Multicast, IGMP and Spanning-Tree

So, I've come across this problem a lot of times, so I thought I'd try and write a post to help others in the same situation.

The situation is this - you have a large network of switches, using spanning-tree to prevent loops, but you are also using the network for multicast streaming. If you have any significant amount of multicast going on (maybe an IPTV system) then you'll be using IGMP snooping on all the switches to make sure that you don't have traffic going where it's not required. You set it up, and everything is working fine.

But then... it breaks. Badly. Your network starts flooding occasionally, for a couple of minutes at a time. During that time, all traffic on the network is delayed at best, and often dropped.

The interactions of IGMP, STP and your large amounts of multicast traffic are killing your network.

Let's break it down to explain the different things that are happening here:

Why does my network grind to a halt?

It's flooded! When using IGMP snooping, the multicast traffic on your network is normally only sent to those people who want to receive it. However, in this situation, your switches are momentarily sending traffic to all ports. There is so much traffic that your switch ports may be running at capacity, or the end-hosts are getting sent so much unwanted multicast traffic that they can't keep up.

So why does IGMP snooping suddenly stop working?

It doesn't. It is choosing to flood your multicast traffic because it thinks that is the best course of action in the given situation. If we look at the debug messages for IGMP snooping:

00:08:15: IGMPSN: mgt: Received topology change on vlan 1
00:08:15: IGMPSN: mgt: Updating all GCEs with flood portset for in Vlan 1

When spanning-tree protocol tells the switch that a topology change has occurred (more on this below), IGMP snooping will flood your multicast traffic to all ports, assuming that if the topology has changed and your traffic is mission-critical, then it had better send it to all ports to make sure it gets to your end user!

But I don't want that...

Ok, no problem - you can turn it off. In Cisco switches, you need to add this command to every interface you want to stop the flooding on.

 no ip igmp snooping tcn flood

That probably means all your edge ports, and potentially some of your uplink trunks, although these should probably be high enough bandwidth to be able to cope with all your multicast! This command is basically telling your switches "Don't flood traffic when you receive a topology-change notification (TCN)".

What is this topology change anyway? I didn't change anything!

Spanning-tree protocol, although very useful, can be very tricky to get configured correctly, and can cause you a lot of problems. When any switch believes a topology change has occurred, it will send a notification to the root bridge. When the root bridge receives this, it sets the topology-change (TC) bit in its BPDUs, to notify the whole of the rest of the network that a topology change has occurred.

So why are they happening if my network isn't changing?

Spanning tree will send a topology-change-notification (TCN) whenever it believes a topology change has occurred. If you already understand spanning-tree, you will know that any port, as it comes up, will go through two different states, "learning" and "listening", before finally entering the "forwarding" state and starting to operate normally. Any port transitioning in or out of this forwarding state will trigger a TCN. However, if a port is configured with "portfast" it will skip the "listening" and "learning" states and jump straight to "forwarding", without triggering a TCN. So, put simply, any port going up or down, anywhere on your network, that is not in portfast mode, will trigger a TCN, as shown below:

Without Portfast:

02:30:04: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
02:30:05: set portid: VLAN0001 Fa0/1: new port id 8001
02:30:05: STP: VLAN0001 Fa0/1 -> listening
02:30:06: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
02:30:20: STP: VLAN0001 Fa0/1 -> learning
02:30:35: STP: VLAN0001 sent Topology Change Notice on Gi0/1
02:30:35: STP: VLAN0001 Fa0/1 -> forwarding

With Portfast:

02:29:10: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
02:29:11: set portid: VLAN0001 Fa0/1: new port id 8001
02:29:11: STP: VLAN0001 Fa0/1 ->jump to forwarding from blocking
02:29:12: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

As you can see, portfast is very desirable, because not only does it stop unwanted TCNs, but it also means your ports will come up much faster. If you're anything like me, you already put all your access ports into portfast, just because you want them to come up fast. However, you may not put your edge trunk ports (perhaps for a server, wireless AP or VoIP phone) into portfast.

And this will fix all my problems?

Not necessarily. It's possible that you are getting legitimate topology changes within your network. For example, I have seen an occasion where a faulty fibre link was causing an interface flap for an unused switch on the edge of a network. You can track down the source of your TCNs by using "debug spanning-tree events" on your switches. Start with the root bridge, and when your TCN occurs, you should see something like this:

02:38:48: STP: VLAN0001 Topology Change rcvd on Fa0/24

So, work out which switch is on Fa0/24, log into that and run debugs there. Repeat the process until you find the port that is flapping. A quicker way of doing this is to set up all your switches to log debug messages to a syslog server, and turn on spanning-tree event debugs on all the switches at the same time, and then you only have to see a single TCN, rather than having to keep waiting for it to occur. I'll put up another post about syslogs on Cisco another time.

I'm still confused, how do I stop this flooding happening?!

The quickest way is to add the "no ip igmp snooping tcn flood" command to all your interfaces. If you want to stop the underlying cause, make sure all ports where a single device is connected are set up with "spanning-tree portfast" for access ports or "spanning-tree portfast trunk" for trunk ports. Don't do this for links to switches - they should be set up as part of your spanning tree.


I hope this is useful to some people. I've dealt with this situation quite a few times, and the first few it took me a while to figure out what was happening. If you want to understand this further, Cisco has a very helpful page about this here.

Edit: See my next entry for info on some of the Linux commands I used to test multicast in my lab.
Update: See my newer post about IGMP Query Solicitation