route information. After the adjacency is established, the DVMRP route table is
created. Route information is exchanged via route reports. It is important to remember that the
DVMRP route table is stored separately from the unicast routing table. The DVMRP route table
is more like a unicast route table than the multicast route table that was shown earlier in this
chapter. A DVMRP table contains the layer 3 IP network of the multicast source and the next
hop toward the source.
Because the DVMRP table has this form, it works perfectly with source tree distribution,
as discussed earlier. Using the information in the DVMRP table, the tree for the source can be
established. In addition, the router uses this information to perform the Reverse Path Forwarding
check to verify that the multicast data coming into the interface is coming in an interface that
leads back to the source of the data. DVMRP uses SPT for its multicast forwarding.
Figure 19.16 shows how DVMRP works. You can see that not every router in the network
is a DVMRP router. Notice also that the adjacencies are established over tunnel interfaces.
DVMRP information is tunneled through an IP network. On either end of the tunnel, information
is learned and exchanged to build a multicast forwarding database or route table.