The short story of the “ip default-network” command

Brian Dennis wrote a long post about the unexpected side effects of the ip default-network command. The Cisco documentation describes the “side effects” but in an even more obscure manner.

What's really happening is this:

  • If the parameter of the ip default-network command is a major network, it specifies the default route (how it gets inserted into the routing protocol you're using is a completely different story).
  • If the parameter is a subnet of a major network, it specifies the default subnet for the network.

In any case, it's an obscure leftover from the classful days that should probably never be used today outside of a CCIE lab.

3 comments:

  1. Could anybody explain what is the reason for such behavior of ip default-network command when subnetwork is specified? I've asked about it in the comment under the orginal article a couple of days ago, but unfortunately my post is still waiting for moderate.

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  2. There are many things in life that are the way they are for no apparent reason :) Default-network is one of them and there are plenty more of them in Cisco IOS :))

    Seriously, it's a leftover from days when we worked with classful IP routing and routing protocols did not carry subnet masks in the routing updates (IGRP, RIPv1, BGPv3). It should not be used any longer.

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  3. Thanks for the explanation. Now I know why I couldn't make myself understood that behavior :)

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.