Multicast broadcast traffic forwarding to wrong interface

m45595400 · ‎07-11-2007

I have a simple 2 router back-to-back serial setup with a LAN on each end. Each router has 1 FastEthernet interface and 2 Serial.

I have multicast enabled and pim set to dense mode.

I have a client sending a MADCAP discover packet to find a server - on the same LAN this works fine. If I move the client to the remote subnet it fails.

On the router at the client end I see the following MROUTE output for 239.255.255.254 (the standard MADCAP discover address)

(*, 239.255.255.254), 02:42:00/stopped, RP 0.0.0.0, flags: D

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

Serial1, Forward/Dense, 02:41:59/00:00:00

Serial0, Forward/Dense, 02:42:00/00:00:00

and for the client address (192.168.101.23) I see this entry

(192.168.101.23, 239.255.255.254), 02:41:00/00:01:49, flags: T

Incoming interface: FastEthernet0, RPF nbr 0.0.0.0

Outgoing interface list:

Serial0, Forward/Dense, 02:41:00/00:00:00

Serial1, Forward/Dense, 02:41:00/00:00:00

that router also shows this PIM routing table

PIM Neighbor Table

Neighbor Interface Uptime/Expires Ver DR

Address Prio/Mode

192.168.12.1 Serial1 03:02:27/00:01:35 v2 1 / S

the other end of the serial link I see this PIM routing table

PIM Neighbor Table

Neighbor Interface Uptime/Expires Ver DR

Address Prio/Mode

192.168.12.2 Serial1 03:03:52/00:01:27 v2 1 / S

so PIM is happy for the intra router link.

on the server end router I see the following MROUTE info for 239.255.255.254

(*, 239.255.255.254), 02:43:38/stopped, RP 0.0.0.0, flags: D

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

Serial1, Forward/Dense, 02:43:38/00:00:00

Serial0, Forward/Dense, 02:43:38/00:00:00

and for the client entry I see

(192.168.101.23, 239.255.255.254), 02:43:39/00:01:40, flags: T

Incoming interface: Serial1, RPF nbr 192.168.12.2

Outgoing interface list:

Serial0, Forward/Dense, 02:43:39/00:00:00

So it appears that the multicast packets are being routed out of Serial0, but the server is on FastEthernet0.

Why doesn't FastEthernet0 appear on the MROUTE for 239.255.255.254 when it does for some other group addresses on the same router?

(e.g.

(*, 239.0.9.0), 02:45:25/00:02:41, RP 0.0.0.0, flags: DC

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

Serial1, Forward/Dense, 02:45:25/00:00:00

Serial0, Forward/Dense, 02:45:25/00:00:00

FastEthernet0, Forward/Dense, 02:45:25/00:00:00

)

Thanks,

Mark.

bgibson · ‎07-11-2007

Can you show us what sh ip igmp group and sh ip igmp int have to say?

m45595400 · ‎07-12-2007

No need now - I got it working thanks.

Your suggestion started me thinking down a different path. The fact is there's nothing special about the 239.255.255.254 address that MADCAP uses, and the server hadn't joined that group, so the router had no idea that it would have been interested in receiving those packets.

So I added an "ip igmp join-group 239.255.255.254" to the fa0 interface on each router and hey presto everything started working.

Thanks for the nudge in the right direction.

Mark.