BGP protocol operation relies on the exchange of BGP messages between peers. 7450 ESS, 7750 SR, 7950 XRS, and most other routers, support the following message types: Open message, Update message, Keepalive message, Notification message, and Route refresh message.
The minimum BGP message length is 19 bytes and the maximum is 4096 bytes. BGP messages appear as a stream of bytes to the underlying TCP transport layer, and so there is no direct association between a BGP message and a TCP segment. One TCP segment can include parts of one or more BGP messages. Immediately after session setup, the initial value for the maximum TCP segment size that can be sent toward a specific peer is the minimum of the following:
the MSS option value in the TCP SYN received from the peer, when the connection was established
the IP MTU of the initial outgoing interface used to route packets to the peer, minus 40 bytes (IPv4) or minus 60 bytes (IPv6)
the TCP MSS configuration value in the BGP configuration of the peer (if there is such a configuration and it specifies a value other than ip-stack)
As time elapses, the maximum sending segment size can fall below the initial value if path MTU discovery (PMTUD) is active on the session. PMTUD lowers the segment size when ICMP unreachable or packet-too-big messages are received. These messages indicate that the IP MTU of the link could not forward the unfragmentable packet and this IP MTU minus 40 (IPv4) or minus 60 (IPv6) bytes sets the new maximum segment size value.