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WGS3 Layer 3 Switch User’s Manual
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6.4.3 GMRP Protocol
GARP Multicast Registration Protocol (GMRP) allows network devices to register end-stations with
multicast groups. GMRP requires that any participating network devices or end-stations comply with the
IEEE 802.1p standard. Compliant end-stations can request to receive traffic from a multicast group
simply by issuing a join packet that includes a known multicast address. When the join packet reaches a
port on the switch, it configures this port to receive multicast traffic for the requested group, and then
issues a similar join packet to all other ports on the switch, informing them that incoming multicast traffic
for the stated group is to be forwarded to the requesting port.
6.4.4 DVMRP Routing Protocol
The Distance-Vector Multicast Routing Protocol (DVMRP) behaves somewhat similar to RIP. A router
supporting DVMRP periodically floods its attached networks to pass information about supported
multicast services along to new routers and hosts. Routers that receive a DVMRP packet send a copy
out to all paths (except the path back to the origin). These routers then send a prune message back to
the source to stop a data stream if the router is attached to a LAN that does not want to receive traffic
from a particular multicast group. However, if a host attached to this routing switch issues an IGMP
message indicating that it wants to subscribe to the concerned multicast service, this switch will use
DVMRP to build up a source-rooted multicast delivery tree that allows it to prevent looping and determine
the shortest path to the source of this multicast traffic.
When this switch receives the multicast message, it checks its unicast routing table to locate the port that
provides the shortest path back to the source. If that path passes through the same port the multicast
message was received on, then this switch records path information for the concerned multicast group in
its routing table and forwards the multicast message on to adjacent routers, except for the port through
which the message arrived on. This process eliminates any potential loops from the tree and ensures
that the shortest path (in terms of hop count) is always used.