10.2 Dynamic Routing

Dynamic routing occurs when routers talk to adjacent routers, informing each other of what networks each router is currently connected to. The routers must communicate using a routing protocol, of which there are many to choose from. The process on the router that is running the routing protocol, communicating with its neighbor routers, is usually called a routing daemon. As shown in Figure 9.1, the routing daemon updates the kernel's routing table with information it receives from neighbor routers.

The use of dynamic routing does not change the way the kernel performs routing at the IP layer, as we described in Section 9.2. We called this the routing mechanism. The kernel still searches its routing table in the same way, looking for host routes, network routes, and default routes. What changes is the information placed into the routing table ”instead of coming from route commands in bootstrap files, the routes are added and deleted dynamically by a routing daemon, as routes change over time.

As we mentioned earlier, the routing daemon adds a routing policy to the system, choosing which routes to place into the kernel's routing table. If the daemon finds multiple routes to a destination, the daemon chooses (somehow) which route is best, and which one to insert into the kernel's table. If the daemon finds that a link has gone down (perhaps a router crashed or a phone line is out of order), it can delete the affected routes or add alternate routes that bypass the problem.

In a system such as the Internet, many different routing protocols are currently used. The Internet is organized into a collection of autonomous systems (ASs), each of which is normally administered by a single entity. A corporation or university campus often defines an autonomous system. The NSFNET backbone of the Internet forms an autonomous system, because all the routers in the backbone are under a single administrative control.

Each autonomous system can select its own routing protocol to communicate between the routers in that autonomous system. This is called an interior gateway protocol (IGP) or intradomain routing protocol. The most popular IGP has been the Routing Information Protocol (RIP). A newer IGP is the Open Shortest Path First protocol (OSPF). It is intended as a replacement for RIP. An older IGP that has fallen out of use is HELLO ” the IGP used on the original NSFNET backbone in 1986.

The new Router Requirements RFC [Almquist 1993] states that a router that implements any dynamic routing protocol must support both OSPF and RIP, and may support other IGPs.

Separate routing protocols called exterior gateway protocols (EGPs) or interdomain routing protocols are used between the routers in different autonomous systems. Historically (and confusingly) the predominant EGP has been a protocol of the same name : EGP. A newer EGP is the Border Gateway Protocol (BGP) that is currently used between the NSFNET backbone and some of the regional networks that attach to the backbone. BGP is intended to replace EGP.