External Border Gateway Protocol (eBGP) is when BGP is run between two different Autonomous Systems
- BGP is a path vector protocol
- It makes routing decisions based on the Best Path Algorithm
- BGP exchanges routing information with a neighbor (peer) router
- BGP uses Autonomous Systems. BGP routers in other ASs are typically under the control of another company. For example, your company has a BGP router that peers with the ISPs BGP router. Your BGP router will be in a different AS than the ISP router. Those routers participate in eBGP.
- If you had two BGP routers in your AS than they would talk to each other via iBGP.
- BGP uses TCP port 179
- BGP assigns the first valid path as the current best path
- BGP then compares the best path with the next path in the list, until it reaches the end of the list of valid paths
- There are 15 rules determining the best path. You don’t need to know these for the CCNA. Just understand that it goes through a process from 1 to 15 to figure out the best path.
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