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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
1.4 AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 C613-50032-01 REV D
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
a set of PAs, including the AS_Path. The AS_Path PA associated with a prefix/ length lists
the ASNs that would be part of an end-to-end route for that prefix as learned using BGP or
BGP4+.
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.