Wireless Access

Reply
Highlighted
Contributor I

Campus AP - PMTUD

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?


Accepted Solutions
Highlighted
Contributor I

Re: Campus AP - PMTUD

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).

 

 

 

View solution in original post


All Replies
Highlighted
Guru Elite

Re: Campus AP - PMTUD

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


*Answers and views expressed by me on this forum are my own and not necessarily the position of Aruba Networks or Hewlett Packard Enterprise.*
ArubaOS 8.5 User Guide
InstantOS 8.5 User Guide
Airheads Knowledgebase
Airheads Video Knowledge Base
Remote Access Point Solution Guide
ArubaOS Consolidated Release Notes
ArubaOS 8 ViA VPN Solution Guide
Highlighted
Contributor I

Re: Campus AP - PMTUD

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).

 

 

 

View solution in original post

Highlighted
Guru Elite

Re: Campus AP - PMTUD

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


*Answers and views expressed by me on this forum are my own and not necessarily the position of Aruba Networks or Hewlett Packard Enterprise.*
ArubaOS 8.5 User Guide
InstantOS 8.5 User Guide
Airheads Knowledgebase
Airheads Video Knowledge Base
Remote Access Point Solution Guide
ArubaOS Consolidated Release Notes
ArubaOS 8 ViA VPN Solution Guide
Highlighted
Contributor I

Re: Campus AP - PMTUD

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

Search Airheads
cancel
Showing results for 
Search instead for 
Did you mean: