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XGS-4526/4528F/4728F User’s Guide
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H A P T E R
3 9
DVMRP
This chapter introduces DVMRP and tells you how to configure it.
39.1 DVMRP Overview
DVMRP (Distance Vector Multicast Routing Protocol) is a protocol used for routing
multicast data within an autonomous system (AS). This DVMRP implementation is
based on draft-ietf-idmr-dvmrp-v3-10. DVMRP provides multicast forwarding
capability to a layer-3 switch that runs both the IPv4 protocol (with IP Multicast
support) and the IGMP protocol. The DVMRP metric is a hop count of 32.
IGMP is a protocol used for joining or leaving a multicast group. You must have
IGMP enabled when you enable DVMRP; otherwise you see the screen as in
Figure
175 on page 351 .
39.2 How DVMRP Works
DVMRP uses the Reverse Path Multicasting (RPM) algorithm to generate an IP
Multicast delivery tree. Multicast packets are forwarded along these multicast tree
branches. DVMRP dynamically learns host membership information using Internet
Group Management Protocol (IGMP). The trees are updated dynamically to track
the membership of individual groups.
1
Initially an advertisement multicast packet is broadcast (“B” in the following
figure).
2
DVMRP-enabled Layer-3 devices that do not have any hosts in their networks that
belong to this multicast group send back a prune message (“P”).
3
If hosts later join the multicast group, a graft message (“G”) to undo the prune is
sent to the parent.