104
PIM-DM does not require a DR. However, if IGMPv1 runs on any shared-media LAN in a PIM-DM
domain, a DR must be elected to act as the IGMPv1 querier for the LAN. For more information about
IGMP, see "Configuring IGMP."
IMPORTANT:
IGMP must be enabled on the device that acts as the receiver-side DR. Otherwise, the receiver
hosts attached to the DR cannot join any multicast groups.
Figure 40 DR election
As shown in
, the DR election process is as follows:
1.
The routers on the shared-media LAN send hello messages to one another. The hello
messages contain the DR priority for DR election. The router with the highest DR priority is
elected as 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 priority in hello messages.
If the DR fails, its PIM neighbor lifetime expires and the other routers will initiate to elect a new DR.
RP discovery
An RP is the core of a PIM-SM domain. For a small-sized, simple network, one RP is enough for
multicast forwarding throughout the network. In this case, you can specify a static RP on each router
in the PIM-SM domain. However, in a PIM-SM network that covers a wide area, a huge amount of
multicast data is forwarded by 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. An RP
is dynamically elected from the multiple C-RPs through the bootstrap mechanism. An elected RP
provides services for a different multicast group. For this purpose, you must configure a bootstrap
router (BSR). A BSR acts as the administrative core of a PIM-SM domain. A PIM-SM domain has
only one BSR, but can have multiple candidate-BSRs (C-BSRs). If the BSR fails, a new BSR can be
automatically elected from the C-BSRs and 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.
Join message
Ethernet
Ethernet
RP
DR
DR
Hello message
Register message
Source
Receiver
Receiver
