Corporate.Net internetRx By Chris Lewis Q: Several divisions in my company have deployed their own intranets, which we now need to interconnect. None of the groups wants to relinquish administrative control of its network to another group, yet we need the networks to communicate in a reliable and controlled fashion. I have heard that this problem exists on the Internet and was solved by using exterior gateway protocols to connect the different ISP networks. Is this a reasonable solution? What are our options? A: At the top level, there are two types of routing protocols: Interior and exterior gateway protocols. Interior gateway protocols, such as the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF), share complete routing information between network devices within the same Autonomous System (AS ), which is a collection of routers under a common administration. An exterior gateway protocol shares only minimal routing information between networks in separate ASes. In this situation, each division's intranet would be considered a separate AS. The methods to consider for exterior gateway routing include the Exterior Gateway Protocol (EGP), the Border Gateway Protocol (BGP) and static routing. In the beginning, there was EGP. It has a number of failings, however, not the least of which is the absence of metric information to choose an optimal route. If there is more than one route between ASes, packets can get caught in routing loops and never be delivered. For example, in the diagram "Route Confusion" (above), routers R1 and R2 in AS1 each must route traffic to AS2. Because of the lack of a metric, R1 may decide that the route it will use to networks within AS2 is via R2, while R2 may decide to get to networks within AS2 via R1. Packets destined for networks in AS2 would bounce continuously between R1 and R2. BGP was introduced to improve EG P. Its main feature is its reliable transport protocol, which ensures that route updates are received. BGP also implements a "keep-alive" mechanism, which alerts BGP routers if neighboring routers fail. BGP does not transmit metrics with its route updates. It does transmit a path for each AS that lists the ASes it will visit on the way to the destination AS. You can control the selection of these paths by manually assigning administrative weights, thereby directing traffic over higher bandwidth or infrequently used links. BGP avoids the circulating-packet problem of EGP. An experienced network administrator will usually seek to minimize any manual configuration. In the case of exterior routing, this approach may differ, and my preferred method would be static routing, which requires manual configuration. It of fers a number of advantages over exterior gateway protocols when routing between ASes. These advantages include complete flexibility over the advertisement of subnets and their next-hop routers, and the fact that routing protocol traffic does not travel over the link connecting the ASes. The downsides are obvious. Static routes do not adapt to link failures, and manual configuration can be a headache to maintain. EGP is out of vogue these days. If you have more than six ASes to interconnect, BGP is worth considering. However, for most private intranets, static routing is the best solution for connecting different ASes.

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