Wireless Access

All-Decade MVP 2020

Anyone doing Jumbo? Any Tricks or Traps?


We're eventually looking to get jumbo frames working between our

225s/275s and the controllers, once we manage to plumb jumbo

across the distro/edge.


As far as I can see by the manual, this is specific to AP communications

tunnels, and would not affect the MTU used by other services on the

controller, e.g. ssh, web, Airwave communications, RADIUS, syslog.  Is this

a correct assessment?  Or if not, is the controller going to properly

PMTUD all these services?


The manual seems to infer that all that is needed is one firewall

setting, and then to set the MTU on port channels/interfaces on the

controller.  When we initially installed the controller we turned all these

bells on, even though we knew the distro layer would swat down

the traffic, and PMTUD seems to have done the trick and kept everything

more or less happy since then, minus of course the HP switches whining

about the DF probe packets..


We do note, however, that even though jumbo is enabled on both

the controller 10G uplinks, and the portchannel to which they belong,

a show interfaces command will show the portchannel at 9216 but the

member ports still have MTU 1500 showing (and do not show the

Jumbo support status line).  We also wonder whether the MTU setting on

the SVI for the vlan serving AP tunnels will need attention.


Interested to hear if anyone has got this working and whether there

were any additional pitfalls or things to look out for.


Trusted Contributor I

Re: Anyone doing Jumbo? Any Tricks or Traps?

Did you get this figured out?  Looking at enabling jumbo frames as well.

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All-Decade MVP 2020

Re: Anyone doing Jumbo? Any Tricks or Traps?


Current status is this:


1) We've been assured by TAC that no settings to the SVI are needed

(or desireable) just the firewall setting and the individual ports/bundles.


2) There are several places to look to verify whether PMTU is succeeding

at the frame size you expect.

   A) "show ap debug system-information" will spit out lots of stuff including

   PMTU values and MTU values for uplinks.

   B) "show ap bss-table" has an mtu column

   C) "show datapath tunnel | include <ap-ip>" will give mtu values for various

       tunnels constructed between the AP and controller, and most of those

       tunnels are AFAICT supposed to negotiate the jumbo MTU.

   D) Show datapath frame has some Jumbo related counters.


3)  We have had mixed results on these indicators.  We've seen baby giant

MTUs on A,B,C for every AP we expect jumbos to reach.  We've seen

a >=9k MTU on A for all those, but only a baby giant MTU on B and C unless

the AP is directly attached to the controller.  It's unclear to us exactly what

D is counting.  We do not know whether these discrepancies are cosmetic or

a real issue.


4) Next step is to play with the D indicator, TAC has requested us to do some

tests which which I hope to get to shortly.


All-Decade MVP 2020

Re: Anyone doing Jumbo? Any Tricks or Traps?



Today we have been able to confirm that:


If the path between an AP and the controller is clean for 9216 frame sizes, tunnels will build and display an MTU of 9000.


If the path between an AP and the controller is clean for frame sizes of 9000, tunnels will build and display an MTU of 1578.


Note that some vendors switches do not support jumbo frame sizes above 9000, so you won't be doing 9000-MTU AP tunnels across a Cisco catalyst 29xx for example.


These latest tests were performed on


We have a ticket in, and hopefully Aruba will address the issue of smaller-than-9216 jumbo paths only using MTU 1578.


All-Decade MVP 2020

Re: Anyone doing Jumbo? Any Tricks or Traps?

And another update: At least for MTU paths of 9000, fixes this.  Whether you can go lower than 9000 we won't be testing.



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