Enterasys Networks X-PeditionTM manual Neighbor Discovery and DR Election

Describing the XSR’s PIM-SM v2 Features

Figure 7-4 Phase 3 Topology: Shortest Path Tree Between Sender and Receiver

	RP
Native Packet	Native Packet

(S,G)	Sender
Join
Native Packet

Native	(S,G) Join
Packet

	RP
(S,G,RPT)		(S,G)
Prune	Native	Prune
	Packets	Sender
(S,G,RPT)
Prune	(S,G,RPT)
	Prune

Native

Packets

Receiver	Receiver	Receiver	Receiver
	RP
	Native
	Packet		Sender
		Native
		Packet

ReceiverReceiver

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-intervalcommand. 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-thresholdcommand.

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-prioritycommand.

7-10 Configuring PIM-SM and IGMP