Install guide

10 Software Reference
Rendezvous point Each multicast group must have a rendezvous point (RP). The RP forms the
root of the group’s distribution tree. The designated router for a multicast
sender sends multicast packets towards the RP. Designated routers with group
members connected to them send join messages towards the group’s RP. The
RP candidate with the lowest priority is elected from all the RP candidates for a
group. If the RP becomes unavailable, the remaining RP candidates elect a new
RP.
Note that software release versions prior to 2.7.3 did not correctly support the
PIM hash mask length option. As a result, the RP selection calculation differs
between this release and release versions prior to 2.7.3. If a network contains
switches running a mixture of versions, this leads to incorrect forwarding
behaviour. To avoid this issue, either ensure that all devices on the network
correctly support the hash mask length option (recommended), or ensure that
the following both hold:
■ The hash mask length option on all BSR candidates is configured to 4 bits.
This implies that all BSR candidates must be running 2.7.3 or later.
■ All RP candidates use a common prefix of 224.0.0.0/240.0.0.0. This has the
side effect of collapsing all groups to use a single PIM RP.
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
standardised 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
neighbours. When a switch receives a Hello message, it records the interface, IP
address, priority for becoming a designated router, and the timeout for the
neighbour’s information. The switch sends Hello messages regularly at the
Hello Time interval.