ip default-network

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May 31st, 2008
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Hi,


In the IOS version that I use on my 2621 routers, when I use the ip default-network command and flag one of the major classful networks that I have as a candidate default. It propagtes a default route to its neighbors in the same routing domain leaving its own gateway of last resort as 'not set'. Neighbors have their gateway of last resort set to be 0.0.0.0 via <next-hop> when using RIP.


If instead of specifying only a major network number, I specify a subnet and major network number in two separate ip default-network commands - strange things happen.


1) The gateway of last resort of the local router changes to the major network via subnet specified in ip default-network.

2) A static route that can only be removed using no ip default-network <subnet> appears in the config.

3) It no longer propagates the default route - which I thought was the point of this command.


Can anyone explain the purpose of providing a subnet for the ip default-network command? I don't get it.


R

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russell.witteri... Sun, 06/01/2008 - 02:21
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Thanks for the link Frank. It helped.


As far as I can understand through reading this document and my own experimentation, when you provide a subnet of a major classful network to the ip default-network command and then flag the major network as well.


router(config)#ip default-network 3.3.3.0

router(config)#ip default-network 3.0.0.0


It changes the gateway of last resort locally, but doesn't propagate it to other routers.


It looks like an alternative way of setting the local gateway of last resort to be a specific network other than 0.0.0.0. It doesn't say in the document, if it's supposed to propagate itself as the default route or not? My testing indicates that it doesn't.


I've also determined that static routes only get propagated with RIP when you specify an exit interface and flag the major network using the classful network number.


router(config)#ip route 3.3.3.0 255.255.255.0 fa 0/0

router(config)#ip default-network 3.0.0.0


If I use the next hop address in the static route it doesn't get propagated.


R

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