Multicast Routing Basics

The main function of any routing protocol is to help routers forward a packet in the right direction, causing the packet to keep moving closer to its desired destination, ultimately reaching its destination. To forward a unicast packet, a router examines the packet's destination address, finds the next-hop address from the unicast routing table, and forwards the packet through the appropriate interface. A unicast packet is forwarded along a single path from the source to the destination.

The top part of Figure 20-1 shows how a router can easily make a decision about forwarding a unicast packet by consulting its unicast routing table. However, when a router receives a multicast packet, as shown at the bottom of Figure 20-1, it cannot forward the packet because multicast IP addresses are not listed in the unicast routing table. Also, routers often have to forward multicast packets out multiple interfaces to reach all receivers. These requirements make the multicast forwarding process more complex than unicast forwarding.

Figure 20-1 Multicast Routing Problem

Unicast Packet Destination Address: 10.1.6.3

Source Address: 10.1.1.10

Multicast Packet Destination Address: 226.1.1.1

Source Address: 10.1.1.10

"I know how to handle this packet." Router# show ip route

Gateway of last resort Is not set 10.0.0.0/24 Is subnetted, 3 subnets C 10.1.1.0 Is directly connected, FastEthernet0/0 R 10.1.6.0 [120/1] via 10.1.4.3, 00:00:10, Serial0/1

C 10.1.4.0 Is directly connected, Serial0/1

Unicast Packet Destination Address: 10.1.6.3

Source Address: 10.1.1.10

Multicast Packet Destination Address: 226.1.1.1

Source Address: 10.1.1.10

Unicast Packet Destination Address: 10.1.6.3 Source Address: 10.1.1.10

"Who owns the multicast address 226.1.1.1? Where should I forward this packet?"

Figure 20-1 shows that the router has received a multicast packet with the destination address 226.1.1.1. The destination address represents a dynamically changing group of recipients, not any one recipient's address. How can the router find out where these users are? Where should the router forward this packet?

An analogy may help you to understand better the difficulty of multicast routing. Assume that you want to send party invitations through the mail, but instead of creating dozens of invitations, you create only one. Before mailing the invitation, you put a destination address on it, "This envelope contains my party invitation," and then drop it in a mailbox. When the postal system examines the destination address on your envelope, where should it deliver your envelope? And because it is only one invitation, does the postal system need to make copies? Also, how can the postal system figure out to which addresses to deliver the copies? By contrast, if IP multicast were the post office, it would know who you want to invite to the party, know where they are located, and make copies of the invitation and deliver them all to the correct addresses.

The next few sections discuss solutions for forwarding multicast traffic and controlling the distribution of multicast traffic in a routed network.

Continue reading here: Overview of Multicast Routing Protocols

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