Cisco router with dhcp client could kill MS DHCP server ???

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Jun 27th, 2010
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Hi all,

Last week a router was connected to our LAN. The router belongs to a 'mobile' class solution.

This router has DHCP server configured at its local side and dhcp client toward its LAN side.

The next day we were in big trouble. Not one client on the LAN could connect.

Shortly afterwards it was discovered that the default gateway in all the scopes configured on a MS DHCP server.

The strange thing, to me, was that I got several dhcp-snooping messages (SNMP) from a switch

on which I had configured with dhcp-snooping. Unauthorized server !!

I was surprised because was the message contaned the ip-address from the router but later on the

message contained zero's.

Furthermore strange because the router only has dhcp server configured for its inside.

Has anyone the same problem or is known with this issue.

How can I resolve it if it is a Cisco related problem?



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jennyjohn Mon, 06/28/2010 - 04:04
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What was the IP your clients were getting? Was it from the router DHCP pool?

What was error you got on the switch on with dhcp-snooping was enabled?

JLaay-Diode_2 Mon, 06/28/2010 - 23:40
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The IP-addresses came from the dhcp-pool as configured in the router.

No dhcp-relay configured (see attachment).

The warning on the switch with dhcp-snooping enabled gave the message that

an unauthorized dhcp server was signaled (on a trusted interface, the up-link).

On the MS DHCP server (from the logging I got from the quys over there):

DNS renew request from the, so from the netword address of the subnet !!

Update failes.

Several renew requests and failures.


Greetz Jaap



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