topic Re: WLC Multicast and FlexConnect (rev. 2017) in Wireless

WLC Multicast and FlexConnect (rev. 2017)

Johannes Luther — Mon, 05 Jul 2021 14:34:55 GMT

Hi board,
I want to wrap up some discussion from 2012 (https://supportforums.cisco.com/t5/other-wireless-mobility-subjects/multicast-client-on-flexconnect-ap/m-p/2093220) and some official Cisco documents regarding FlexConnect and Multicast forwarding (simple because I am kinda confused) 🙂

So here are some facts:

Fact 1: The 5520 does only support Multicast forwarding (except IPv6 RAs) in combination with Multicast-mode-Multicast (reference [1])

strangley enough, table 4 (Platform Support for Global Multicast and Multicast Mode) disappeared in all later release notes. I guess this limitation for the 5520 still exists with the latest WLC code.

Fact 2: Forewarding of Multicast traffic in centrally switched WLANs is only supported with Multicast-mode-Unicast (reference [2], [3], [4])

This means, that with the 5520 WLC, there is no support for Multicast applications for FlexConnect centrally switched WLANs, right? Or is there some outdated or incorrect documentation?

However, other statements are in the same and different documentations

Example 1: Mobility Design Guide 8.1

"FlexConnect APs allow the local termination of WLANs at the network edge rather than at the WLC,
and the multicast behavior is controlled at that edge. If a FlexConnect WLAN is terminated on a WLC
and multicast is enabled on that WLC, multicast is delivered to that FlexConnect WLAN, if the
CAPWAP multicast group is allowed to extend to the FlexConnect network location."

In short this means: If the WAN is Multicast capable, then you're ready to go (or am interpreting this wrong?)

Example 2: WLC Software Configuation 8.2

"FlexConnect APs do not join the multicast group when the Multicast mode is Multicast-Multicast and CAPWAP has IPv4 and IPv6. For Cisco 5508 and 8510 WLCs, you can disable the Multicast-Multicast mode and enable the Multicast-Unicast mode. For Cisco Flex 7510 WLC, there is no Multicast-Multicast configuration. For FlexConnect APs in Multicast-Multicast mode joined with central switching clients, there is reduction of 0-13 percent in data throughput. "

First sentence states, that Multicast-Multicast is not supported. The last sentence states, that Multicast-Multicast IS possible but with a reduction of data throghput (technically I don't get the point, why there is a throughput degradance)

You get my point? What is correct here? What is the correct, valid and Cisco supported design for 5520 FlexConnect deployments with centrally switched WLANs?

Fact IS, that FlexConnect APs are joining the AP multicast group:

Switch#show ip igmp snooping groups
Vlan Group Type Version Port List
-----------------------------------------------------------------------
10 239.255.128.40 igmp v2 Gi0/1

Gi0/1 is a port to a FlexConnect AP and 239.255.128.40 is the configured AP Multicast group

AP Ouput:

AP#show capwap mcast
CAPWAP MULTICAST
Multicast Group: 239.255.128.40, Source: 10.1.2.3
[..]

AP#show capwap reap status
[...]
AP Mode: REAP, Connected
[...]

So obviously the 8.2.150.0 deployment from above is pretty buggy, or there is a documentation bug, because the AP actually joins the Multicast group.

Edit: Found the hint in the CG of SW version 7.4:

"FlexConnect APs [...] Can join in Multicast-Multicast mode from Release 8.0 onwards"

I'm pretty sure, that FlexConnect with centrally switched WLANs and Multicast-mode-Multicast actually works, but I haven't verified this in the lab yet. But even if this is working, I cannot use it in production, because officially it's not supported by Cisco (see references 1-4).

Keep you posted about my testing results. However it would be good if someone who knows about this problem, or shares my findings, or proves me wrong could answer.

If some wireless guy from Cisco reads this - a statement from your side would be highly appreciated 🙂

Cheers and thanks for reading my load of rubbish 🙂

Johannes

References:

[1] AireOS 8.3 release notes

[2] Doc: Multicast support on different WLC platform when using Local and Flexconnect AP Mode

"Flexconnect mode AP can't join Multicast group address configured at WLC"

"Flex central switched APs can't receive Multicast packets since it doesn't support Unicast mode. Use Local mode AP or Flex local switching as workaround in this case."

[3] WLC 8.2 configuration guide

"The controller can send multicast packets in the form of unicast or multicast packets to the access point. In FlexConnect mode, the access point can receive multicast packets only in unicast form. "

[4] Mobility Design Guide 8.1

"In FlexConnect mode, the AP can receive multicast packets only in unicast form."

Re: WLC Multicast and FlexConnect (rev. 2017)

Johannes Luther — Tue, 29 Aug 2017 08:52:53 GMT

I checked a few scenarios in the lab an here are my findings:

Test:

Wireless clients joins Multicast group (via iperf). Wired clients is Multicast sender (ping).

Verification of Multicast receiving via tcpdump on wireless client.

Prerequesites:

WLC Global Multicast must be enabled
WLC Multicast mode must be Multicast (5520 WLC)
Network between WLC and AP must support IPv4 Multicast routing (PIM-SM in my case)

Working scenarios: Wireless client receives Multicast packets and IPv6 SLAAC is working (an RA is a Multicast)

FlexConnect local switching with static VLAN-WLAN mapping via FlexConnect groups

Client is bridged at AP and receives Multicast over locally assigned VLAN ID.

FlexConnect local switching with dynamic VLAN assignment (AAA override) AND Multicast override

Command: config flexconnect group <group_name> multicast overridden-interface enable

Client is bridged at AP and receives Multicast over locally assigned VLAN ID (on AP or FlexConnect group level).

FlexConnect central switching with dynamic VLAN assignment (AAA override)

Client is bridged at WLC and receives Multicast over dynamically assigned VLAN ID.

FlexConnect central switching with dynamic VLAN assignment (AAA override) AND Multicast interface

Client is bridged at WLC and receives Multicast over the statically configured Multicast interface. The dynamically assigned interface / VLAN via AAA is not relevant for Multicast in this case.

FlexConnect central switching with static VLAN assignment

Client is bridged at WLC and receives Multicast over the statically configured dynamic interface / VLAN.

FlexConnect VLAN based central switching (centrally switched) with dynamic VLAN assignment

Client is bridged at WLC and receives Multicast over dynamically assigned VLAN ID.

Not working scenarios: Wireless client does not receive Multicast packets and IPv6 SLAAC is not working

FlexConnect local switching with dynamic VLAN assignment (AAA override)

VLAN is created using an ACL assignment. Unicast communication works as expected.

Multicast is NOT forwarded by the AP.

In every case the AP joins the AP Multicast group.

SW version: 8.2

Re: WLC Multicast and FlexConnect (rev. 2017)

Javier Contreras — Wed, 30 Aug 2017 07:40:25 GMT

Hi Johannes

Here the distinction is mostly on Supported=Tested, so let's go by parts.

Basically:

Flex with multicast-unicast mode central switching, is supported (because it is tested) scenario.

Flex with multicas-multicast mode, central switching, is not tested, thus not officially supported.

Does it work? yes, it should, as I am not aware of any explicit blocking point at AP level for this not to work, but if a bug is found on this specific topology/feature combination, it would be treated as best effort fix.

Why it is not a tested scenario: because few, if any, customers would have a WAN scenario supporting multicast routing, so this has never been a priority to add as specific test case.

For local switching, the multicast mode at WLC should not have any impact, as the traffic forwarding is handled by AP directly on the VLAN.

There are test scenarios to validate that WLC should not consume/generate multicast for those vlans on flex AP local switching scenarios (if WLC sends multicast to them from the central site, that is a bug, and must be fixed)

Re: WLC Multicast and FlexConnect (rev. 2017)

Johannes Luther — Wed, 30 Aug 2017 11:20:17 GMT

Hey Javier,
thank you for the answer.

Flex with multicas-multicast mode, central switching, is not tested, thus not officially supported.
Does it work? yes, it should, as I am not aware of any explicit blocking point at AP level for this not to work, but if a bug is found on this specific topology/feature combination, it would be treated as best effort fix.

Yeah, this is what I suspected (unfortunately 🙂 ). I tested it and it seems to be working quite good. Now it's up to the customer to decide whether only Cisco official supported designs are used or not.
However, FlexConnect centrally switched WLANs with Multicast user traffic were supported with the 5508 (Multicast-mode-Unicast). Now, with the 5520/8540 this formerly supported scenario is deprecated and there is no officially supported solution for that, except deploying WLCs in the remote locations. But I agree, that this is a corner-case and not many customers actually require Multicast application support over Flex central.

For local switching, the multicast mode at WLC should not have any impact, as the traffic forwarding is handled by AP directly on the VLAN.

Correct ... this is what I tested as well. The only exception is, that Multicast will not work with AAA overridden VLANs for locally switched VLANs (as documented). The only workaround is the multicast-override CLI switch. But this is all clear and well documented.

There are test scenarios to validate that WLC should not consume/generate multicast for those vlans on flex AP local switching scenarios (if WLC sends multicast to them from the central site, that is a bug, and must be fixed)

Interesting! I didn't check this. However, this would be a strange scenario / error case. The WLC would have to send IGMP joins towards the wired network with the corresponding dynamic interface. Normally I define a "dummy" dynamic Interface in the WLAN configuration for locally switched WLANs.

Thanks again for your detailled answer!

Re: WLC Multicast and FlexConnect (rev. 2017)

Javier Contreras — Wed, 30 Aug 2017 14:15:51 GMT

>Now, with the 5520/8540 this formerly supported scenario is deprecated and there is no officially supported solution for that, except deploying WLCs in the remote locations.

well... for 5520, we added support for multicast-unicast scenario in I think, 83MR2 (if I am not mistaken)

it is only supported on limited size networks (AP count), as the multicast frame replication simply can't scale to the number of APs that those platforms can support on a fully loaded scenario

So there is a solution for 5520 as long as the network is not large

regards

Re: WLC Multicast and FlexConnect (rev. 2017)

Johannes Luther — Wed, 30 Aug 2017 15:11:50 GMT

Hi Javier,

I guess the Multicast-Unicast support is limited to RAs and ARPs.

(config network passive-client arp-unicast-forwarding enable).

The 8.5 configuration guide still states, that enabled "Global Multicast" is only supported with Multicast-mide-Multicast.

Re: WLC Multicast and FlexConnect (rev. 2017)

Ryan Curry — Mon, 11 Mar 2019 15:39:06 GMT

Sorry to drum up a year and a half old thread, but has anyone gotten the Flex Local Switching with Dynamic VLAN Assignment working? I've got a group of roughly 25 APs that are in a flex group where users are reporting no longer able to broadcast from their wireless phones. I suspect this is because I've recently moved them to this exact scenario (trying to reduce my WLC footprint). Any insight would be appreciated.