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Multicast Routing
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CLI – This example displays known DVMRP routes.
Configuring PIM-DM
Protocol-Independent Multicasting (PIM) provides two different modes of operation:
sparse mode and dense mode. Sparse mode (SM) is designed for networks where
the probability of multicast group members is low, such as the Internet. Dense mode
(DM), on the other hand, is designed for networks where the probability of multicast
group members is high, such as a local network.
PIM-DM is a simple multicast routing protocol that uses flood and prune to build a
source-routed multicast delivery tree for each multicast source-group pair. It is
simpler than DVMRP because it does not maintain it’s own routing table. Instead, it
uses the routing table provided by the unicast routing protocol enabled on the router
interface. When the router receives a multicast packet for a source-group pair,
PIM-DM checks the unicast routing table on the inbound interface to determine if this
is the same interface used for routing unicast packets to the multicast source
network. If it is not, the router drops the packet and sends a prune message back out
the source interface. If it is the same interface used by the unicast protocol, then the
router forwards a copy of the packet to all the other interfaces for which is has not
already received a prune message for this specific source-group pair.
DVMRP holds the prune state for about two hours, while PIM-DM holds it for only
about three minutes. This results in more flooding than encountered with DVMRP,
but this the only major trade-off for the lower processing overhead and simplicity of
configuration for PIM-DM.
Configuring Global PIM-DM Settings
PIM-DM is used to route multicast traffic to nodes which have requested a specific
multicast service via IGMP. It uses the router’s unicast routing table to determine if
the interface through which a packet is received provides the shortest path back to
the source. This is done on a per hop basis back toward the source of the multicast
delivery tree. PIM-DM uses three different techniques to dynamically reconfigure the
multicast spanning tree: broadcasting, pruning, and grafting.
To use PIM-DM, you must enable it globally for the router as described below, and
for each interface that will support multicast routing as described in the next section.
Also note that IGMP must be enabled to allow the router to determine the location of
group members.
Console#show ip dvmrp route 4-306
Source Mask Upstream_nbr Interface Metric UpTime Expire
--------------- --------------- --------------- --------- ------ ------ ------
10.1.0.0 255.255.255.0 10.1.0.253 vlan1 1 84438 0
10.1.1.0 255.255.255.0 10.1.1.253 vlan2 1 84987 0
10.1.8.0 255.255.255.0 10.1.0.254 vlan1 2 19729 97
Console#