Overview

The mrouted Routing Daemon

of the multicast datagrams. You can achieve this by using topological knowledge of the network to implement a multicast forwarding algorithm called Truncated Reverse Path Broadcasting (TRPB).

mrouted structures routing information in the form of a pruned broadcast delivery tree that contains routing information. mrouted structures routing information only to those subnets that have members of the destination multicast group. In other words, each router determines which of its virtual network interfaces are in the shortest path tree. In this way, DVMRP can determine if an IP multicast datagram needs to be forwarded. Without such a feature, the network bandwidth can easily be saturated with the forwarding of unnecessary datagrams.

Because DVMRP routes only multicast datagrams, you must handle routing of unicast or broadcast datagrams using a separate routing process.

To support multicasting across subnets that do not support IP multicasting, DVMRP provides a mechanism called tunnelling. Tunnelling forms a virtual point-to-point link between pairs of mrouted routers by encapsulating the multicast IP datagram within a standard IP unicast datagram using the IP-in-IP protocol (IP protocol number 4). This unicast datagram, containing the multicast datagram, is then routed through the intervening routers and subnets. When the unicast datagram reaches the tunnel destination, which is another mrouted router, the unicast datagram is stripped away and the mrouted daemon forwards the multicast datagram to its destinations.

Figure 1-1 shows a tunnel formed between a pair of mrouted routers.

Figure 1-1 Tunnel Made with mrouted Routers

Multicast

DVMRP Tunnel

Nonmulticast

 

DVMRP Tunnel Multicast

Transmitter

Endpoint

Endpoint

Recipient

Node

Router

Router

Node

M

R1

R2

N

Tunnel

18

Chapter 1