Peering is when you exchange routes with another BGP speaking device. There are two types of peering sessions:
INTERNAL PEERS (iBGP)
An Internal peer is a BGP speaking neighbor who has the same Autonomous System (AS) number as you do. An internal peer will only pass on the best routes it knows from it's own connections.
EXTERNAL PEERS (eBGP)
External peers have different Autonomous System (AS) numbers. An external peer will pass on all the best routes it knows or has learned from any other peer to all other directly connected external peers. This is what I mean when I say eBGP is a 'gossipy' protocol. Routers speaking eBGP gab everything they know to their neighbors unless you install a gag (a route filter).
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