Reference Guide

Border Gateway Protocol IPv4 (BGPv4) | 207
A stub AS is one that is connected to only one other AS.
A transit AS is one that provides connections through itself to separate networks. For example as seen in
Figure 10-1, Router 1 can use Router 2 (the transit AS) to connect to Router 4. ISPs are always transit ASs,
because they provide connections from one network to another. The ISP is considered to be “selling transit
service” to the customer network, so thus the term Transit AS.
When BGP operates inside an Autonomous System (AS1 or AS2 as seen in Figure 10-1), it is
referred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP operates
between
Autonomous Systems (AS1 and AS2), it is called External BGP (EBGP Exterior Border
Gateway Protocol).
IBGP provides routers inside the AS with the knowledge to reach routers external to
the AS. EBGP routers exchange information with other EBGP routers as well as IBGP routers to maintain
connectivity and accessibility.
Figure 10-1. BGP Autonomous Zones
BGP version 4 (BGPv4) supports classless interdomain routing and aggregate routes and AS paths. BGP
is a path vector protocol - a computer network in which BGP maintains the path that update
information takes as it diffuses through the network. Updates traveling through the network and
returning to the same node are easily detected and discarded.
BGP does not use traditional Interior Gateway Protocol (IGP) matrix, but makes routing decisions based
on path, network policies and/or rulesets. Unlike most protocols, BGP uses TCP as its transport protocol.
Since each BGP routers talking to another router is a session, a BGP network needs to be in “full mesh”.
This is a topology that has every router directly connected to every other router. For example, as seen in
Figure 10-2, four routers connected in a full mesh have three peers each, six routers have 5 peers each, and
eight routers in full mesh will have seven peers each.
AS 1
AS 2
Router 2
Router 1
Router 3
Router 4
Router 7
Router 6
Router 5
Interior BGP
Exterior BGP
Interior BGP