Border Gateway Protocol (BGP)
The Border Gateway Protocol (BGP) is a path vector exterior gateway routing protocol used principally by ISPs to establish routing between autonomous systems.
- designed to be used between routing domains in a mesh internetwork
- as such, is used as the routing protocol on the Internet
- primarily between ISPs
- EIGRP and OSPF are used for communications between routers within a single routing domain (autonomous system)
- BGP is primarily used for routing between autonomous systems
- an AS is designed to hide the complexity of private networks from the public Internet
- if all Internet locations had to be propagated to all Internet routers
- routing tables would be too large to process
- if all Internet locations had to be propagated to all Internet routers
- edge routers for each AS exchange only as much network-reachability information as is required to access other autonomous systems (the AS path)
- rather than networks and hosts within each AS
- prioritizes stability and can be slow to converge
- Autonomous system numbers (ASN) are allocated to ISPs by IANA via the various regional registries
- works with classless network prefixes called Network Layer Reachability Information (NLRI)
- path selection is based on multiple metrics
- hop count
- weight
- local preference
- origin
- community
- works over TCP on port 179