Came across this one while studying for the JNCIP-ENT.

http://www.juniper.net/techpubs/en_US/junos12.1/topics/usage-guidelines/routing-configuring-mtu-discovery-for-bgp-sessions.html

Quick Summary:

• mtu-discovery is disabled by default
• TCP MSS for non-direct peerings: 512-byte
• thus convergence can be improved by enabling mtu-discovery