VLAN DHCP Pools

Unanswered Question
Feb 4th, 2009

The 1800 series router is able to manage multiple DHCP pools on VLAN through it's intergrated 8 port switch. Am I able to connect a layer 3 24 port managed switch to the router to obtain IP address for the VLAN's from the specific DHCP pools?

I can't see why this wouldn't work I just want to be sure before I implement this as a solution.

I have this problem too.
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Giuseppe Larosa Wed, 02/04/2009 - 06:25

Hello Mike,

once you have a L2 trunk carrying all the user Vlans working between C1800 and the catalyst you should be fine.

You need to define the vlans on both devices (not sure VTP work on C1800 etherswitch).

Hope to help

Giuseppe

Yudong Wu Wed, 02/04/2009 - 10:26

The other way you can do is:

1. have a layer 3 link between 1800 and layer 3 switch.

2. configure vlan interfaces for all vlans with ip address on layer 3 switch. and add "ip helper-address x.x.x.x" under each vlan interfaces.

3. configure related DHCP pool for each vlan on 1800 router.

DHCP request from the client will be relayed to 1800 router by its vlan interface. IP address of vlan interface will be included in DHCP request so that DHCP server on 1800 will know which IP pool should be used. HTH.

ClearWaveIT Thu, 02/05/2009 - 06:10

Thanks kwu2

We are looking to implement this into a business centre with 20+ rooms. The way i was looking at setting this up was to have 20+ dhcp pools. Each connection on the switch will correspond to a dhcp pool.

Now so that i'm sure, Can I set the vlan's to obtain the ip's from the dhcp pools on the router, or do i have to have identical vlan's on the router also?

I pretty sure from what you have told me is what all I needed to know but I wanted to be clear before I spend money on this equipment.

Yudong Wu Thu, 02/05/2009 - 07:45

It is depend on how you would like to implement it.

If you would like to use 1800 router as default gateway for all vlans, you need to configure a layer 2 trunk between the switch and 1800 router and configure subinterface (it's IP will be default gateway IP) for each vlan on 1800 router. The switch will be configured as pure layer 2 switch. This is what was suggested by giuslar.

Since you have a layer 3 switch, you can also use the switch as default gateway for each vlan. You need to create vlan interface for each vlan, ip address of vlan interface will be the default gateway IP for its vlan. "ip helper-address" need to be added under each vlan interface to point to dhcp server which is your 1800 router. (By the way, Cisco layer 3 switch can also work as a DHCP server). You just need a layer 3 connection between 1800 router and swtich. You don't need to configure subinterface for each vlan on the router by this way. HTH.

ClearWaveIT Wed, 02/11/2009 - 03:57

Would you be able to compile the config scripts we need for this setup? How much would it cost?

Yudong Wu Wed, 02/11/2009 - 07:35

Sorry. You can find a local Cisco partner to help you on this.

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