Tip

Advantages of static routing

Advantages of static routing
Scott M. Ballew

Dynamic routing sounds great. Routers update each other on the routes they have available so packets travel from one to the other automatically using the links available. But wait -- doesn't this take a lot of overhead? This tip, excerpted from

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Managing IP Networks with Cisco Routers, published by O'Reilly and Associates, gives details.

Static routing has some enormous advantages over dynamic routing. Chief among these advantages is predictability. Because the network administrator computes the routing table in advance, the path a packet takes between two destinations is always known precisely, and can be controlled exactly. With dynamic routing, the path taken depends on which devices and links are functioning, and how the routers have interpreted the updates from other routers.

Additionally, because no dynamic routing protocol is needed, static routing doesn't impose any overhead on the routers or the network links. While this overhead may be minimal on an FDDI ring, or even on an Ethernet segment, it could be a significant portion of network bandwidth on a low-speed dial-up link. Consider a network with 200 network segments. Every 30 seconds, as required by the RIP specification, the routers all send an update containing reachability information for all 200 of these segments. With each route taking 16 octets of space, plus a small amount of overhead, the minimum size for an update in this network is over three kilobytes. Each router must therefore send a 3KB update on each of its interfaces every 30 seconds. As you can see, for a large network, the bandwidth devoted to routing updates can add up quickly.

Finally, static routing is easy to configure on a small network. The network administrator simply tells each router how to reach every network segment to which it is not directly attached.

For more information on Managing IP Networks with Cisco Routers, or to buy the book, click here.

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This was first published in March 2001

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