D-Link DES-3326 User Manual - Multicast Spanning Trees
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts
103
Multicast Spanning Trees
A multicast delivery tree that spans the entire network with a
single active link between routers (or subnetwork) is called a
multicast spanning tree. Links (or branches) are chosen such that
there is only one active path between any two routers. When a
router receives a multicast packet, it forwards the packet on all
links except the one on which it was received. This guarantees
that all routers in the network will receive a copy of the packet.
The only information the router needs to store is whether a link is
a part of the spanning tree (leads to a router) or not.
Multicast spanning trees do not use group membership
information when deciding to forward or drop a given multicast
packet.
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