Chapter 20
| Multicast Routing
Overview
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maintaining its own multicast routing table, making it routing protocol
independent.
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. As
mentioned above, it does not maintain it’s own routing table, but instead, uses the
routing table provided by whatever unicast routing protocol is 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 an
Assert message back out the source interface. An Assert winner is then selected to
continue forwarding traffic from this source. On the other hand, 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. Although this results in more flooding than encountered with
DVMRP, this is the only major trade-off for the lower processing overhead and
simplicity of configuration for PIM-DM.
Configuring PIM-SM
PIM-SM uses the router’s local unicast routing table to route multicast traffic, not to
flood it. It only forwards multicast traffic when requested by a local or downstream
host. When service is requested by a host, it can use a Reverse Path Tree (RPT) that
channels the multicast traffic from each source through a single Rendezvous Point
(RP) within the local PIM-SM domain, and then forwards this traffic to the
Designated Router (DR) in the local network segment to which the host is attached.
However, when the multicast load from a particular source is heavy enough to
justify it, PIM-SM can be configured to construct a Shortest Path Tree (SPT) directly
from the DR up to the source, bypassing the RP and thereby reducing service delays
for active hosts and setup time for new hosts.
PIM-SM reduces the amount of multicast traffic by forwarding it only to the ports
that are attached to receivers for a group. The key components to filtering multicast
traffic are listed below.
Common Domain – A common domain must be set up in which all of the multicast
routers are configured with the same basic PIM-SM settings.
Bootstrap Router (BSR) – After the common domain is set, a bootstrap router is
elected from this domain. Each time a PIM-SM router is booted up, or the multicast
mode reconfigured to enable PIM-SM, the bootstrap router candidates start
flooding bootstrap messages on all of their interfaces (using reverse path
forwarding to limit the impact on the network). When neighboring routers receive
bootstrap messages, they process the message and forward it out through all
interfaces, except for the interface on which this message was received. If a router
receives a bootstrap message with a BSR priority larger than its own, it stops
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