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New Member

Redundant DHCP across subnets...

I've been tasked with recommending a redundant DHCP solution in our corporate building. Here's the layout:

16 floors, each with a floor server doing DHCP and F&P for the PC's on that floor. Each floor is it's own subnet (ie. 172.16.6.x, indicates the 6th floor and x is the host).

Anyway, each floor has one or more hubs and a Cisco 2924 switch. All floor switches are connected to a Cisco 6509 in the data center on floor 13 (172.16.13.x)

To test, I created a backup DHCP scope on our domain PDC which is also in the data center on floor 13. Then I disabled the scope on floor 6 and tried to release and renew from a laptop plugged into the 2924 on floor 6. NOTHING! :-(

Any ideas on what I need to make this work? The 2924 is a layer 2 device so it should forward any broadcasts, but what about the 6509? Is there special config I need to do on that switch since it's a multilayer switch?

Thanks!

Dave

4 REPLIES
New Member

Re: Redundant DHCP across subnets...

I have not worked with the 6509, but I would dare to say you would have to configure a DHCP relay. The reason I think this is that even though it is a layer 3 switch it would still control the broadcast. Otherwise it would be a very noisy network. There must be in the existing configuration a DHCP helper (relay) set up.

New Member

Re: Redundant DHCP across subnets...

Where are your floor 6 clients getting DHCP from now? Is it a device on the local broadcast domain? For a backup, throw an ip helper-address on the interface closest to the client, at the router that gets you to the backup dhcp server. feel free to email me if i can help.

shaun.skillin@inetexperts.net

New Member

Re: Redundant DHCP across subnets...

Hi Shaun:

Each floor has it's own server which does DHCP and F&P for just that floor. If the floor server is down, we don't want the clients to be unable to work if they don't get an IP address.

I guess the ip helper would have to go on the 6509 switch, since the 2900 series switches (on the floors) are only layer 2 switches.

Dave

New Member

Re: Redundant DHCP across subnets...

As skiilin reccommends that the ip-helper address needs to be configured on the appropriate interface to direct your broadcast to a unicast to the backup dhcp server. this will sort it out.

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