Same Vlan Defined

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Sep 14th, 2007
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Can the Same VLAN be defined and functional on 2 different Switch with out causing a ISSUE. ie. I have a vlan 100 up and running I want to create the same vlan on a differnet switch its already in the core and up and running. will doing this cause any issues on my network. Want to be sure.

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glen.grant Fri, 09/14/2007 - 09:40
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I wouldn't say that is a good practice as your evironment will just become confusing in a troubleshooting scenario . With the amount of different vlan numbers that you can use there is really no reason to duplicate vlans anywhere in your network . If your network is of any size you should keep track of what vlans are being used and not duplicate them elsewhere .

janet.maxwell Fri, 09/14/2007 - 10:01
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I totally Understand. This is inpreparation of moving from a 3750 switch to a 6504E. We will have both functioning. for a brief period t then the 3750 will be taken out of production with the vlan functional on. So we have to the same vlan on the New switch. I am concerned about possible spanning tree issues; or even possible causing a outage. This will only be demporary if i know there will not be a outage. SO I can create the same vlan w/the same vlan ip address w/o a issue

glen.grant Fri, 09/14/2007 - 10:33
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Not knowing how your layout is , it would be hard to say . I would look at your network and decide if your spanning tree root is where you want it to be . Obviously you can't use the same ip address on 2 different switches that are connected but they can be in the same subnet range .

cratejockey Mon, 09/17/2007 - 08:37
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If I understand what you saying here, you are trying to migrate from a 3560 Core to a 6506 Core and your want to have them both running at once so you dont have a network outage for the cutover. If this is what you are shooting for you could bring them up as HSRP peers. This would alow you to failover the VLAN Gateway to the new switch when it comes time for your cutover. Not sure if this is exatly what your looking for but I hope it helps.

jrensink78 Thu, 09/20/2007 - 05:43
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Are you using VTP currently? If so, why not just add this switch to the VTP domain as a client? Then it will have all of the VLANs defined by the VTP server.

There shouldn't be issues in having multiple switches with the same VLAN. They are already all using VLAN 1. If you want to allow devices on VLAN 100 to communicate between the switches without getting routing into the picture, just make sure you set up a compatible trunk between the 2 switches.

If you don't setup a trunk between the switches, then the 2 VLANs will be isolated from each other from a layer 2 perspective.


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