%ACE-3-400001: MSS mismatch from 192.168.100.1:80 (1460) to 193.126.127.28:56380 (1380)

[ajtm]

Hi,

I'm receiving a lot of these messages in a ACE4710 cluster. 192.168.100.1:80 is the VIP, 193.126.127.28:56380 is the client. Already tried to set the mss with this:

parameter-map type connection mymap

  set tcp mss min 0 max 1380

policy-map multi-match L4_policymap

  class vip_PRDWEB_http

    loadbalance vip inservice

    loadbalance policy L7_policymap

    loadbalance vip icmp-reply active

    connection advanced-options mymap

  class vip_PRDWEB_https

    loadbalance vip inservice

    loadbalance policy L7_policymap

    loadbalance vip icmp-reply active

    connection advanced-options mymap

interface vlan 903

  description ### Client Side VLAN ###

  service-policy input L4_policymap

But it doesn't work...

Thanks for you help

[Daniel Arrondo Ostiz]

Hi,

If you want to reduce the MSS on the ACE, what you need to do is add the command "mtu 1420" under the vlan interface. This will lower the MSS advertised by the the ACE to 1380 and should get rid of the mismatch message.

I hope this helps

Daniel

[ajtm]

I've set the mtu on client side vlan and I'm still receiving the messages.

Thanks

Antonio

[Daniel Arrondo Ostiz]

Hi Antonio,

Have you actually confirmed what is the MSS that the ACE is sending back to the client when you try to access the VIP? A traffic capture would be useful for that.

Daniel

[jlamousn · Accepted solution]

What ACE software version are you running?  you may be running into the following defect which may cause this syslog message to be incorrectly generated .  The message indicates that the MSS on the front end is lower, then the mss used on the backend.  That is perfectly ok and no changes on the ace configuration is required, since the ace will simply use the lower mss from the client on the backend. 

This syslog message would be a problem if it indicated that the client was using a higher mss then what is used on the backend.  That scenario should be the only scenario that should trigger this syslog message, since it is actually a problem, but because of the following defect other scenarios can incorrectly trigger this syslog message.

Defect CSCtr12755 Fixed in A4(2.1a) and A4(2.2)
Joel Lamousnery