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Figure 42 DR election
As shown in
, the DR election process is as follows:
1.
Routers on the multi-access network send hello messages to one another. The hello messages
contain the router priority for DR election. The router with the highest DR priority becomes the
DR.
2.
The router with the highest IP address wins the DR election under one of following conditions:
{
All the routers have the same DR election priority.
{
A router does not support carrying the DR-election priority in hello messages.
When the DR fails, a timeout in receiving a hello message triggers a new DR election process among
the other routers.
RP discovery
The RP is the core of a PIM-SM domain. For a small-sized, simple network, one RP is enough for
forwarding information throughout the network, and you can statically specify the position of the RP
on each router in the PIM-SM domain. In most cases, however, a PIM-SM network covers a wide
area, and a huge amount of multicast traffic must be forwarded through the RP. To lessen the RP
burden and optimize the topological structure of the RPT, you can configure multiple candidate-RPs
(C-RPs) in a PIM-SM domain, among which an RP is dynamically elected through the bootstrap
mechanism. Each elected RP is designated to a different multicast group range. For this purpose,
you must configure a bootstrap router (BSR). The BSR acts as the administrative core of the PIM-SM
domain. A PIM-SM domain can have only one BSR, but can have multiple candidate-BSRs
(C-BSRs). If the BSR fails, a new BSR is automatically elected from the C-BSRs to avoid service
interruption.
NOTE:
•
An RP can provide services for multiple multicast groups, but a multicast group only uses one RP.
•
A device can act as a C-RP and a C-BSR at the same time.
As shown in
, each C-RP periodically unicasts its advertisement messages (C-RP-Adv
messages) to the BSR. An advertisement message contains the address of the advertising C-RP
and the multicast group range to which it is designated. The BSR collects these advertisement
messages and organizes the C-RP information into an RP-set, which is a database of mappings
between multicast groups and RPs. The BSR then encapsulates the RP-set in the bootstrap
messages (BSMs) and floods the bootstrap messages to the entire PIM-SM domain.
