What does the command 'neighbor neighbor IP update-source int type' accomplish?

The command 'neighbor neighbor IP update-source int type' is used to set the source address for BGP (Border Gateway Protocol) neighbor adjacency. This command is particularly crucial when the BGP router needs to specify which specific interface or IP address to use for sending updates to a neighbor.

In scenarios where a BGP peer is not directly connected—such as when establishing connections over VPNs or when BGP routers reside on different networks—using the update-source command allows the router to indicate which local IP address it should utilize for communication with its BGP peer. This is especially important in EGP (Exterior Gateway Protocol) setups where BGP typically relies on directly connected interfaces by default.

By designating a specific source-based interface, this command ensures that packets sent to the neighbor are sourced from the proper IP address, thereby maintaining correct neighbor relationships and facilitating successful BGP routing updates and advertisements.