Diff b/w Layer2/3 vlan

Unanswered Question
May 29th, 2007


got confused,

I configured L2 vlan on 2 catalyst switches, on 1st there are clients on 2nd there is DHCP &DNS

is it necessary to create L3(interface vlan too).

I have this problem too.
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mohammedmahmoud Tue, 05/29/2007 - 11:52


You'll need to do Inter-VLAN routing, and use the ip-helper address command on the VLAN that doesn't have the DHCP server on it, to do inter-VLAN routing you have 2 options, if the switch is layer3 capable configure the VLAN interfaces (SVIs), and if not you'll need an external router to do a router-on-a-stick Inter-VLAN routing.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Richard Burts Tue, 05/29/2007 - 12:01


I believe that we need to understand your situation a bit better to be able to really answer it. I understood your original post that you had created a single VLAN on 2 switches and that one switch connected clients and the other switch connected servers for DHCP and DNS. If there is a single VLAN and if you do not need access to resources outside that VLAN then you do not need any SVIs or anything and the devices within the VLAN will be able to communicate.

If the situation is that there are multiple VLANs then the points that Mohammed makes are correct. Each VLAN needs to be connected to a layer 3 interface (either on a layer 3 switch or on a router) to be able to do inter VLAN routing. And the layer 3 interfaces will need ip helper-address so that the clients on other VLANs can access the DHCP server on its VLAN.



mohammedmahmoud Tue, 05/29/2007 - 12:22

Hi Rick,

You are absolutely right when saying that we need to understand the situation better :), i did jump to the Inter-VLAN according to Sohail's words "I configured L2 vlan on 2 catalyst switches".

Sohail, please never hesitate to ask more questions.


Mohammed Mahmoud.


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