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Craig_05_144607's avatar
Craig_05_144607
Icon for Nimbostratus rankNimbostratus
Feb 18, 2014

Destination Unreachable "reduce MTU size" message not passing back through F5 LTM for MS Exchange traffic

MS Exchange mail traffic from our F5 LTM is reaching a tunnel which is requiring the Exchange Servers to adjust the MTU. An ICMP message "Destination Unreachable" which contains the reduce MTU message is not passing through the F5 to the Exchange Servers. When the F5 is removed from the process the Destination Unreachable message reduce the MTU makes it back to the Exchange servers and they reduce the MTU and traffic flows back correctly. How is this to be accomplished within the F5?

 

application delivery
devops
iRules
LTM

3 Replies

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  • Mui_64918's avatar
    Mui_64918
    Icon for Cirrus rankCirrus
    Feb 18, 2014

    There is a bug with MSS adjust, check the version you are running. In F5 you can work around by changing the MTU for the vlan to the lowest MTU of the TUNNEL or you can selectively set for the VLAN or the path.

     

  • Craig_05_144607's avatar
    Craig_05_144607
    Icon for Nimbostratus rankNimbostratus
    Feb 18, 2014

    Thank you for your reply. I'm new to the F5 LTM. Only mail to a single office is going through this specific tunnel. Would I be required to create a separate Vlan for all exchange traffic to and from this site and connecting to a new VIP with the same pool of exchange servers?

     

  • Mui_64918's avatar
    Mui_64918
    Icon for Cirrus rankCirrus
    Feb 18, 2014

    If its for a specific office and if you have a specific subnet then you can add a route to it and specify an MTU for that route.

     

    tmsh create net route 10.10.22.0/24 gw 30.30.30.158 mtu 1450