Multicast over frame-relay can be tricky when you try to run it over a hub and spoke topology. In a previous lesson I described the issue when you are using auto-rp and your mapping agent is behind a spoke. This time we’ll take a look at PIM NBMA mode.

Let me show you the topology that I will use to explain and demonstrate this to you:

Above you see 3 routers. R1 is the hub router, R2 and R3 are my spokes. We are using point-to-multipoint frame-relay so there is only a single subnet. R1 is also the RP (Rendezvous Point)

PIM treats our frame-relay network as a broadcast media. It expects that all routers can hear each other directly. This however is only true when we have a full mesh…using a hub and spoke topology like the network above this doesn’t apply because there is only a PVC between the hub and spoke routers. The spoke routers are unable to reach each other directly, they have to go through the hub router.

This causes a number of issues. First of all, whenever a spoke router sends a multicast packet it will be received by the hub router, but the hub router doesn’t forward it to other spoke routers because of the RPF rule (never send a packet out of the interface you received it on). One of the methods of dealing with this problem is by using point-to-point sub-interfaces as its solves the split horizon problem.

The other problem is that spoke routers don’t hear each others PIM messages. For example, let’s say that R2 and R3 are both receiving a certain multicast stream. After awhile there are no users behind R2 that are interested in this stream and as a result R2 will send a PIM prune message to R1.

If R3 still has active receivers it would normally send a PIM override message to let R1 know that we still want to keep receiving the multicast stream. R1 however assumes that the prune message from R2 is heard by all PIM routers but this is not the case in our hub and spoke topology…only the hub router has received it, R3 never heard this PIM prune message. As a result R1 will prune the multicast stream and R3 will not receive anything anymore…

PIM NBMA mode solves these issues that I just described to you. Basically it will tell PIM that the frame-relay network should be treated as a collection of point-to-point links, not as a multi-access network.  Let’s take a look at the example above and configure it so you can see how it works.

OSPF has been configured to advertise the loopback0 interface of R1 so that we can use it as the IP address for the RP. Let’s start by enabling PIM on the interfaces:

R1(config)#interface serial 0/0
R1(config-if)#ip pim sparse-mode

R2(config)#interface serial 0/0
R2(config-if)#ip pim sparse-mode 

R3(config-if)#interface serial 0/0
R3(config-if)#ip pim sparse-mode

This will activate PIM on all serial interfaces. Let’s verify that we have PIM neighbors:

R1#show ip pim neighbor 
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
      S - State Refresh Capable
Neighbor          Interface                Uptime/Expires    Ver   DR
Address                                                            Prio/Mode
192.168.123.3     Serial0/0                00:03:51/00:01:21 v2    1 / DR S
192.168.123.2     Serial0/0                00:04:04/00:01:35 v2    1 / S

That’s looking good. Now let’s configure the RP:

R1(config)#ip pim rp-address 1.1.1.1

R2(config)#ip pim rp-address 1.1.1.1

R3(config)#ip pim rp-address 1.1.1.1

I will use a static RP as it saves the hassle configuring auto-RP and a mapping agent. Let’s configure R3 as a receiver for the 239.1.1.2 multicast group address, I will use R2 as a source by sending pings:

R3(config-if)#ip igmp join-group 239.1.1.2

R2#ping 239.1.1.2 repeat 9999

Type escape sequence to abort.
Sending 9999, 100-byte ICMP Echos to 239.1.1.2, timeout is 2 seconds:
.....

As you can see nothing no packets are arriving. Let’s take a closer look to see what is going on:

R3#show ip mroute 239.1.1.2
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

(*, 239.1.1.2), 00:01:40/00:02:27, RP 1.1.1.1, flags: SJPCL
  Incoming interface: Serial0/0, RPF nbr 192.168.123.1
  Outgoing interface list: Null

R3 has registered itself at the RP but doesn’t receive anything.

R1#show ip mroute 239.1.1.2
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

(*, 239.1.1.2), 00:02:10/00:03:18, RP 1.1.1.1, flags: SJC
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    Serial0/0, Forward/Sparse, 00:02:10/00:03:18

(192.168.123.2, 239.1.1.2), 00:01:38/00:02:00, flags: PJT
  Incoming interface: Serial0/0, RPF nbr 0.0.0.0
  Outgoing interface list: Null

R1 is receiving traffic from R2 but doesn’t forward it out of the same interface to R3 (Serial0/0).