When an OSPF router is first activated, it uses OSPF's "hello protocol" to discover any neighbors to which it is connected. It then exchanges link-state information with these routers in the form of LSAs (link-state advertisements). Using this information, each router creates a database that consists of every interface, its corresponding neighbor and a metric representing the speed of that interface. Each router then uses LSAs to pass this information along to all neighboring routers. Every LSA that a router receives from a neighbor is passed along to its other neighbors in turn until every router receives the LSAs of every other router in the network.

It's important to note that a link-state database is distinct from a routing table. From LSA information, each router calculates a path to every destination on the network, building a tree with itself at the root. This comprises its SPF (shortest path first) tree, which forms the basis of the routing table. LSAs are exchanged every 30 minutes, unless there is a change in network topology. If an interface goes down, for example, the information is propagated across the network at once.

If there is a redundant path, the convergence will last as long as it takes to recalculate the SPF tree and update the routing tables for the affected network. This can happen in a few seconds or less, depending on the size of the network. Because of these calculations, routers running OSPF require more CPU resources. This becomes even more critical on an unstable network where interfaces are up and down frequently and a lot of CPU resources are required on the router. The flooding of LSAs may also cause problems on WAN links if they happen too frequently.

One way that OSPF compensates for increased CPU and memory demands is by dividing the network into separate, hierarchical domains, called areas. Routers exchange LSAs only with other routers in their own areas. There is also a backbone area known as Area 0. All areas must be adjacent to Area 0. A border between two areas is defined on an ABR (area border router). ABRs have at least one interface in Area 0 and one interface in another non-backbone area.

The best designed OSPF networks contain contiguous networks to each area, which can be summarized on the backbone through VLSMs (variable-length subnet masks). This makes it possible to describe multiple networks in one routing table entry. The rule of thumb is that there should be about 50 routers per area, but this can vary based on the number of interfaces per router and their stability.

Where RIP is unable to consider the speed of interfaces in determining the best path through the network, OSPF is able to consider a "cost" that is derived by the speed of each interface. However, the formula for determining the cost is not standardized so the default settings may vary.

In all, OSPF is a powerful alternative to RIP, but it makes many demands on router resources and requires more planning. If you're running RIP and it isn't causing you any problems, it may pay to stick with it. But if you want to take advantage of redundant links on your network with a standards-based protocol, OSPF is the way to go.

Enhanced Interior Gateway Routing Protocol

Like OSPF, EIGRP routers discover their neighbors and exchange hello packets. EIGRP sends hello packets every five seconds. If three are missed, the neighboring router is considered dead and alternative routes are used. EIGRP also sends incremental updates regarding topology changes on an as-needed basis. And unlike RIP, it doesn't use bandwidth to advertise regular updates.

EIGRP calls the next router in the path toward a network destination a "successor." It also keeps track of the next-hop routers that can provide loop-free backup routes, which it calls "feasible successors." This information is tracked in a topology table. If a route becomes unavailable, the topology table can be quickly consulted for feasible successors. If it finds one, convergence is instantaneous. If none are found, the router will start querying local neighbors for another route, and update its topology table and router table accordingly.

For instance, when a link state changes on a local router, it will recompute its topology table based on the new information. Where OSPF would immediately flood the change in link state to every router on the network, EIGRP will only involve routers that are directly affected by the change. This uses bandwidth as well as router CPU resources more efficiently. Also, EIGRP won't use more than 50% of the available bandwidth, resulting in big gains on low-bandwidth WAN links. Another advantage of EIGRP is that it supports Novell/IPX and AppleTalk environments. This could mean less training for workers in multiprotocol environments.

Editor's Note: Next week, we'll take routing to the next level with a discussion of Multihoming with BGP.