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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 117
Reverse Path Broadcasting (RPB)
The Reverse Path Broadcasting (RPB) algorithm is an
enhancement of the multicast spanning tree algorithm. RPB
constructs a spanning tree for each multicast source. When
the router receives a multicast packet, it then checks to
determine if the packet was received on the shortest path back
from the router to the source. If the packet was received on the
shortest path back to the source, the packet is forwarded on all
links except the link on which the packet was received. If the
packet was not received on the shortest link back to the source,
the packet is dropped.
If a link-state routing protocol is in use, RPB on a local router
can determine if the path from the source through the local
router to an immediately neighboring router. If it is not, the
packet will be dropped at the next router and the packet
should not be forwarded.
If a distance-vector routing protocol is in use, a neighboring
router can either advertise its previous hop for the source as
part of its routing update messages. This will ‘poison-reverse’
the route (or have the local router prune the branch from the
multicast source to the neighboring router because the
neighboring router has a better route from the source to the
next router or subnetwork).
Since multicast packets are forwarded through the shortest
route between source and destination, RPB is fast. A given
router also does not need information about the entire
spanning tree, nor does it need a mechanism to stop the
forwarding of packets.
RPB does not use multicast group membership information in
its forwarding decisions.