Troubleshooting PIM-SM issues on a LAN segment

Below is the topology for this lab. R1 is the Mapping Agent and the RP. PIM-SM is enabled everywhere except the link between R1 and R3.

All routers also have the following debug command:

debug ip pim 239.0.0.1

Let's take a look at what happens R5 joins group 239.0.0.1

R5(config)#int f0/0
R5(config-if)#ip igmp join-group 239.0.0.1
Mar 1 00:51:50.599: PIM(0): Check RP 1.1.1.1 into the (*, 239.0.0.1) entry

R4#ping 239.0.0.1 re 5 sou s1/0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:
Packet sent with a source address of 172.12.14.4
.....
R4#

Hmmm....a quick check of the RP mapping and everyone knows about 1.1.1.1 (R1) as the RP. Let's take a look at the mroute table on R1:

R1#sho ip mrou 239.0.0.1 | be \(
(*, 239.0.0.1), 00:01:38/stopped, RP 1.1.1.1, flags: SP
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list: Null

(172.12.14.4, 239.0.0.1), 00:01:38/00:01:46, flags: PT
Incoming interface: Serial1/2, RPF nbr 0.0.0.0
Outgoing interface list: Null

R1 is seeing the packets from R4 but it's outgoing interface list is NULL. Let's take a look at R2's mroute table:

R2#sho ip mrou 239.0.0.1 | be \(
(*, 239.0.0.1), 00:03:27/00:02:57, RP 1.1.1.1, flags: SP
Incoming interface: Serial1/0, RPF nbr 172.12.12.1
Outgoing interface list: Null

NULL also, what gives? Let's wait and see if we get any debugs on R2:

R2#
00:55:42: PIM(0): Received v2 Join/Prune on FastEthernet0/0 from 172.12.25.6, not to us
00:55:42: PIM(0): Building Periodic Join/Prune message for 239.0.0.1

Interesting...It appears that R6 has become the DR for this segment and is responsible for sending (*,G) joins to the RP. R2 is hearing them, but ignoring them...why? What exactly is in the packet that tells R2 its not for us. Well since this is a dynamips lab, we can find out!

Here is a screenshot of the packet capture:

We can see that when R6 sends this join it is using a multicast address of 224.0.0.13. But inside of the PIM packet we can see R6 specifies an upstream neighbor of 172.12.25.3 which is R3.

Also on R6 we see the following debug messages:

R6#
*Mar 1 01:02:48.847: PIM(0): Building Periodic Join/Prune message for 239.0.0.1
*Mar 1 01:02:48.847: PIM(0): Insert (*,239.0.0.1) join in nbr 172.12.25.3's queue
*Mar 1 01:02:48.851: PIM(0): Building Join/Prune packet for nbr 172.12.25.3
*Mar 1 01:02:48.855: PIM(0): Adding v2 (1.1.1.1/32, 239.0.0.1), WC-bit, RPT-bit, S-bit Join
*Mar 1 01:02:48.859: PIM(0): Send v2 join/prune to 172.12.25.3 (FastEthernet0/0)

Can we fix this? Of course!

R6(config)#ip mroute 1.1.1.1 255.255.255.255 172.12.25.2

*Mar 1 01:05:52.019: PIM(0): Building Periodic Join/Prune message for 239.0.0.1
*Mar 1 01:05:52.019: PIM(0): Insert (*,239.0.0.1) join in nbr 172.12.25.2's queue
*Mar 1 01:05:52.023: PIM(0): Building Join/Prune packet for nbr 172.12.25.2
*Mar 1 01:05:52.027: PIM(0): Adding v2 (1.1.1.1/32, 239.0.0.1), WC-bit, RPT-bit, S-bit Join
*Mar 1 01:05:52.027: PIM(0): Send v2 join/prune to 172.12.25.2 (FastEthernet0/0)

Ping now:

R4#ping 239.0.0.1 re 5 sou s1/0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:
Packet sent with a source address of 172.12.14.4

Reply to request 0 from 172.12.25.5, 244 ms
Reply to request 1 from 172.12.25.5, 104 ms
Reply to request 2 from 172.12.25.5, 72 ms
Reply to request 3 from 172.12.25.5, 52 ms
Reply to request 4 from 172.12.25.5, 44 ms

But wait! There's one more solution. We can make R2 the DR for the segment (Remove the mroute on R6 and clear the mroute table on R2):

R2(config)#int f0/0
R2(config-if)#ip pim dr-priority 300000

01:07:09: PIM(0): Changing DR for FastEthernet0/0, from 172.12.25.6 to 172.12.25.2 (this system)
01:07:09: %PIM-5-DRCHG: DR change from neighbor 172.12.25.6 to 172.12.25.2 on interface FastEthernet0/0 (vrf default)
01:07:09: PIM(0): Check RP 1.1.1.1 into the (*, 239.0.0.1) entry
01:07:09: PIM(0): Building Triggered Join/Prune message for 239.0.0.1
01:07:09: PIM(0): Insert (*,239.0.0.1) join in nbr 172.12.12.1's queue
01:07:09: PIM(0): Building Join/Prune packet for nbr 172.12.12.1
01:07:09: PIM(0): Adding v2 (1.1.1.1/32, 239.0.0.1), WC-bit, RPT-bit, S-bit Join
01:07:09: PIM(0): Send v2 join/prune to 172.12.12.1 (Serial1/0)

R2#sho ip mrou 239.0.0.1 | be \(
(*, 239.0.0.1), 00:01:28/00:02:31, RP 1.1.1.1, flags: SJC
Incoming interface: Serial1/0, RPF nbr 172.12.12.1
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:01:28/00:02:31

It's always good to have more than one solution up your sleeve :)