Multicast Forwarding

Like any other router, the two fundamental functions of a multicast router are route discovery and packet forwarding This section addresses the unique requirements of multicast forwarding, and the next section looks at the requirements for multicast route discovery

Unicast packet forwarding involves forwarding a packet toward a certain destination Unless certain policies are configured, a unicast router is uninterested in the source of the packet The packet is received, the destination IP address is examined, a longest-match route lookup is performed, and the packet is forwarded out a single interface toward the destination

Instead of forwarding packets toward a destination, multicast routers forward packets away from a source This distinction may sound trifling at first glance, but it is actually essential to correct multicast packet forwarding A multicast packet is originated by a single source but is destined for a group of destinations At a particular router, the packet arrives on some incoming interface, and copies of the packet may be forwarded out multiple outgoing interfaces

If a loop exists so that one or more of the forwarded packets makes its way back to the incoming interface, the packet is again replicated and forwarded out the same outgoing interfaces The result can be a multicast storm, in which packets continue to loop and be replicated until the TTL expires It is the replication that makes a multicast storm potentially so much more severe than a simple unicast loop Therefore, all multicast routers must be aware of the source of the packet and must only forward packets away from the source

A useful and commonly used terminology is that of upstream and downstream Multicast packets should always flow downstream from the source to the destinations, never upstream toward the source To ensure this behavior, each multicast router maintains a multicast forwarding table in which (source, group) or (S, G) address pairs are recorded Packets from a particular source and destined for a particular group should always arrive on an upstream interface and be forwarded out one or more downstream interfaces By definition, an upstream interface is closer to the source than any downstream interface, as illustrated by Figure 5-18 If a router receives a multicast packet on any interface other than the upstream interface for that packet's source, it quietly discards the packet

Figure 5-18 By Identifying Upstream and Downstream Interfaces in Relation to Each Multicast Source, Routers Avoid Multicast Routing Loops

51 downstream interface

52 upstream interface

51 upstream interface

52 downstream interface

51 downstream interface

52 downstream interface

Figure 5-18 By Identifying Upstream and Downstream Interfaces in Relation to Each Multicast Source, Routers Avoid Multicast Routing Loops

51 downstream interface

52 upstream interface

51 upstream interface

52 downstream interface

51 downstream interface

52 downstream interface

S2 downstream interface S1 upstream interface

51 downstream interface

52 upstream interface

S2 downstream interface S1 upstream interface

51 downstream interface

52 upstream interface

Of course, the router needs some mechanism for determining the upstream and downstream interfaces for a given (S, G) This is the job of the multicast routing protocol

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