BGP and BGP4+ Introduction
BGP does not send any more route updates than it absolutely has to. When routers first peer up, they exchange the route table data that they wish to inform each other of. Thereafter, they only send each other route information if anything changes.
BGP and BGP4+ ASNs and AS_Path Attributes
BGP and BGP4+ uses BGP path attributes (PAs). PAs define information about a path, or route, through a network. Some BGP and BGP4+ PAs describe information that is used to choose the best BGP or BGP4+ route. PAs are also used for other purposes, such as preventing routing loops.
If no BGP or BGP4+ PAs have been explicitly set, BGP and BGP4+ routers use the BGP and BGP4+ AS_Path (autonomous system path) PA when choosing the best route among competing routes. The AS_Path PA itself has many subcomponents, one of which is the autonomous system number (ASN)
The integer ASN identifies one organization that considers itself autonomous from other organizations. Each company with a network that connects to the Internet can be considered to be an autonomous system and can be assigned an ASN. (IANA assigns unique ASNs.) Each ISP has an ASN. Some large ISPs have multiple ASNs.
When a router uses BGP or BGP4+ to advertise a route, the prefix/ length is associated with
aset of PAs, including the AS_Path. The AS_Path PA associated with a prefix/ length lists the ASNs that would be part of an
BGP and BGP4+ use the AS_Path for two key functions:
■Choose the best route for a prefix based on the shortest AS_Path.
■Prevent routing loops.
| Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches |
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1.4 | AlliedWare PlusTM Operating System - Software Version |
