1. Border Gateway Protocol advertises , learns , and choosed the best paths inside the global Internet.When two ISPs connect , they typically use BGP to exchanges routing information. Enterprises sometimes use BGP to exchange routing information with one or more ISPs, allowing the enterprise routers to learn Internet routes.
2. BGP uses the robust best-path algorithm to choose the best BGP path(route) using rules that extend far beyond just choosing the route with the lowest metric.
3. BGP does establish a neighbor relationship before exchanging topology information with a neighboring router. BGP does not require neighbors to be attached to the same subnet. BGP routes use a TCP connection (port 179) between the routers to pass BGP messages, allowing neighboring routers to be on the same subnet or to be separated by several routers .
4. Instead of choosing the best route just by using an interger metric , BGP uses a more complex process , using a variety of information , called BGP path attributes (PAs), which are exchanged in BGP routing updates much like IGP metric information.
5. The integer BGP ASN uniquely identifies one organization that considers itself autonomous from other organization. Each company whose enterprise network connects to the Internet can be considered to be an autonomous system and can be assigned a BGP ASN. (IANA/ICANN also assigns globally unique ASNs) Additionally , each ISP has an ASN , or possibly several , depending on the size of the ISP.
6. BGP uses the AS_PATH to perform two key functions : Choose the best route for a prefix based on the shortest AS_PATH (fewest number of ASNs listed). ; Prevent routing loops.
7. BGP routers prevent routing loops using the ASNs listed in the AS_PATH , When a BGP router receives an update , and a route advertisement lists an AS_PATH with its own ASN , the router ignores that route . This is because the route has route already been advertised through the local ASN; to believe the route and then advertise it further might cause routing loops.
8. A BGP router behaves differently in several ways depending on whether the peer is an iBGP or eBGP peer . The differences include different rules about what must be true before the two routers can become neighbors , differnet rules about which routes the BGP best-path algorithm chooses as best , and even some different rules about how the routers update the BGP AS_PATH PA.
9. 16-bit ASN Assignment Categories from IANA
0 Reserved
1---64495 Assignable by IANA for public use
64496---64511 Reserved for use in documentation
64512---65534 private use
65535 Reserved