PIM Assert

The PIM Assert mechanism is used to shutoff duplicate flows onto the same multiaccess network. Routers detect this condition when they receive an (S,G) packet via a multi-access interface that is already in the (S,G) OIL. This causes the routers to send Assert Messages.

In this scenario the workstation attached to R6 has joined group 239.6.6.6. A multicast feed is started and both R1 and R4 begin sending the mcast.

With 'debug ip pim' enabled on R1 and R4, it can be seen that a PIM assert exhange is initiated between them.

ON R1
*Jun 8 06:18:49.419: PIM(0): Send v2 Assert on FastEthernet0/0 for 239.6.6.6, source 155.1.58.2, metric [80/65]
*Jun 8 06:18:49.423: PIM(0): Assert metric to source 155.1.58.2 is [80/65]
*Jun 8 06:18:49.423: PIM(0): We win, our metric [80/65]


ON R4
*Jun 8 06:18:49.359: PIM(0): Received v2 Assert on FastEthernet0/0 from 155.1.1
48.1
*Jun 8 06:18:49.367: PIM(0): Assert metric to source 155.1.58.2 is [80/65]
Router_4#
*Jun 8 06:18:49.371: PIM(0): We lose, our metric [90/2172416]

The winner of the assert exchange is the router with best (AD,Metric). In the above case, R1 has an AD of 80 and R4 has an AD of 90. R1 wins!

As a result R4 prunes the S,G entries in its routing table


Router_4#s ip mroute 239.6.6.6
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report,
Z - Multicast Tunnel, z - MDT-data group sender,
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 239.6.6.6), 00:04:43/stopped, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial2/1, Forward/Sparse-Dense, 00:04:43/00:00:00
FastEthernet0/0, Forward/Sparse-Dense, 00:04:43/00:00:00

(150.1.8.8, 239.6.6.6), 00:00:56/00:02:05, flags: PT
Incoming interface: Serial2/1, RPF nbr 155.1.46.5
Outgoing interface list:
FastEthernet0/0, Prune/Sparse-Dense, 00:00:56/00:02:03

(155.1.58.2, 239.6.6.6), 00:00:56/00:02:05, flags: PT
Incoming interface: Serial2/1, RPF nbr 155.1.46.5
Outgoing interface list:
FastEthernet0/0, Prune/Sparse-Dense, 00:00:56/00:02:03


On R1 the S,G entries remain with an 'A' by them denoting Assert winner!

Router_1#s ip mroute 239.6.6.6
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report,
Z - Multicast Tunnel, z - MDT-data group sender,
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 239.6.6.6), 00:05:01/stopped, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial2/0, Forward/Sparse-Dense, 00:05:01/00:00:00
FastEthernet0/0, Forward/Sparse-Dense, 00:05:01/00:00:00

(150.1.8.8, 239.6.6.6), 00:01:14/00:01:46, flags: T
Incoming interface: Serial2/0, RPF nbr 155.1.0.5
Outgoing interface list:
FastEthernet0/0, Forward/Sparse-Dense, 00:01:14/00:00:00, A

(155.1.58.2, 239.6.6.6), 00:01:14/00:01:46, flags: T
Incoming interface: Serial2/0, RPF nbr 155.1.0.5
Outgoing interface list:
FastEthernet0/0, Forward/Sparse-Dense, 00:01:14/00:00:00, A