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Use 'admin distance' to assist in migration

Using the 'admin distance feature can aid in migrations.

Migrating a medium-sized or larger network from one routing protocol to another is one of the trickier projects you're likely to encounter in your IT career. These are quite common as well, as companies are forever doing buyouts and acquisitions and mergers. Companies also have a curious habit of starting small and deploy the simplest thing their administrators can imagine, and then outgrowing the technology. But whatever the reason, the question on everyone's mind is "How do we accomplish this migration without major disruption and with some degree of confidence?"

The easiest way is usually to use the "administrative distance" feature. This is a number between 0 and 255 that represents "believability" or confidence in the routing protocol. By default, each protocol is assigned a number and lower is more preferred.

This feature exists primarily to solve a problem where a router is running two protocols (for instance, EIGRP and RIP) and one protocol has a route that says "to get to network A, send the packet to a next hop of X" and the other protocol has a route that says "to get to network A, send the packet to a next hop of Y". Which way should the router send the packet? The answer depends on the administrative distance. The router will send the packet to whichever protocol is more preferred.

However, what this allows a network administrator to do is to configure a second routing protocol with a high administrative distance, and allow it to run at the same time as the production routing protocol. This way, he can observe the behavior, and verify that all sites are getting both sets of routes. Then, when it comes time to make the big change, simply delete the old protocol's configuration and the new one will take over.

Be careful though as there are several gotchas:

  • You may need to set the distance artificially high on your new protocol. Be sure to check the default setting for all protocols involved.
  • Administrative distance is local to the box. Configuring it on one router does NOT change the behavior of any other routers in the network. If you make a change, make it the same EVERYWHERE,
  • And be doubly careful that you start making the changes at remote sites first, lest you lose connectivity and can't get into remote routers to make the change.

Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.


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