Wireless Multicast Routing

[Thomas Brancato]

I am looking to understand how CAPWAP works with multicast routing. Since CAPWAP traffic is tunneled and the IP header is sourced from APs destined to the WLC (and vice versa), it would seem to me that multicast routing would only need to be enabled on the WLC and VLANs, and not need to be enabled on the routers between the WLC and APs. But in production I had to enable PIM and multicast routing on R0 interfaces and R1 Interfaces in the attached diagram in order for the Wireless client to receive multicast traffic from the Server Multicast Sender. All clients on the same WLAN connected to APs off of Switch A, B or 0, already worked prior to configuration.

Would someone please explain this?

Also, is there a way to view AP IGMP/PIM membership? I had to troubleshoot this using an Ethernet connection then add the client back to wireless to see if it worked.

[Rasika Nayanajith]

But in production I had to enable PIM and multicast routing on R0 interfaces and R1 Interfaces in the attached diagram in order for the Wireless client to receive multicast traffic from the Server Multicast Sender.

I believe those interfaces are L3. So multicast to work you need to enable PIM on all those L3 interfaces including SVI where multicast support required. Remember for multicast functionality relying on underlying IGP & end of the day CAPWAP is another IP header. Here is a very basic setup to check this

http://mrncciew.com/2012/11/08/test-yourself-basic-multicast/

Also, is there a way to view AP IGMP/PIM membership?

Here is another reference post which include some multicast related show commands in wireless setup

http://mrncciew.com/2012/11/17/configuring-multicast-on-wlc/

Also note that "video stream" feature to be configured for proper multicast service in wireless environment.

http://mrncciew.com/2012/12/24/understanding-videostream-feature/

HTH

Rasika

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[Thomas Brancato]

Your post on configuring Multicast on WLC was exactly what I was looking for in relation to IGMP detail on the WLC. This allowed me to view that the AP is part of the multicast group. 

show network multicast mgid detail ###

I conducted a PCap and the actual Frame and IP header did not have anything related to multicast. Below the CAPWAP section, the 802.11 header had multicast info and so did the IPv4 header. 

So my real question is, why is Multicast required on R0 and R1, when the actual IP header source and dest are the AP and WLC, and does not contain multicast information?

[Thomas Brancato]

I am still very interested in the answer to why is Multicast required on R0 and R1, when the actual IP header source and dest are the AP and WLC, and does not contain multicast information?

[Rasika Nayanajith]

Hi Thomas,

Here we go, I hope you understand this.

For basic multicast control plane functionality there are two protocols required.

1. IGMP - between host & router (multicast receivers to their LAN segment gateway)
2. PIM - between router to router (transit devices between sender & receiver gateway, use to build multicast routing table to forward data plane multicast traffic)

Let's say you have 2 wireless clients connected to two different AP require to receive a multicast feed. Then those two AP notify the WLC, that its client require to get the multicast stream (let's say 239.255.255.200). So WLC will use configured multicast address for WLC to AP communication (239.255.255.101 in below example). Look at the packet going from WLC to AP in the below post

http://mrncciew.com/2012/12/24/understanding-videostream-feature/ 

In your case your AP vlan segment is limited at R1. Without configuring PIM on R0 & R1, those routers will not have multicast routing table to forward multicast traffic coming from your WLC to those AP. Also when forwarding multicast traffic, a router will check RPF (reverse path forwarding) to ensure incoming interface of multicast packet (based on multicast routing table) is same as outgoing interface to the multicast source (based on unicast routing table). Unless this RPF pass, multicast packet will be dropped. 

So it is important to enable PIM on all interfaces that multicast traffic can flow in your network.

Hope this clear & good enough for more than 3 stars :)

Rasika

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[Thomas Brancato]

Thanks for the response Rasika. I have attached a PCap to illustrate the issue. This PCap was Taken on the R0 interface to Switch A. The important thing about this PCap is that it shows that there is no multicast referenced below the CAPWAP layer, which is different than your PCap which was conducted inside an L2 environment. On the unencapsulated portion it is 100% unicast Source-AP, Dest-WLC