some about multicast PIM

sullan

Multicast:
1.PIM-DM,DR elected:
PIM-DM Designated Router select by hello package,which router with the highest ip address.
others:can uses DR-Priority option to elected as the DR.(default priority=1);that the router with
highest ip address will be elected as DR.
elected base ip address is PIMv1.
DR-Priority only support for PIMv2.
2.PIM-DM source Distribution tree.
SPTs are built on the fly,using the flood-and prune mechanism. but it is unlike DVMRP,which uses
is own umlticast routing table and a special poison reverse mechanism to initially construct a
minimal spanning distribution tree.
3.PIM traffic forwarding
RPF:in PIM,the RPF check is performed by using the infomation in the unicast routing table.
When RPF chack that there are equal cost paths to a network,the router chooses only one interface
to the recivers. other wards: A router can have only ONE incoming interface for any entry in its
multicast routing table. (select highest next-hop ip address)
can use show ip rpf on cisco router to see it.
4. SPTs prune
know about:Prume overwrite.
the router immediately start a 3-second Prune-delay when  received Prune message on a multi-
access network.if on overriding join message is received to cancel this timer,the prune takes
place when the timer expires.
PIM prunes have a lifetime of 3 minutes.
5.PIM-DM Assert.
if a router receives a multicast package via an interface in the outgoing interface list
associated with a multicast source,send a PIM Assert message out the interface to resolve which
router will continue forwarding this traffic.
first,routers will examine the metric in the PIM Assert elected the frowarding router.
second.if the metric is same,the highest ip address router will win.
6.PIM Grafting.
send Grafting to updream router,that updream router report a ack-grafting message.
7.state refresh.
PIM-DM protocal specification that would all but eliminate the periodic flood-and prune behavior
that the broadcast tree do not time out. The first-hop router send (S,G)state-Refresh message do
it.
8.Summury
PIM-DM RPF chack base unicast route table.unlike DVMRP base multicast table.and PIM-DM no itself
Multicast route table.routing base unicast.



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