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Configuring multicast routing and
forwarding
Overview
The following tables are involved in multicast routing and forwarding:
•
Multicast routing table of each multicast routing protocol, such as the PIM routing table.
•
General multicast routing table that summarizes multicast routing information generated by
different multicast routing protocols. The multicast routing information from multicast sources to
multicast groups are stored in a set of (S, G) routing entries.
•
Multicast forwarding table that guides multicast forwarding. The optimal routing entries in the
multicast routing table are added to the multicast forwarding table.
RPF check mechanism
A multicast routing protocol uses reverse path forwarding (RPF) check to ensure the multicast data
delivery along the correct path and to avoid data loops.
RPF check process
A multicast router performs the RPF check on a multicast packet as follows:
1.
The router chooses an optimal route back to the packet source separately from the unicast,
MBGP, and static multicast routing tables.
The term "packet source" means different things in different situations:
For a packet that travels along the SPT, the packet source is the multicast source.
For a packet that travels along the RPT, the packet source is the RP.
For a bootstrap message originated from the BSR, the packet source is the BSR.
For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see
2.
The router selects one of the three optimal routes as the RPF route as follows:
If the router uses the longest prefix match principle, the route with the highest subnet mask
becomes the RPF route. If the routes have the same mask, the route with the highest route
preference becomes the RPF route. If the routes have the same route preference, the
unicast route becomes the RPF route.
For more information about the route preference, see
Layer 3—IP Routing Configuration
Guide
.
If the router does not use the longest prefix match principle, the route with the highest route
preference becomes the RPF route. If the routes have the same preference, the unicast
route becomes the RPF route.
The RPF route contains the RPF interface and RPF neighbor information.
If the RPF route is a unicast route or MBGP route, the outgoing interface is the RPF
interface and the next hop is the RPF neighbor.
If the RPF route is a static multicast route, the RPF interface and RPF neighbor are
specified in the route.
3.
The router checks whether the packet arrived at the RPF interface. If yes, the RPF check
succeeds and the packet is forwarded. If not, the RPF check fails and the packet is discarded.
