LevelOne GTL-2691 - Manual (Page 706)

C HAPTER 21 | Multicast Routing
Overview
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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 re-configured 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
advertising itself as a BSR candidate. Eventually, only the router with the
highest BSR priority will continue sending bootstrap messages.
Rendezvous Point (RP) – A router may periodically sends PIMv2
messages to the BSR advertising itself as a candidate RP for specified