DHCP Problem

Unanswered Question
Jan 11th, 2008
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i m using 3550-MLS and 2600-Router.

My router is acting as a DHCP server to the clients which are connected to 3550.

The problem is when any client comes up(which is attached to 3550) it issues a DHCP discover message to DHCP server, DHCP server replies to the message wd the

offer. Now this offer is getting lost when it is coming into the uplink port of 3550 which is connected to 2600 router.

Plz help.

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mchin345 Thu, 01/17/2008 - 09:29
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In some scenarios, the clients connected to the switch do not obtain the IP address because DHCP requests can be duplicated and sourced from multiple VLANs if a loop exists in the network.

Istvan_Rabai Thu, 01/17/2008 - 18:17
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Hi Shubham,

The interface toward the client on your 3550 already has an ip address on it right? Which puts the segment of your host into a specific subnet, let's say /24, or the like.

1. Make sure the ip helper-address on this interface is configured and is pointing to the 2600 router (the DHCP server). (There should be a route in the 3550 switch toward the specific interface of the DHCP server. In your case the DHCP server is a next hop.)

2. On the DHCP server under "ip dhcp pool" config, make sure you configured the subnet for DHCP with the "network" command, e.g "network /24".

3. On the DHCP server (2600 router) there should be a route pointing back to the client's subnet, to in the example. You can put this route into the 2600 router by advertising it from your 3550 MLS, or by configuring a static route.

In this case your DHCP offer will not be lost.

Please acknowledge me if this config works for you.

Use the debug ip dchp ... commands to troubleshoot and see how it works.

Best regards: Istvan

Istvan_Rabai Thu, 01/17/2008 - 22:24
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Hi Shubham,

I forgot to tell you one more thing:

Make sure you have excluded the ip address of the 3550 client facing interface from the DHCP pool. Otherwise the DCHP server may make an offer to the client for the same ip address, which would be overlapping.

This happens frequently when the router interface address is the 1st useable address in the subnet. In this case for example DHCP will offer this ip address as the 1st one. This could also prevent DHCP from working.

Use the "ip dhcp excluded-address" global config command.



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