Question on VLANs and DHCP

Unanswered Question
Aug 22nd, 2008

I have a design question that I need help with. I have a flat network that is in the beginning stages of being split up into VLANs. Imagine two 6500's with networks on either end and both of these networks are in the native VLAN. In between the two 6500's is an etherchannel that is VLAN'd to VLAN100 and EIGRP is setup. This etherchannel between the two 6500's is setup to pass VLAN 100 and VLAN 1. The Native VLAN has a DHCP server running on it and people on both sides and get DHCP addresses.

What I would like to do is take one side of the network and VLAN it to a new VLAN, lets call it VLAN 50, and setup an IP Helper to dish out a new DHCP scope. No problem so far. However I also have an unknown number of devices that have static IP's on that side of the network. So I would like to be able to pass their VLAN 1 traffic and still have all the PC's pick up the new DHCP scope and not the old DHCP scope.

Is there a way to do this without having to identify every device that has a static IP and the switchport it is plugged into? Basically I would like to pass VLAN on the new network but not the DHCP scope from the native VLAN, just the new scope from the new VLAN.

We dont know how many statics are in place, and the idea is to migrate them but operations in this area are 24/7 and business critical so they need to be able to switch over fast. We wanted to keep the statics running and migrate them one at a time. My understanding tells me we are going to have to identify all the statics first and move them all at once. ugh..

Thanks

e-

**the attachment is very basic layout.

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Edison Ortiz Fri, 08/22/2008 - 05:50

When you create a Vlan, you are creating a new broadcast domain, not an IP subnet.

When creating Vlan 50, I'm assuming you've created a new IP subnet and at the DHCP server assigned a new scope.

With that said, you can have devices in the same broadcast domain (Vlan) with different IP address. Those devices can be from the new Vlan 50 IP subnet and also devices from the old Vlan 1 IP subnet, both residing in the same broadcast domain.

The trick is having a gateway for those devices using the Vlan 1 IP subnet directly connected in the Vlan 50 broadcast domain.

You also need to make sure, you know longer have the Vlan 1 IP subnet running on the Vlan 1 broadcast domain. This can create some strange routing problems.

So to recap, you can have Vlan 50 broadcast domain holding Vlan 50 and Vlan 1 IP Subnet during this migration.

HTH,

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Edison.

Eric Hansen Fri, 08/22/2008 - 05:59

Yes I create a new scope for VLAN 50 on my DHCP server, that scope is 10.100.1.0/24 but that DHCP server resides in VLAN1 which is in the IP subnet 10.80.1.0/24

I understand what your saying about the broadcasts domains. So as long as I dont have an ip helpder for 10.80.1.0/24 i shouldnt have to worry about PC's picking up the DHCP VLAN1 scope. I'll put a helper in for VLAN 50's scope 10.100.1.0/24

But the static IP's that are still in VLAN1's IP Subnet but physically in VLAN50's IP subnet, they should still be able to talk across the network to their gateway as long as I pass VLAN1 on the trunk?

I'm a little confused by your statement..

"You also need to make sure, you know longer have the Vlan 1 IP subnet running on the Vlan 1 broadcast domain. This can create some strange routing problems."

can you elaborate on that?

thanks

eric

Edison Ortiz Fri, 08/22/2008 - 07:50

So as long as I dont have an ip helpder for 10.80.1.0/24 i shouldnt have to worry about PC's picking up the DHCP VLAN1 scope. I'll put a helper in for VLAN 50's scope 10.100.1.0/24

The devices will reside in Vlan 50 broadcast domain with static IP address for Vlan 1 IP subnet. They won't use the ip helper-address and if they do, they will get the IP from the Vlan 50 IP Subnet.

But the static IP's that are still in VLAN1's IP Subnet but physically in VLAN50's IP subnet, they should still be able to talk across the network to their gateway as long as I pass VLAN1 on the trunk?

You need to place a gateway in the Vlan 50 broadcast domain with a Vlan 1 IP subnet and this gateway must have reachability to the Vlan 50 IP subnet and the rest of the network.

You can use the current Vlan 50 Layer 3 Vlan and just add a secondary IP address there that belongs to the Vlan 1 IP Subnet.

"You also need to make sure, you know longer have the Vlan 1 IP subnet running on the Vlan 1 broadcast domain. This can create some strange routing problems."

can you elaborate on that?

If the Layer3 switch is planning to hold both subnets, you can't have the same IP subnet allocated to 2 broadcast domains.

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Edison.

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