Understanding Bgp
Short summary
How BGP works is it will create a tree of possible path
The path to the destination (in this case AS1 — 207.126.0.0/16) is called AS_PATH. In this case we have 2 AS_PATH:
- AS4, AS2, AS1
- AS6, AS5, AS3, AS1
Which leads to AS1. BGP will choose the shortest path which is AS4, AS2, and AS1
Loop detection
AS (Autonomous System) detects a loop by checking if itself is reported in the AS_PATH. Taken this example:
We can see that AS7 will reject the path that has itself in it.
Understanding show ip bgp command
The Network column is the destination, each * is the same destination.
For example in here to reach to 3.0.0.0 we have multiple next hop from 217.75.96.60 to 193.0.0.56
The Path column shows which AS it goes through. We can see that in the end it all goes to AS 80 which indicate our destination is at AS 80.
The >* is the shortest path that the algorithm choose.
When 2 neighbors first establish a BGP peer connection, they exchange their entire BGP routing table at first. After that they will exchange incremental updates.