- Enterasys Security Router User's Guide

Describing the XSR’s PIM-SM v2 Features
XSR User’s Guide 7-11
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-interval command.
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-Set
messages and RP-Adv messages. 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-border
command. Bootstrap router candidates can be configured with
ip pim bsr-candidate and 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.