Simple definition
BGP peering establishes a session between routers to exchange network reachability information and routing attributes.
Purpose and use
BGP peering lets two routers exchange prefixes and routing attributes, either within one autonomous system or between different systems.
How does it work?
BGP runs over TCP, normally using destination port 179. Routers negotiate capabilities before exchanging accepted advertisements.
Example and verification
If a session is established but useful routes are absent, check address families, filters and the prefixes actually advertised.
Points to watch
An Established session does not prove end-to-end forwarding. Insufficient filtering can also propagate unwanted advertisements.
In short
Peering establishes a route exchange; import, export and forwarding policies determine its actual usefulness.