Internet Routing Architectures, Second Edition

Overlapping Protocols: Backdoors

With different IGPs and EGPs working together to achieve routing, routes can be learned via different protocols; choosing one protocol over another affects how the traffic flows. For example, if traffic follows a RIP route, it might traverse one link, whereas if it follows an external BGP route, it might end up on another link. Backdoor links offer an alternative IGP path that can be used instead of the external BGP path. IGP routes that can be reached over the backdoor link are called backdoor routes. With the existence of such alternative routes, a mechanism that gives one protocol preference over other protocols is needed. Cisco Systems offers a preference parameter called the administrative distance of a ...