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Apr 8th, 2006
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I need to bridge the both building with the same IP subnet but different VLAN id. (within a cloud).

Can that be achieved??

Building A - VLAN 1 (

Building B - VLAN 5 (


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Roberto Salazar Sat, 04/08/2006 - 08:02
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Yeap, put the switch's port in building A that connects to switch in Building B in vlan 1 and put the switch port in bldg. B that connects to Bldg. A in vlan 5. for example.

Switch bldg. A 1/1 (vlan 1)-----vlan 5 1/1 switch bldg. B.

Of course all of the PC's in Bldg. B will have to be in vlan 5. The ports will have to be access ports, this will work becuase the packets coming from both switch will not have a vlan tag hence it arrives on the port of each switch it will be assumed that the packet will belong to that port's vlan - vlan 1 in bldg. A or vlan 5 in bldg. B.

pciaccio Sun, 04/09/2006 - 07:48
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The previous reply is correct. You can run a bridge with different VLAN numbers on both ends, however, keep in mind that you will see VLAN MISMATCH errors on both switches. This is fine...

Francois Tallet Mon, 04/10/2006 - 08:38
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Other possibilities if you want to merge these two vlans so that they have connectivity (I may not have fully understood the scenario)

-vlan translation -> this allows you to translate a vlan into another on a port. Supported with some restrictions on different platforms but should allow you to use a trunk instead of connecting to access ports in two different vlans.

-bridge groups: you can create SVIs and use a bridge group to merge the two vlans. Here also, there will be some platform dependent considerations as most Cisco switches would do this in software.




A small query regding vlan translation.

I make a trunk port and allow vlan 1,2,3,4,5 to

pass through it. Can I configure vlan translation in such a way that the vlan 1,2,3 that is coming to the trunk from one port exit the other side of the port as vlan 5.If so,pls point to some documentation.

Is there some documentation which gives confguration examples on creating bridge groups in merging 2 vlans.

Thanks in advance,


Francois Tallet Tue, 04/11/2006 - 10:10
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Hi Thomas,

No, you won't be able to merge 3 vlans into one. Vlan translation only allows you to convert a vlan tag into another.

What are you trying to achieve exactly then? I thought you had a kind of administrative issue (two independent sites with their own vlan assignment), but it seems you are trying to merge lots of vlans into one. There is no real use in merging vlans. The vlan ID has in fact no influence on the functionality of the vlan. A simple solution to your problem is to just configure all your vlans 1,2,3 and 5 as a single vlan 1 for instance. If this is because you want several different SVI with different IP subnets, you can create secondary interfaces on a single SVI for example. Anyway, you'll have to tell us what you have in mind;-)

As far as bridging is concerned, this is a generic IOS configuration. Here is a link to the config guide of an arbitrary IOS release:

Here too, bridging between vlans generally only makes sense if you want to do this selectively (like bridging IPX but routing IP). If you want to bridge all the protocols between two vlans, then it's better to use a unique vlan instead of two.




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