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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Autonomous Systems (AS)

BGP autonomous systems (ASs) are a collection of nodes under common administration with common network routing policies.

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 identifies 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), defined 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 traffic 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.

When BGP operates inside an AS (AS1 or AS2, as seen in the following illustration), it is referred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP operates between ASs (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 1. Interior BGPIllustration of an interior BGP

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 diffuses 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.

Because each BGP router 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. Each BGP router within an AS must have iBGP sessions with all other BGP routers in the AS. For example, a BGP network within an AS needs to be in “full mesh.” As seen in the following illustration, four routers connected in a full mesh have three peers each, six routers have five peers each, and eight routers in full mesh have seven peers each.
Figure 2. BGP Routers in Full Mesh Illustration of BGP routers in full mesh.

The number of BGP speakers each BGP peer must maintain increases exponentially. Network management quickly becomes impossible.

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