LevelOne GTL-2691 User Manual

Page 706

HAPTER

| Multicast Routing

Overview

– 706 –

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