Designated Routers - Cisco ASR 9000 Series Configuration Manual

Implementing Layer-3 Multicast Routing on Cisco IOS XR Software

Designated Routers

Designated Routers
Cisco routers use PIM-SM to forward multicast traffic and follow an election process to select a designated
router (DR) when there is more than one router on a LAN segment.
The designated router is responsible for sending PIM register and PIM join and prune messages toward the
RP to inform it about host group membership.
If there are multiple PIM-SM routers on a LAN, a designated router must be elected to avoid duplicating
multicast traffic for connected hosts. The PIM router with the highest IP address becomes the DR for the LAN
unless you choose to force the DR election by use of the dr-priority command. The DR priority option allows
you to specify the DR priority of each router on the LAN segment (default priority = 1) so that the router with
the highest priority is elected as the DR. If all routers on the LAN segment have the same priority, the highest
IP address is again used as the tiebreaker.
Figure 5: Designated Router Election on a Multiaccess Segment, on page 88 illustrates what happens on a
multiaccess segment. Router A (10.0.0.253) and Router B (10.0.0.251) are connected to a common multiaccess
Ethernet segment with Host A (10.0.0.1) as an active receiver for Group A. As the Explicit Join model is used,
only Router A, operating as the DR, sends joins to the RP to construct the shared tree for Group A. If Router
B were also permitted to send (*, G) joins to the RP, parallel paths would be created and Host A would receive
duplicate multicast traffic. When Host A begins to source multicast traffic to the group, the DR's responsibility
is to send register messages to the RP. Again, if both routers were assigned the responsibility, the RP would
receive duplicate multicast packets.
If the DR fails, the PIM-SM provides a way to detect the failure of Router A and to elect a failover DR. If the
DR (Router A) were to become inoperable, Router B would detect this situation when its neighbor adjacency
with Router A timed out. Because Router B has been hearing IGMP membership reports from Host A, it
already has IGMP state for Group A on this interface and immediately sends a join to the RP when it becomes
the new DR. This step reestablishes traffic flow down a new branch of the shared tree using Router B.
Additionally, if Host A were sourcing traffic, Router B would initiate a new register process immediately
after receiving the next multicast packet from Host A. This action would trigger the RP to join the SPT to
Host A, using a new branch through Router B.
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