Cisco IOS Cookbook, 2nd Edition

Introduction

Multicast routing differs from unicast routing in several ways. The most important differences are in the ways that multicast routers use source and destination addresses. A multicast packet is addressed to a special IP address representing a group of devices that can be scattered anywhere throughout a network. Since the destinations can be anywhere, the only reliable way to eliminate loops in multicast routing is to look at the reverse path back to the source. So, while unicast routing cares where the packet is going, multicast routing also needs to know where it came from.

For this reason, multicast routing protocols such as Protocol Independent Multicast (PIM) always work with the source address and destination group simultaneously. The usual notation for a multicast route is (Source, Group), as opposed to the unicast case in which routes are defined by the destination address alone. We have already mentioned that this is necessary for avoiding loops, but the router also needs to keep track of both source and group addresses in each multicast routing table entry because there could be several sources for the same group.

For example, in the NTP chapter, we discussed how a central device can be configured to send time synchronization information as a multicast. In that chapter, we also explained why it was important to have more than one NTP server. So even in a simple multicast example like this, it is quite likely that the routers will need to forward packets to the ...