Hi
I have really random problem as follows:
- Backbone Router Interface set to MTU 9216
- 3850 WAN Switch between the Back Bone Router and ACI Leaf with MTU 1500
- Leaf Configured to Run BGP with the Backbone Router across 3850 WAN switch
BGP Endpoints negotiate TCP MSS Value of 9216 even though Path MTU Discovery is enabled (IP Unreachable enabled on BB Router as well)
Even When I change MTU Value on the Fabric side (Leaf) to 1500 it doesn't seem to have any effect.
My next option is to increase System MTU on 3850 WAN switch but the maximum is 9198 which is 18 bytes short so it may not work anyway unless I change MTU on the BB Router to be less then 9216.
I have a TAC opened for it but didn't find the solution yet. I have to send a lot more routes to the ACI Fabric and at the moment to get around the flapping BGP I only send 1 default route so it only works because there is no requirement to send larger packets with 1 route.
As soon as I stop the filter and send lets say 100 routes the updates are encapsulated in larger MTU and never get into the destination which causes constant BGP reset.
