Multicast-PIM Assert

If multiple multicast routers share a single segment, all of them could flood this segment with the same multicast traffic. Thus, traffic duplication occurs.To avoid this situation, only one router is allowed to flood traffic on the shared segment.

When a router detects that someone is sending traffic for the same source IP/destination group on the segment, where it has an active (S,G) state for the same group/source, it immediately originates a PIM Assert message. This message contains the source IP, the group, and the path cost to the source. The path cost is a touple (AD, Metric), where AD is the administrative distance of the routing protocol used to look up the source IP, and Metric is that same protocol’s metric to reach the source. The router with the best (lowest) AD value on the segment wins the assertion. 
If the ADs are equal, metric is used as tie-breaker. If both the AD and the metric are the same, the router with the highest IP address wins.

R1(config-if)#do show run int f0/0
Building configuration...

Current configuration : 143 bytes
!
interface FastEthernet0/0
 ip address 123.1.1.1 255.255.255.0
 ip pim dense-mode
 ip igmp join-group 224.1.1.1
 duplex auto
 speed auto
end

R5#ping 224.1.1.1 re 10
Type escape sequence to abort.
Sending 10, 100-byte ICMP Echos to 224.1.1.1, timeout is 2 seconds:
Reply to request 0 from 123.1.1.1, 184 ms
Reply to request 0 from 123.1.1.1, 208 ms
Reply to request 1 from 123.1.1.1, 104 ms
Reply to request 2 from 123.1.1.1, 112 ms

R2#
*Mar  1 00:57:14.763: PIM(0): Send v2 Assert on FastEthernet0/0 for 224.1.1.1, source 5.5.5.5, metric [110/21]                                                         
*Mar  1 00:57:14.763: PIM(0): Assert metric to source 5.5.5.5 is [110/21]
*Mar  1 00:57:14.767: PIM(0): We win, our metric [110/21]
*Mar  1 00:57:14.767: PIM(0): (5.5.5.5/32, 224.1.1.1) oif FastEthernet0/0 in Forward state
*Mar  1 00:57:14.839: PIM(0): Received v2 Assert on FastEthernet0/0 from 123.1.1.3
*Mar  1 00:57:14.839: PIM(0): Assert metric to source 5.5.5.5 is [110/21]
*Mar  1 00:57:14.839: PIM(0): We lose, our metric [110/21]
*Mar  1 00:57:14.843: PIM(0): Prune FastEthernet0/0/224.1.1.1 from (5.5.5.5/32, 224.1.1.1)
R3#
*Mar  1 00:57:25.107: PIM(0): Send v2 Assert on FastEthernet0/0 for 224.1.1.1, source 5.5.5.5, metric [110/21]                                                         
*Mar  1 00:57:25.107: PIM(0): Assert metric to source 5.5.5.5 is [110/21]
*Mar  1 00:57:25.111: PIM(0): We win, our metric [110/21]
*Mar  1 00:57:25.111: PIM(0): (5.5.5.5/32, 224.1.1.1) oif FastEthernet0/0 in Forward state
*Mar  1 00:57:25.115: PIM(0): Received v2 Assert on FastEthernet0/0 from 123.1.1.2
*Mar  1 00:57:25.119: PIM(0): Assert metric to source 5.5.5.5 is [110/21]
*Mar  1 00:57:25.119: PIM(0): We win, our metric [110/21]
*Mar  1 00:57:25.119: PIM(0): (5.5.5.5/32, 224.1.1.1) oif FastEthernet0/0 in Forward state