Wireless Access

I am curious whether there is any mechanism for a Campus AP to perform PMTUD to figure out whether an MTU lower than 1500 bytes exists between itself and a controller (under AOS 6 or 8).

I believe such a mechanism exists to identify whether Jumbo frames are supported, but not for an MTU lower than 1500 bytes.

Furthermore, it seems that the GRE IPv4 for CAP to Controller traffic does not have the DF bit set which would result in fragmentation of traffic.

Is there any form of dynamic MTU discovery that should be working in this case, or is it up to the administrator to define the SAP MTU in cases where it is lower than 1500 bytes?

It is up to the administrator to set it lower than 1500.

Actually that isn't true - as long as the upstream router is configured to send unreachables (ICMP Type 3, Code 4) i.e. Fragmentation Needed the AP SAP MTU gets set automatically.

I tested this by setting the IP MTU on an HP MSR router between an AP and VMC to 1400 bytes.

When 'ip unreachables enable' was configured, the router sends an ICMP message stating 'Fragmentation needed, MTU of next hop: 1400'.

show ap debug system-status ap-name TEST-CAP

MTU Discovery
-------------
IP Probes Responses Last Sent Last Rcvd jumbo:tx/rx 9KB: tx/rx
-- ------ --------- --------- --------- ----------- ----------
192.168.10.40 7 0 532 0 7/0 0/0

Switch MTU, Update sent time, Ack receive time, Current time (Time in seconds since AP booted)
1400 532 532 569

When unreachables was set disabled, there is no ICMP message from the router and the AP sets its MTU to 1500:

MTU Discovery
-------------
IP Probes Responses Last Sent Last Rcvd jumbo:tx/rx 9KB: tx/rx
-- ------ --------- --------- --------- ----------- ----------
192.168.10.40 2 1 225 225 2/0 0/0

Switch MTU, Update sent time, Ack receive time, Current time (Time in seconds since AP booted)
1500 225 225 228

There is still a failed probe for the initial GRE packet the AP sends with a length of 1578 (to find out if the network supports jumbo frames or not).

You are correct, but it is certainly not guaranteed.  Ask anyone who has had to deploy over a WAN network.

That's exactly why I've been looking into this