Introduction to BGP (1)

1. Border Gateway Protocol advertises, learns, and choosed the best paths inside the global internet.when both ISPs Conne CT, they typically use BGP to exchanges routing information. Enterprises sometimes use BGP to exchange routing information and one or more ISPs, allowing the enterprise routers to Le ARN Internet routes.

2. BGP uses the robust Best-path algorithm to choose the best BGP Path (route) using rules which extend far beyond just Choo Sing 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 being attached to the same subnet. BGP routes use a TCP connection (port 179) between the routers to pass BGP messages, allowing neighboring routers The same subnet or to is separated by several routers.

4. Instead of choosing the best route just by using a interger metric, BGP uses a more complex process, using a variety of information, called BGP path Attributes (PAs), which is exchanged in BGP routing updates much like IGP metric inform ation.

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 is considered to be a autonomous system and can be igned a BGP ASN. (Iana/icann also assigns globally unique Asns) Additionally, each ISP have an ASN, or possibly several, depending on the size of the ISP.

6. BGP uses the As_path to perform, key functions:choose the best route for a prefix based on the shortest as_path (f Ewest number of Asns listed). ; Prevent routing loops.

7. BGP routers prevent routing loops using the Asns listed in the As_path if a BGP router receives an update, and a R Oute advertisement lists an as_path with its own ASN, the router ignores that route. This was 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 was an IBGP or EBGP peer. The differences include different rules about what must is true before the the routers can become neighbors, differnet ru Les about which routes the BGP Best-path algorithm chooses as best, and even some different rules on how the routers U Pdate the BGP as_path PA.

9.16-bit ASN assignment Categories from IANA

0 Reserved

1---64495 assignable by IANA

64496---64511 Reserved for use in documentation

64512---65534 private use

65535 Reserved

Introduction to BGP (1)