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DVMRP

This chapter introduces DVMRP and tells you how to configure it.

30.1 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 133 on page 247.

30.2 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.

1Initially an advertisement multicast packet is broadcast (“B” in the following figure).

2DVMRP-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”).

3If hosts later join the multicast group, a graft message (“G”) to undo the prune is sent to the parent.

4The final multicast (“M”) after pruning and grafting is shown in the next figure.

 

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