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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Multicast Routing Protocols
This section contains an overview of two multicast routing protocols – Distance Vector Multicast
Routing Protocol (DVMRP), and Protocol Independent Multicast-Dense Mode
(PIM-DM). The most commonly used routing protocol (not a multicast routing protocol), the Routing
Information Protocol, is discussed in a later section.
Distance Vector Multicast Routing Protocol (DVMRP)
The Distance Vector Multicast Routing Protocol (DVMRP) was derived from the Routing Information
Protocol (RIP) with the introduction of multicast delivery trees constructed from information about the
‘distance’ from the local router back toward the multicast source. DVMRP uses an RPM algorithm to
construct its multicast delivery trees.
The first multicast packet received by a multicast router using DVMRP is flooded to all interfaces except
the one on which the packet was received. Subsequent prune messages are used to prune branches of
the delivery tree that are either not on the shortest path back to the multicast source, or that have no
active multicast group members. A ‘graft’ message is added that allows a previously pruned branch of
the multicast delivery tree to be reactivated. This allows for lower latency when a leaf router adds a new
member to a multicast membership group. Graft messages are forwarded one hop (one router) back at a
time toward a multicast source until they reach a router that is on an active branch of the multicast
delivery tree.
If there is more than one multicast router on a network, the one that has the shortest path back to the
multicast source is elected to forward multicast packets from that source. All other routers will discard
multicast packets from that source. If two multicast routers on a network have the same distance back
to a multicast source, the router with the lowest IP address is elected.
DVMRP also supports tunnel interfaces, where two multicast routers are connected through a router
that cannot process multicast packets. This allows multicast packets to cross networks with routers
that are not multicast-aware.
Protocol-Independent Multicast – Dense Mode
There are two protocols in Protocol Independent Multicast (PIM), Protocol Independent Multicast-Dense
Mode (PIM-DM) which is used when the multicast destinations are closely spaced, and Protocol
Independent Multicast-Sparse Mode (PIM-SM) which is used when the multicast destinations are
spaced further apart. PIM-DM is most commonly implemented in an intranetwork (LAN) where the
distance between users is minimal.
Routing Protocols
Routing Information Protocol (RIP)
The Routing Information Protocol is a distance-vector routing protocol. There are two types of network
devices running RIP – active and passive. Active devices advertise their routes to others through RIP
messages, while passive devices listen to these messages. Both active and passive routers update their
routing tables based upon RIP messages that active routers exchange. Only routers can run RIP in the
active mode.
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