All transit autonomous systems are required to carry traffic originating from or destined for locations outside of that AS. For the transit AS to meet this requirement, there has to be interaction and coordination between BGP and the IGP that this particular AS uses. Such a configuration requires special care to ensure consistency of routing information throughout the AS. The topology of the Internet can be viewed as a series of connections among stub networks, multihomed networks, and transit autonomous systems. A transit AS is a multihomed AS that contains more than one connection to the outside world and that allows traffic not originating in that AS to travel through it. This course introduces the concept of the multihomed transit AS and how BGP exchanges routing information inside the AS and between neighboring autonomous systems. It also explains the requirement for IBGP within the multihomed transit AS.

Configuring a BGP network in a transit services configuration requires special care to ensure consistency of routing information throughout the AS. Understanding the interaction between EBGP and IBGP is crucial to successfully configuring and troubleshooting the transit AS.

A transit AS requires interaction between EBGP and IBGP and between IBGP and an IGP in the transit AS. This course describes packet forwarding through a transit AS and discusses the requirements for successful packet forwarding, such as recursive route lookup and an IGP in the transit AS. This course concludes with a discussion of the interaction between IBGP and an IGP running within the transit AS.

Introducing a transit backbone into a BGP network can create unique troubleshooting challenges. This course introduces IBGP monitoring commands and troubleshooting techniques for solving the most common IBGP problems that you might encounter in a transit backbone. Common problems with IBGP, as discussed in this course, occur when IBGP sessions do not reach the established state, when routing information that is received via IBGP is never selected and when the best BGP route is never installed in the routing table.