PIM-SM Configuration
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
36.3
Bootstrap router
Each PIM-SM network must have at least one bootstrap router (BSR) candidate, unless all
routers in the domain are configured statically with information about all RPs in the domain.
Every router that is a BSR candidate periodically sends a Bootstrap Candidate Advertisement
message to advertise that it is available as a bootstrap router candidate. The BSR candidates in
the network elect the router with the highest preference value to be the bootstrap router. The
elected bootstrap router listens to PIM Candidate RP Advertisement messages specifying RP
candidates for multicast groups. It maintains a list of RP candidates, and sends a bootstrap
message every BSM interval, specifying all the multicast groups in the PIM network, and their
rendezvous point candidates. Each router uses this information and a standardized hash
mechanism to determine the RP for each group.
In summary:
■
Each
multicast group
must have at least one rendezvous point candidate
■
Each
PIM-SM domain
must have at least one Bootstrap Router candidate, unless all routers
in the domain are configured statically with information about all RPs in the domain
■
Each
subnetwork
must have at least one Designated Router candidate.
PIM hello
messages
When PIM is enabled on a switch, it sends out a PIM
Hello
message on all its PIM enabled
interfaces, and listens for Hello messages from its PIM neighbors. When a switch receives a
Hello message, it records the interface, IP address, priority for becoming a designated router,
and the timeout for the neighbor’s information. The switch sends Hello messages regularly at
the Hello Time interval.
Operation of PIM Sparse Mode
Once roles are established, multicast routing follows specific phases:
1.
Rendezvous point tree
2.
Register stop
3.
Shortest path tree
While multicast routing always begins with phase 1, the designated router for a receiver
determines whether and when to move on to phases 2 and 3, depending on the amount of
traffic from the source.
Rendezvous point
tree
Phase 1 establishes and uses a shared tree rooted at the rendezvous point (RP) to forward all
multicast data to group members.
When an IP host sends an IGMP join message to the local PIM designated router, which is not
the RP for the group, the designated router sends a
PIM
join
message towards the RP for the
group (“upstream”). The designated router determines which router is the RP for the group
from the most recent bootstrap message. Every router the join message passes through
records that there is a group member on the incoming interface. Eventually, the join message
reaches either the RP, or another router that already knows that it has a group member
downstream. If the group has many members, the join messages converge on the RP to form a
rendezvous point tree (RPT). This is called a shared tree because multicast data that is sent to
the group by any sender shares the tree. The multicast receiver’s designated router sends join
messages periodically according to the upstream join timer as long as the IP host is a member
of the group. When the last receiver on a subnetwork leaves the group, the join messages stop,
and their entries timeout on routers that are closer to the RP.
The sender’s designated router encapsulates the multicast data in a unicast packet in a process
called
registering
, and sends these register packets to the group’s RP. When the RP receives the
data, it decapsulates them, and forwards them onto the shared tree.