DHCP helper

Unanswered Question
May 26th, 2008
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Hey all,

I'm facing issues in getting an ip address from a DHCP server in a central site. ip helper-address command is configured on the catalyst switch l3 that am connected to. this is connected to an mpls router goes through mpls cloud to the server. please confirm that we have to ask the MPLS service provider to configure ip helper-address in all their routers till the DHCP network.

Also once we configure the scope in the DHCP server for a particular VLAN network. Will it get the ipaddress from a DHCP server which resides different networks away..

also..;)..would it be just better to configure the l3 switch as a dhcp server..


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Richard Burts Mon, 05/26/2008 - 19:16
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Can you confirm that the ip helper-address is correctly configured with the address of the DHCP server? And can you confirm that there is IP connectivity between your L3 switch to which you connect and the DHCP server (ping to the server from the L3 switch is the easy way to demonstrate this)? And can you confirm that the DHCP server has a correct scope configured for the subnet on the interface of the L3 switch to which you connect?

In my experience these are the 3 most common reasons why end stations fail to receive addresses from DHCP servers. If you confirm that all 3 of these are correct then we will look for other explanations and solutions to the problem.

I would not expect that your provider would need to config ip helper-address on their MPLS routers since your L3 switch should just generate a unicast packet to the server address. The MPLS routers just need to forward it. You might want to check with the provider and verify that their routers would forward this traffic (check to make sure that they do not have access lists or other filtering that would deny it).



zanub Mon, 05/26/2008 - 20:00
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All the initial 3 questions you asked has been confirmed already. Correct DHCP server ip address.I'm able to ping the DHCP server. The correct VLAN network scope has been configured in the DHCP server and activated.

I'll will check with the provider whether anything is blocked.I guess maybe DHCP ports mite be blocked?



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