Enterasys Networks Security Router X-PeditionTM - Manual (Page 186)

Describing the XSR’s PIM-SM v2 Features
7-10 Configuring PIM-SM and IGMP
Figure 7-4 Phase 3 Topology: Shortest Path Tree Between Sender and Receiver
Neighbor Discovery and DR Election
PIM-SM neighbor discovery and DR election are performed via Hello messages which are sent
periodically through each PIM-enabled interface. A Hello Timer is kept for each interface whose
timeout event will trigger sending a Hello message. You can set the interval between Hello
messages with the
ip pim query-interval command. Hello messages use the multicast address
224.0.0.13 (ALL-PIM-ROUTERS) as its destination address. Hello message must be sent out all
PIM-active interfaces, including physical point-to-point ports. The Time-To-Live value of the
Hello should be set to 1 with the
ip multicast ttl-threshold command.
For each PIM interface, a PIM-enabled router keeps a list of active neighbors from which Hello
messages are received. When the router receives Join/Prune or Assert messages from an interface,
the sources of the messages are checked against the neighbor list for this interface. If the message
does not come from one of its neighbors, it is dropped. So if a router needs to send a Join/Prune or
Assert message to an interface on which it has not sent a Hello message, it should send a Hello
message first without waiting for the Hello timer to expire. Before an interface goes down or
changes its IP address, a Hello message with zero HoldTime should be sent immediately to let its
neighbors immediately remove itself from their neighbor lists.
On each LAN, whether it is a shared media or point-to-point link, a Designated Router (DR) is
elected to act on behalf of the hosts in the same LAN for PIM-SM. A DR is chosen from all active
neighbors on the interface; if all have their DR priority (a 32-bit unsigned number within the Hello
message) present, the one with the largest value is preferred. If there is more than one neighbor
share the largest DR priority, the one with the largest IP address is selected as the DR. You can set
DR priority with the
ip pim dr-priority command.
RP
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Sender
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(S,G) Join
(S,G)
Join
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Prune
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Prune
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