Concept Guide
BGP Autonomous Systems
BGP autonomous systems (ASs) are a collection of nodes under common administration with common network routing policies. Devices in
the AS use IGP to communicate with one another. For devices in dierent AS to communicate, they need to use EGP. BGP is the EGP
which allows the devices to communicate.
Each AS has a number, which an internet authority already assigns. You do not assign the BGP number.
AS numbers (ASNs) are important because the ASN uniquely identies each network on the internet. The Internet Assigned Numbers
Authority (IANA) has reserved AS numbers 64512 through 65534 to be used for private purposes. IANA reserves ASNs 0 and 65535 and
must not be used in a live environment.
You can group autonomous systems into three categories (multihomed, stub, and transit), dened by their connections and operation.
• multihomed AS — is one that maintains connections to more than one other AS. This group allows the AS to remain connected to the
Internet in the event of a complete failure of one of their connections. However, this type of AS does not allow trac from one AS to
pass through on its way to another AS. A simple example of this group is seen in the following illustration.
• stub AS — is one that is connected to only one other AS.
• transit AS — is one that provides connections through itself to separate networks. For example, in the following illustration, Router 1
can use Router 2 (the transit AS) to connect to Router 4. Internet service providers (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.
The devices within an AS (AS1 or AS2, as seen in the following illustration) exchange routing information using Internal BGP (IBGP Internal
Border Gateway Protocol), whereas the devices in dierent AS communicate using External BGP (EBGP External 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 17. BGP Topology with autonomous systems (AS)
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 updated information takes as it diuses through the network. Updates traveling
through the network and returning to the same node are easily detected and discarded.
BGP does not use a 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.
Border Gateway Protocol (BGP)
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