Overview
NOTE: PIM has gained generalacceptance among a largenumber of multicast-enabled
networks. We recommend that you use PIM rather than DVMRP for applications that
are not otherwise required to run DVMRP.
DVMRP is a dense-mode multicasting protocol and therefore uses a broadcast and prune
mechanism. The protocol builds a source-rooted tree (SRT) in a similar way to PIM dense
mode. DVMRP routers flood datagrams to all interfaces except the one that provides
the shortest unicast route to the source. DVMRP uses pruning to prevent unnecessary
sending of multicast messages through the SRT.
A DVMRP router sends prune messages to its neighbors if it discovers that:
•
The network to which a host is attached has no active members of the multicast group.
•
All neighbors, except the next-hop neighbor connected to the source, have pruned the
source and the group.
When a neighbor receives a prune message from a DVMRP router, it removes that neighbor
from its (S,G) pair table, which provides information to the multicast forwarding table.
When a host on a previously pruned branch attempts to join a multicast group, it sends
an IGMP message to its first-hop router. The first-hop router then sends a graft message
upstream.
Identifying Neighbors
In this implementation of DVMRP, a neighbor is a directly connected DVMRP router. When
you enable DVMRP on an interface, the associated VR adds information about local
networks to its DVMRP routing table. The VR then sends probe messages periodically
to learn about neighbors on each of its interfaces. To ensure compatibility with other
DVMRP routers that do not send probe messages, the VR also updates its DVMRP routing
table when it receives route report messages from such routers.
Advertising Routes
As its name suggests, DVMRP uses a distance-vector routing algorithm. Such algorithms
require that each router periodically inform its neighbors of its routing table. DVMRP
routers advertise routes by sending DVMRP report messages. For each network path, the
receiving router picks the neighbor advertising the lowest cost and adds that entry to its
routing table for future advertisement.
The cost, or metric, for this routing protocol is the hop count back to the source. The hop
count for a network device is the number of routers on the route between the source and
that network device.
Table 6 on page 123 shows an example of the routing table for a DVMRP router.
Copyright © 2010, Juniper Networks, Inc.122
JunosE 11.2.x Multicast Routing Configuration Guide
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