CBT data distribution

CBT data distribution
Host A
Host F
Host B Host C
Host D Host E
Source for
224.2.125.125
Member Member
Member
New member of
224.2.125.125
Router 1
Router 2
Router 3
Host G
Router 5
Router 4
Forward
multicast data Graft
request
IGMP membership report
Forward multicast data
Host A
Host F
Host B Host C
Host D Host E
Source for
224.2.125.125
Member Member
Member
Non-member
Router 1
Router 2
Router 3
Host G
Router 5
Router 4
CBT CBT
CBT
CBT
CBT
(CME)
Routing Multicast Traffic 617
It is important to see the difference between this sparse mode method and the dense mode
method. In sparse mode operation, routers are members of the tree only if they have active members
directly connected. Notice in Figure 19.20 that Router 5 is not participating. Dense mode
operates on the initial premise that all PIM neighbors have active members directly connected.
The tree changes when the directly connected routers request to be pruned from the tree.
A CBT router might become part of the tree after a host sends an IGMP Membership Record
to the directly connected router. The router then sends a join tree request to the core router. If
the request reaches a CBT tree member first, that router will add the leaf router to the tree and
begin forwarding multicast data.
Pruning the tree is done much the same way. When there are no more active members on
a router’s interfaces, the router sends a prune request to the upstream router. The answering
router removes the interface from the forwarding cache if it is on a point-to-point circuit, or it
waits for a timer to expire it if is on a shared access network. The timer gives enough time for
other CBT routers on the segment to override the prune request.