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Dell Configuration Guide for the S4048–ON System 9.14.2.5

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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 different 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 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.

The devices within an AS (AS1 or AS2, as seen in the following illustration) exchange routing information using Internal BGP (IBGP), whereas the devices in different AS communicate using External BGP (EBGP). 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. BGP Topology with autonomous systems (AS)
BGP Topology with autonomous systems (AS)

BGP version 4 (BGPv4) supports classless interdomain routing (CIDR) 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.

Since each BGP router talking to another router is a session, an IBGP 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 illustration below, 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 IBGP 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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