PIM - MA Placement in a Frame Relay network

When placing the Mapping Agent in a frame relay hub and spoke environment always aim to locate it at the hub or behind it.

PIM by default will assume NBMA interfaces are broadcast capable. However, by default, when a spoke sends a multicast message the hub will not replicate this to other spokes, obeying the split horizon rule. This can in part be solved by the placement of the ‘ip pim nbma-mode’ command on the hub. It is worth noting however that this only fixes sparse-mode traffic. Dense mode traffic will NOT be replicated.

Hence this poses a problem for auto-rp information which uses dense mode to mcast groups 224.0.1.39 and 224.0.1.40.

If the RP and mapping agent are placed on a spoke then auto-rp messages will only reach the hub node. If the mapping agent is on the hub then RPs could be located on the spokes as long as the announces reach the hub.

There are a couple of resolutions to this problem. First use sub-interfaces on the hub, or secondly create multicast enabled tunnels between the spokes.

The tunnel config for the spokes is shown here.

Router_1#
interface Tunnel0
ip address 155.1.20.20 255.255.255.0
ip pim sparse-mode
tunnel source Loopback1
tunnel destination 150.1.3.3
tunnel mode ipip

Router_3#
interface Tunnel0
ip address 155.1.20.21 255.255.255.0
ip pim sparse-mode
tunnel source Loopback0
tunnel destination 150.1.1.1
tunnel mode ipip

Another caveat is that if the tunnel is not included in the IGP routing then static multicast routes will be required pointing at the tunnel to ensure RPF checks don’t fail.

R1
Ip mroute 150.1.1.1 255.255.255.255 tu0


Note. The problem with the disssemination of traffic to mcast group 224.1.0.40 can be seen on the hub (R5) as the frame relay serial interface S2/0 is missing in the OIL interface list.


(150.1.1.1, 224.0.1.40), 01:14:19/00:02:34, flags: LT
Incoming interface: Serial2/0, RPF nbr 155.1.0.1
Outgoing interface list:
Loopback0, Forward/Sparse, 01:14:19/00:00:00
Serial2/1, Forward/Sparse, 01:14:19/00:00:00
FastEthernet0/0, Forward/Sparse, 01:14:19/00:00:00