IGMP Multicast Packets Not being Forwarded

DWNetworkSprout · ‎11-03-2022

Greetings All,

I'm currenting working on a Cisco 9500-48Y4C 25 Gigabit switch with Amsterdam 17.3.1 IOS installed. We are using this switch so that our output source (custom FPGA through a 10 Gbe Core) can send out UDP data packets to our data capture computer. However, we are running into an issue were the packets are not being forwarded on the switch.

IGMP PIM is enabled in sparse-mode via our Vlan interface (ID 1100 in this case), and the packets are being registered as multicast packets on the interface that the output source is plugged into on the switch. However, nothing comes out on the Data Capture Computer's interface on the switch. The two interfaces that are in use are configured as below.

interface TwentyFiveGigE1/0/1
  switchport trunk allowed vlan 1100
  switchport mode trunk
  switchport block multicast
  switchport block unicast
  no keepalive
  no lldp transmit
  no ip igmp snooping tcn flood

interface TwentyFiveGigE1/0/2
  switchport trunk allowed vlan 1100
  switchport mode trunk
  switchport block multicast
  switchport block unicast
  no keepalive
  no lldp transmit
  no ip igmp snooping tcn flood

interface Vlan 1100
  ip address 192.168.100.241 255.255.255.0
  no ip redirects
  no ip unreachables
  no ip proxy-arp
  ip pim sparse-mode

When the FPGA sends out its data packets to the switch, it comes into the interface (which I see using 'show int Twe1/0/1) as multicast packets, and with no errors, but nothing gets to Twe1/0/2. However, when I use a custom written program to send out multicast packets from our Data Capture computer to the switch (through a separate 10 Gbe interface on it) to the switch and back to the Data Capture computer, it goes through perfectly fine and we are able to receive and capture the data.

We have been down sending the IGMP join command, to enabling the IGMP checksum on the command, to various settings on the Cisco switch, but nothing makes the FPGA packets come broadcast out on the other interface. The only other thing I've noted is that 'show ip igmp groups' does not show the multicast ip group (230.1.1.10) of our packets, but only lists a 240.x.x.x address that seems to be attached to the Vlan 1100 interface, but I didn't set that at all. However the packets from the Data Capture computer come through fine despite this.

Any help here would be greatly appreciated. Thank you very much.

georgehewittuk1 · ‎11-03-2022

Based on your config I assume your trying to stop flooding… best bet is looking at the ports igmp snooping is detecting a request for the feed from the relative ports… debug ip igmp snooping vlan

Other thoughts are debugging igmp and pcaps on the clients on the igmp join workout a difference.

DWNetworkSprout · ‎11-04-2022

Thank you, I will take a look at those. is "debug ip igmp snooping vlan" an actual command, or do I drop "debug" from that?

I also don't understand your last sentence. Would it be possible to expand on that more.

Yes, we are trying to stop flooding on our ports, I presume so that everything doesn't get blasted all the time. We are using a previously working config with some small mods due to the switch we are currently using and the differences in the IOS (I had to learn to implement 'ip pim sparse-mode' in our setup now, as that was something that wasn't needed beforehand when we used a Catalyst 4500 Series to do this).