VLANs and overflow DHCP

[Armegeden]

Hello,

We have numerous VLAN's setup with the ip-helper address to tell them where to request DHCP from.

For certain VLAN's, we've had to put in a secondary network for when the first /27 fills up. The problem is that once the first /27 is filled, any new machine we plug in does NOT get DHCP, as in the VLAN is not using the Secondary range.

interface Vlan100

ip address 10.0.0.33 255.255.255.224 secondary

ip address 10.0.0.1 255.255.255.224

ip helper-address 10.0.0.254

That is an example of a VLAN with a secondary IP range. The first range fills up, but it does not flow into the second.

Am I missing a command? This is my first time doing something like this, so I apologize if it's completely obvious 8(

Or does the problem reside on the DHCP server side?

Any tips/advice greatly appreciated.

[ankurbhasin · Accepted solution]

Hi Armegeden,

I remember ethere was a command "ip dhcp smart-relay" on global config mode which enables the DHCP relay agent to automatically switch to secondary address pools.

Can you try this and update if it works.

Regards,

Ankur

[ankurbhasin]

Hi Friend,

I think it should work because by default the router will forward the DHCP broadcast received on Primary address inserting primary address in the giaddr field of the DHCP packet.

If the DHCP server has a scope or pool configured for the primary network, it will respond; otherwise it will not respond.

Because the ip dhcp smart-relay global configuration command is configured, if the router sends three requests using primary address in the giaddr field, and doesn't get a response, it will move on and start using secondary address in the giaddr field instead.

Without the smart relay functionality, the route only uses primary address in the giaddr field.

Check this link out

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00804412bf.html#wp1090022

HTH

Ankur

[andrew.butterworth]

What DHCP Server are you using? With Microsoft's DHCP Server you can create SuperScopes, these are specifically for use with secondary addresses. Create the individual scopes for the main IP Network and the subsequent secondaries, then group them together as a superscope. Make sure you don't group other scopes in the Superscope though.

I would say though that you should have designed the network to cater for expansion initially. In my opinion secondary addressing is a 'bodge' to get around a problem and should only be used as a temporary fix.

HTH

Andy

[Armegeden]

Ugh, just had a long post typed up and Firefox froze!

Thanx for the help so far, all.

I applied the "ip dhcp smart-relay" to the VTP and everything looks right in the config. The config looks like what I posted before, just "ip dhcp smart-relay" in global.

I tried hooking up a machine to the VLAN and it stalled at "Obtaining IP Address" until it resorted to 169.x.x.x. So I would guess that the actual DHCP server is configured incorrectly.

The way our office is set up, I do not configure the DHCP server (Microsoft 2003 server). I'll get with a Sys Admin and go through it with him, hopefully we'll get it figured out.

Will post back with solution. Again, thanx for the help so far, all!

[Armegeden]

Sorry for the delay, busy at work.

It turns out that the "ip dhcp smart-relay" was the solution. It wasn't working before because the System Admin had put in the wrong scope of IP address for the secondary range, so it was a DHCP issue. The VLAN's secondary IP's are working fine now with the extra global config line.

Thanx all!