Cisco Support Community
Community Member

dhcp problem on wan interface of 871 router

I recently purchased a cisco 871 for use at home with a cox business line.

For initial setup/testing I had connected it to a linksys router but it would not receive an ip address. After beating my head against the wall for an hour, I read somewhere that dhcp will not work with a non-routable ip address on the wan link.

So I went ahead and plugged into my cable modem, and was able to pull an ip address from cox.

The problem is that after a random period of time it would stop routing traffic out the wan interface. All routes still showed up as they should, and I did not touch the config. A release/renew dhcp would fix the problem for another random amount of time (would pull same address).

I then brought it into work with me, and it will not pull a routable ip address from our campus dhcp server. debug dhcp revealed the following error which I was initially getting when hooked into the linksys:

"unknown dhcp problem.. no allocation possible"

if I assign a static ip address on wan interface, everything works fine.

i have also tried this with a completely clean config file (other than ip address dhcp entry on wan interface) with the same results.

linksys router worked with no problems.

any advice would be greatly appreciated.

Cisco Employee

Re: dhcp problem on wan interface of 871 router

Please paste the config of the router.

Are you using a fast ethernet port as a WAN port. if yes, then have you tried using the command " ip address dhcp " under the interface to recieve an ip from the dhcp server.


-amit singh

Community Member

Re: dhcp problem on wan interface of 871 router

yes, and yes.

from a pc connected to my 871, i initiated a ping -t to the gateway router (cable modem). everything works fine initially. but then after a random period of time (anywhere from 5-10 minutes that I observed) the pings begin to fail.

if i then go to my 871 router and issue release dhcp and renew dhcp commands, the pings are then again successful for another random period of time.

Community Member

Re: dhcp problem on wan interface of 871 router

here is config.

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