Why is the BGP foreign address port 1027?
I use BGP to connect Router1 and Router2, but when I show the TCP progress:
Router1>show tcp brief
TCB Local Address Foreign Address (state)
4E976890 10.0.0.1.179 10.0.0.2.1027 ESTABLISHED
We say the BGP is based on TCP port 179. Why is Router2's BGP on port 1027?
Best Answer
One side of the connection will have an arbitrary port number, the other will be on 179.
Cisco Press "BGP Fundamentals" has a good explanation (link)
This is just the same as any other TCP connection: the passive open side sits and waits on a well-known port number; the active open side uses an arbitary port. This makes it much easier to manage many-to-many TCP links.