I enabled IP routing and then got disconnected from telnet

Answered Question
May 29th, 2007

I was connected to a cat 3750 via telnet. The only connection/port live to that switch is to a 3550 (doing L2).

Then when I did 'ip routing', I lost connection via telnet to the 3750 and now I can't get connected to that switch anymore.

My question is, is there any chance that the 'ip routing' config will cause a loop in the remote network? Again, that switch was configured with a default configuration (other than the vlan1 configured there).

I have this problem too.
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Correct Answer by Richard Burts about 9 years 6 months ago

Marlon

It is almost certainly not an issue about a loop (without more detailed information about the topology and the operating environment I would not be able to say for sure that there is no loop). Sundar is almost certainly right that the issue is that before you enabled ip routing the switch had an ip default-gateway which allowed it to access remote subnets. When you enable ip routing it effectively disables the ip default-gateway and without a default route you lost connectivity. Have someone configure an appropriate default route and your connectivity will be restored.

HTH

Rick

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sundar.palaniappan Tue, 05/29/2007 - 16:28

It's hard to say precisely what's causing the problem unless you are consoled into the switch at the time of the problem. However, it appears the 3750 not having a default route is probably the cause of your telnet session disconnecting. Your telnet session is probably sourced from a different subnet than the IP subnet of the 3750 switch you are using to telnet into it. If you are wondering how did it work before IP routing was enabled a layer 2 switch doesn't use a default route instead it uses the default gateway command to route traffic to remote network(s).

Try adding a default route on the 3750 to point to the layer 3 gateway for that VLAN and enable IP routing after that and test connectivity.

HTH

Sundar

news2010a Tue, 05/29/2007 - 16:36

You are right.

I have no console access today but I will ask someone to do that via console.

There is nothing else connected to the switch(besides the 3550), so in my view that's is not going to cause any routing loop there... I hope.

Correct Answer
Richard Burts Tue, 05/29/2007 - 18:49

Marlon

It is almost certainly not an issue about a loop (without more detailed information about the topology and the operating environment I would not be able to say for sure that there is no loop). Sundar is almost certainly right that the issue is that before you enabled ip routing the switch had an ip default-gateway which allowed it to access remote subnets. When you enable ip routing it effectively disables the ip default-gateway and without a default route you lost connectivity. Have someone configure an appropriate default route and your connectivity will be restored.

HTH

Rick

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