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DHCP relay

Unanswered Question
Oct 10th, 2008
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We have two different subnets separated by a router. Trouble is company has an old router, 2620 doesn't support VLAN's, so I just implemented two different subnet ranges, and secondary address on our one horse one ethernet port router, lol. Now clients are getting IP's from the wrong DHCP server. Origianlly I thought it might be DHCP relay enabled on the router, turned it off. But if a client doesnt have an IP address, the servers or the router wouldn't know to which subnet it belongs. Is there a way out without upgrading router to support vlans? I know assigning vlans to ports would eliminate this problem.

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John Blakley Fri, 10/10/2008 - 09:33
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Does your 2620 support subinterfaces? can you post a config?

attrib7575 Fri, 10/17/2008 - 12:00
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I'm pretty sure it supports subinterfaces, but I thought that was only for Vlans and frame relay situations. Here's the config:


attrib7575 Fri, 10/17/2008 - 12:02
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Come to think of it, in the config, it looks like someone has already set up two subinterfaces. Must have been someone before me. They don't have IP addresses though.

John Blakley Fri, 10/17/2008 - 12:05
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So I take it you put the secondary address on to support the other network? That would work, but I believe your DHCP requests will always answer on the 172 network. You really, if you can, need to create subinterfaces and use vlans to separate these. You then can create ACL's on the router to restrict traffic between the networks if you don't want them to see each other.

I may be way off regarding what you're trying to do though. :-)


attrib7575 Tue, 11/04/2008 - 14:34
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Yes, I put the secondary address on to route internet traffic from the second subnet to the internet. I agree VLans would be the way to go, but the router is old and doesn't support 802.1Q. What determines which dhcp service will answer which request?



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