on one of my router(MPLS Service) the BGP datagram size shows as
Datagrams (max data segment is 516 bytes)
and on another router(ISP Service) it shows
Datagrams (max data segment is 536 bytes):
Can anybody tell me the reason of such difference
The mtu of 516 is most likely due to the fact that you use MD5 authentication on this session. MD5 authentication adds 20 bytes of overhead in the TCP header, hence the MSS being reduced by 20 bytes.
Hope this helps,