my customer is peering with 2 ISP’s and has configuration with 2 BGP routers mutually connected with iBGP.
routers are reciving full tables from ISP’s and has 4 byte AS (XXXXXX).

recently we noticed that BGP session between customer and ISP2 is constantly flapping each 3 minutes. logs didn’t show anything useful

%BGP-4-MSGDUMP: unsupported or mal-formatted message received from X.X.X.X

I searched cisco forums and tried to isolate problem by inserting hidden command “neighbor X.X.X.X dont-capability-negotiate”.

%BGP-3-NOTIFICATION: received from neighbor X.X.X.X active 2/2 (peer in wrong AS) 2 bytes 5BA0

by reading this article I assumed that ISP is announcing itself with wrong AS.

problem was finally solved by entering command in global configuration mode (and removing dont-capability-negotiate)

ip tcp mss 1440

To learn something more about TCP MSS go here.