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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
1.42 AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 C613-50032-01 REV D
BGP and BGP4+ Terminology
See the below table for descriptions of BGP and BGP4+ terms used through this chapter:
Term Description
AS number (ASN)
A number that identifies an Autonomous System. Previously, an
ASN was a 16-bit number between 1 and 64,511 (public) and
64,512 and 65,535 (private) assigned to an AS for the purpose of
BGP operation. Since RFC 6793, an ASN is now 32-bit with a
range of decimal values between 1 and 4,294,967,295 .
AS_Path
A BGP path attribute that lists ASNs through which the route has
been advertised.
AS_Sequence
A type of AS_Path segment consisting of an ordered list of ASNs
through which the route has been advertised.
auto summary
A routing protocol feature in which a router that connects to
more than one classful network advertises summarized routes
for each entire classful network when sending updates out
interfaces connected to other classful networks.
Autonomous System
(AS)
A set of routers inside a single administrative authority, grouped
together for the purpose of controlling routing policies for the
routes advertised by that group to the Internet.
BGP best path
algorithm
The process that BGP uses to examine competing BGP paths
(routes) in its BGP table, for a single prefix, choosing one route
as the best route. The best path algorithm results in a choice of a
single route for each prefix as that router’s best BGP path. BGP
compares the current best path against the next path in the list.
BGP peer
A BGP neighbor. A BGP neighbor is a router running BGP that
formed a BGP neighbor relationship to exchange BGP Updates.
BGP peer group
A configuration construct in which multiple neighbors’
parameters can be configured as a group to reduce the length
of the configuration. BGP performs routing policy logic against
one set of Updates for the peer group, improving convergence
time and reducing the CPU load.
BGP synchronization
A feature in which BGP routes cannot be considered to be a best
route to reach a prefix unless that same prefix exists in the
router’s IP routing table as learned via some IGP. When
synchronization is enabled, BGP does not advertise a route
learned via an iBGP peer to external peers, until the route is
learned locally, or via an IGP and is present in the routing table.
When synchronization is disabled, BGP can use and advertise
routes from an iBGP peer to external peers, which are not
present in the routing table.
BGP table
A table inside a router that holds the path attributes known by
BGP on that router.
Border Gateway
Protocol (BGP)
An exterior routing protocol designed to exchange prefix
information between different autonomous systems. The
information includes characteristics called path attributes,
which allow for great flexibility regarding routing choices.