Multihomed Nontransit AS

An AS is multihomed if it has more than one exit point to the outside world. An AS can be multihomed to a single provider or multiple providers. A nontransit AS does not allow transit traffic to go through it. Transit traffic is any traffic that has a source and destination outside the AS. Figure 4-5 illustrates an AS (AS1) that is nontransit and multihomed to two providers, ISP1 and ISP2.

Figure 4-5  Multihomed nontransit AS example.

A nontransit AS would only advertise its own routes and would not advertise routes that it learned from other ASs. This ensures that traffic for any destination that does not belong to the AS would not be directed to the AS. In figure 4-5, AS1 is learning about routes n3 and n4 via ISP1 and routes n5 and n6 via ISP2. AS1 is only advertising its local routes (n1,n2) and is not passing to ISP2 routes it learned from ISP1 or to IPS1 routes it learned from ISP2. This way, AS1 will not open itself to outside traffic, such as ISP1 trying to reach n5 or n6 and ISP2 trying to reach n3 and n4 via AS1. Of course, ISP1 or ISP2 can force their traffic to be directed to AS1 via default or static routing. As a precaution against this, AS1 could filter any traffic coming toward it with a destination not belonging to AS1.

Multihomed nontransit ASs do not really need to run BGP4 with their providers, although it is recommended and most of the time required by the provider. As you will see later on in this book, running BGP4 with the providers has many advantages in controlling route propagation and filtering.

Multihomed Transit AS

A multihomed transit AS has more than one connection to the outside world and can still be used for transit traffic by other ASs (see figure 4-6). Transit traffic (relative to the multihomed AS) is any traffic with an origin and destination that does not belong to the AS.

Figure 4-6  Multihomed transit AS using BGP internally and externally.

Even though BGP4 is an exterior gateway protocol, it can still be used inside an AS as a pipe to exchange BGP updates. BGP connections inside an autonomous system are called Internal BGP (IBGP), whereas BGP connections between autonomous systems are called External BGP (EBGP). Routers that are running IBGP are called transit routers when they carry the transit traffic going through the AS. Routers that run EBGP with other ASs are usually called border routers.

A transit AS would advertise to one AS routes that it learned from another AS. This way, the transit AS would open itself to traffic that does not belong to it. Multihomed transit ASs are advised to use BGP4 for their connections to other ASs and also internally to shield their internal nontransit routers from Internet routes. Not all routers inside a domain need to run BGP; internal nontransit routers could run default routing to the BGP routers, which alleviates the number of routes the internal nontransit routers must carry.

Figure 4-6 illustrates a multihomed transit autonomous system, AS1, connected to two different providers, ISP1 and ISP2. AS1 is learning routes n3, n4, n5, and n6 from both ISP1 and ISP2 and in turn advertising all that it learned, including its local routes, to ISP1 and ISP2. In this case, ISP1 could use AS1 as a transit AS to reach networks n5 and n6, and ISP2 could use AS1 to reach networks n3 and n4.

Border Gateway Protocol Version 4

BGP went through different phases and improvements from its earlier version, BGP1, in 1989 to today's version, BGP4, deployment of which started in 1993. BGP4 is the first version that handles aggregation (CIDR) and supernetting, as discussed earlier in this book.

BGP imposes no restrictions on the underlying Internet topology. It assumes that routing within an autonomous system is done via an intra-autonomous system routing protocol. (For the purposes of this book, intra means routing within an entity, and inter means between entities.) BGP constructs a graph of autonomous systems based on the information exchanged between BGP neighbors. This directed graph environment is sometimes referred to as a tree. As far as BGP is concerned, the whole Internet is a graph of ASs, with each AS identified by an AS number. Connections between two ASs together form a path, and the collection of path information forms a route to reach a specific destination. BGP ensures that loop-free interdomain routing is maintained. Figure 4-7 illustrates this general path tree concept.

Figure 4-7  Example AS_Path tree.