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Johannes Luther

9800 WLC, HSRP and Multicast forwarding

Hi all,

assuming my wireless client IP subnets are created on a non-VSS, stack, or SVL device.

FHRP is realized using HSRP.

So in my wireless VLAN, there are link-local Multicast messages for the group address (HSRPv2).

If I have Multicast support enabled on the WLC, these HSRP packets are encapsulated in CAPWAP (Multicast) and sent to all APs (right?!).

I guess IGMP snooping is not applicable to these addresses, right?

My "fear" is, that these HSRP Multicast frames are sent to all wireless clients. The problem is, that the Multicast frames are sent at a non-HT rate (highest mandatory data rate) and will potentially pollute the air.

So my questions are:

- Are the HSRP packets encapsulated into CAPWAP by the WLC and sent to the APs? (assuming Multicast forwarding is enabled)

- If yes (so the HSRP packets reach the APs)... are the packets sent to the air?


I didn't test this yet. I just want to understand the theory behind this and the documentation state.

Johannes Luther

So, I did some quick & dirty lab testing.

Obviously the HSRP packets are (of course) forwarded on the correspodning VLAN to the WLC.


Catalyst 9800 (17.3)

The WLC does not encapsulate these packets in CAPWAP.


AireOS 5520 (8.10)

Depending on the setting of "config network multicast l2mcast" and "IGMP snooping"

config network multicast l2mcast enabled on specific interface (I think this is the default)

HSRP traffic (version 1 and 2 / IPv4 and IPv6) is encapsulated in CAPWAP and sent to all APs (if multicast mode multicast)


config network multicast l2mcast disabled on specific interface

HSRP traffic (version 1 and 2 / IPv4 and IPv6) is not encapsulated in CAPWAP


I quite don't get why, because HSRP is NOT a layer-2 Multicast. It's a layer-3 link local Multicast.

An example for a layer-2 Multicast is LLDP or CDP.

=> Anyway ... I guess it's a unprecise documentation issue


If IGMP snooping is disabled OR the wireless client is joining the HSRP group address ( for HSRPv2), then the HSRP packets actually reach the WLAN client.


So I don't care about AireOS any more - these days are over.

But for the Cat9k a documentation would be great, which explains WHY the HSRP packets are not encapsulated in CAPWAP by default.


I hate when something like this happens. For each software release I have to verify now whether the (undocumented) behavior changed or not ... bugger.

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