3Com Switch 8800 Advanced Software V5 Configuration Guide

Multicast Routing and Forwarding Overview 505
the destination address. The corresponding routing entry explicitly defines the
RPF interface and the RPF neighbor.
2 Then, the device selects one from these two optimal routes as the RPF route. The
selection is as follows:
■ If configured to use the longest match principle, the device selects the longest
match route from the two; if these two routes have the same mask, the route
selects the route with a higher priority; if the two routes have the same priority,
the device selects the multicast static route.
■ If not configured to use the longest match principle, the device selects the
route with a higher priority; if the two routes have the same priority, the device
selects the multicast static route.
n
The above-mentioned "packet source" can mean different things in different
situations:
■ For a packet traveling along the shortest path tree (SPT) from the multicast
source to the receivers or the source-based tree from the multicast source to
the rendezvous point (RP), "packet source" means the multicast source.
■ For a packet traveling along the rendezvous point tree (RPT) from the RP to the
receivers, "packet source" means the RP.
■ For a bootstrap message from the bootstrap router (BSR), "packet source"
means the BSR.
For details about the concepts of SPT, RPT and BSR, refer to “PIM Configuration”
on page 563.
Assume that unicast routes exist in the network and no multicast static routes
have been configured on Switch C, as shown in
Figure 162. Multicast packets
travel along the SPT from the multicast source to the receivers.
Figure 162 RPF check process
■ A multicast packet from Source arrives to POS5/1/2 of Switch C, and the
corresponding forwarding entry does not exist in the multicast forwarding
table of Switch C. Switch C performs an RPF check, and finds in its unicast
routing table that the outgoing interface to 192.168.0.0/24 is POS5/1/1. This
Receiver
Source
192.168.0.1/24
Receiver
Switch A
Switch B
Switch C
POS5/1/1
POS5/1/2
POS5/1/2
POS5/1/1
Multicast packets
Destination/Mask
IP Routing Table on Switch C
192.168.0.0/24
Interface
POS5/1/1