Enterasys Networks X-PeditionTM PIM Register Message, PIM Join/Prune Message, Assert Processing

Describing the XSR’s PIM-SM v2 Features

PIM Register Message

By the end of PIM-SM phase one, the DR for the sender will encapsulate packets from the sender in a Register message and send it to RP for the multicast group. When the DR receives a RegisterStop message from RP, the RegisterStop timer will begin to maintain the state. Before the RegisterStop timer expires, the DR should send a empty Register message to RP so that RP will respond with another RegisterStop message. When the DR receives the RegisterStop message while the RegisterStop timer is still running, it restarts the RegisterStop timer so that no unnecessary Register messages are sent to RP. The IP ECN bits and DSCP bits of the original packet should be copied into the encapsulated Register packet.

PIM Join/Prune Message

To build and maintain the MDT, PIM-SM uses Join/Prune messages which contain a list of multicast groups and a list of Joined and Pruned sources for each multicast group. A PIM Join can be triggered by a new receiver joining the multicast group and is sent periodically from the downstream to upstream router to maintain the multicast tree. A timer controls the periodic sending of PIM Join messages which can be set with the ip pim message-intervalcommand.

A PIM Prune can be triggered by the last receiver on a subnet exiting the multicast group. A source-specific Prune can be used to bar the shared RP Tree from forwarding traffic from the specific source. This type of Prune used for the RP tree should also be sent from the downstream to upstream router periodically to maintain state. When a router receives a PIM Prune message from its downstream router, it should start a PrunePending timer to wait for a period so that other downstream routers that are still interested in the traffic to override the Prune message with a Join message. It is performed to avoid unnecessary interruption of multicast traffic.

Bootstrap & Rendezvous Point

The bootstrap router is designed primarily to provide all routers in a domain with a common group for RP mappings. It is important for both sender and receiver to have a common tree for data distribution. The mechanism to distribute RP information is carried out using periodic RP-Setmessages and RP-Advmessages. One Bootstrap Router (BSR) is elected within one domain among all routers that have been configured as Candidate BSRs. A router configured as the Candidate RP sends out periodic RP-Adv messages periodically to the elected BSR. This message contains the address of the other advertising Candidate RPs. When the elected BSR receives RP-Adv messages from different Candidate RPs, it hashes a list of RP to Class-D group mappings in a RP-Set message which is periodically sent hop by hop throughout the domain. In this way, all routers in the domain learn the same RP to group mappings, giving them a consistent view of which RP to send join/prunes and registered packets.

The bootstrap domain in which a uniform set of RP-to-group mapping is maintained can be defined by specifying the border interfaces of the bootstrap routers using the ip pim bsr-bordercommand. Bootstrap router candidates can be configured with ip pim bsr-candidateand RP candidates can be defined with ip pim rp-candidate.

Assert Processing

On a shared LAN, it is possible for more than one upstream router in the MDT to forward the same multicast packet into the same LAN. Assert processing avoids duplicating multicast packets. Assert is triggered when a multicast data packet is received on an outgoing interface according to the constructed multicast tree. This occurs if there are two parallel routers sharing forwarding entries from the same group with outgoing packets directed toward the same LAN.

XSR User’s Guide 7-11