Multicast Forwarding And Reverse Path Check - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 34 Configuring IP Multicast Routing
travel hop-by-hop throughout the PIM domain. Because BSR messages contain the IP address of the
current BSR, the flooding mechanism allows candidate RPs to automatically learn which device is the
elected BSR.
Candidate RPs send candidate RP advertisements showing the group range for which they are
responsible directly to the BSR, which stores this information in its local candidate-RP cache. The BSR
periodically advertises the contents of this cache in BSR messages to all other PIM devices in the
domain. These messages travel hop-by-hop through the network to all routers and switches, which store
the RP information in the BSR message in their local RP cache. The routers and switches select the same
RP for a given group because they all use a common RP hashing algorithm.

Multicast Forwarding and Reverse Path Check

With unicast routing, routers and multilayer switches forward traffic through the network along a single
path from the source to the destination host whose IP address appears in the destination address field of
the IP packet. Each router and switch along the way makes a unicast forwarding decision, using the
destination IP address in the packet, by looking up the destination address in the unicast routing table
and forwarding the packet through the specified interface to the next hop toward the destination.
With multicasting, the source is sending traffic to an arbitrary group of hosts represented by a multicast
group address in the destination address field of the IP packet. To determine whether to forward or drop
an incoming multicast packet, the router or multilayer switch uses a reverse path forwarding (RPF) check
on the packet as follows and shown in
1.
2.
3.
Some multicast routing protocols, such as DVMRP, maintain a separate multicast routing table and use
it for the RPF check. However, PIM uses the unicast routing table to perform the RPF check.
Figure 34-6
A check of the routing table shows that the interface on the reverse path to the source is Gigabit Ethernet
interface 0/1, not interface 0/2. Because the RPF check fails, the multilayer switch discards the packet.
Another multicast packet from source 151.10.3.21 is received on interface 0/1, and the routing table
shows this interface is on the reverse path to the source. Because the RPF check passes, the switch
forwards the packet to all interfaces in the outgoing interface list.
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The router or multilayer switch examines the source address of the arriving multicast packet to
determine whether the packet arrived on an interface that is on the reverse path back to the source.
If the packet arrives on the interface leading back to the source, the RPF check is successful and the
packet is forwarded to all interfaces in the outgoing interface list (which might not be all interfaces
on the router).
If the RPF check fails, the packet is discarded.
shows Gigabit Ethernet interface 0/2 receiving a multicast packet from source 151.10.3.21.
Cisco Implementation of IP Multicast Routing
Figure 34-6:
Catalyst 3550 Multilayer Switch Software Configuration Guide
34-9