3Com Switch 4800G 24-Port - Page 703

Multicast Routing and Forwarding Overview 703
destination address. The corresponding routing entry explicitly defines the RPF
interface and the RPF neighbor.
4 Then, the router selects one from these two optimal routes as the RPF route. The
selection is as follows:
5 If configured to use the longest match principle, the router 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 router selects the multicast static route.
6 If not configured to use the longest match principle, the router selects the route
with a higher priority; if the two routes have the same priority, the router 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 629.
Assume that unicast routes exist in the network and no multicast static routes
have been configured on Switch C, as shown in Figure 206. Multicast packets
travel along the SPT from the multicast source to the receivers.
Figure 206 RPF check process
■ A multicast packet from Source arrives on VLAN-interface 1 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 VLAN-interface
Source
192.168.0.1/24
Receive
r
Receive
r
Switch A
SwitchB
SwitchC
Vlan-int2
Vlan-int1
Vlan-int1
Vlan-int2
Multicast packets
Destination/Mask
IP Routing Table on Switch C
192.168.0.0/24
Interface
Vlan -int2