Designated Routers - Cisco XR 12000 Series Configuration Manual

Information About Implementing Multicast Routing
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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 4
(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.
Tip Two PIM routers are neighbors if there is a direct connection between them. To display your PIM
neighbors, use the show pim neighbor command in EXEC mode.
Multicast Configuration Guide
MCC-10
When multicast-intact is enabled on an IGP, all IPv4 destinations that were learned through
link-state advertisements are published with a set equal-cost mcast-intact next-hops to the RIB. This
attribute applies even when the native next-hops have no IGP shortcuts.
In IS-IS, the max-paths limit is applied by counting both the native and mcast-intact next-hops
together. (In OSPFv2, the behavior is slightly different.)
illustrates what happens on a multiaccess segment. Router A (10.0.0.253) and Router B
Implementing Multicast Routing on Cisco IOS XR Software Cisco ASR 9000 Series Routers
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