Reaching the Internet

A fault-tolerant enterprise network will have more than one path to the Internet.

In smaller networks, it's usually best to let the dynamic routing protocol(s) select the closest exit router. If you take this simplification, don't be surprised if perhaps 30 percent of the traffic leaving by one ISP comes back by a different ISP; this is normal Internet behavior -- don't worry about it. Nothing is broken unless you need controlled quality of service (QoS) on the paths. If you need QoS, you will have to negotiate with providers to supply (and monitor) QoS-controlled paths, usually with the MPLS protocol on their links to you.

Simple Internet gateways may need no more than default routes to the ISP. If they need BGP for optimizations, smaller companies should contract for their ISPs to configure this complex protocol. There are nonintuitive restrictions on addressing to make multihoming work.

Moving On