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Need opinion. Span VLANs across multiple access layer switches.

Alexander Simbun

Hello experts,

Need your opinion. Our customer would like to expand their network with minimal impact on their daily operation. Recently, they are in the midst of acquiring 4 office floors in the same building and they would like to extend their existing network to those floors. The current design is all VLANs are span across multiple access layer of switches (Cisco 3760-X) with Cisco 6504-E as its core switch. These access switches are connected to core switch using fiber optics and all VLANs (as stated in the diagram) are carried via VLAN trunks on each fiber optic towards the core switch. Core switch responsible for giving dynamic IPs to every end-user devices in each respective VLANs.

I wonder if it is a good idea to span VLANs across multiple access switches. Kindly enlighten me.



4 Replies 4

harsh mistry


 I had read your question. First you create MST. So,spanning-tree area create small. You can easily manage vlan.

Jon Marshall
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Apologies, should have answered the rest of the question :-)

If you don't span the vlans then you simply need to create new vlans with new IP subnets.

In terms of impact for the switches very little. There is more configuration work in that you need to create new vlans, new L3 vlan interfaces on the 6500 and new DHCP scopes.

That said you also have a firewall and it depends on your setup there whether you would need to do additional configuration on that too for the new IP subnets.

Difficult to say without knowing how you have it setup.


Joseph W. Doherty
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Basically, ditto to what Jon has described.  I.e. on a L3 switch, NOT spanning VLANs across trunk ports is a better design.  Possibly also less likely to disturb existing VLANs as you add new floors.  (For example, if you add a new floor, and you mistakenly [?] change the VLAN root switch.)


BTW, if customer is concerned about minimal impact for adding new floors, are they also concerned about the many single points of failure?  For example, if the 6504 crashes?

Jon Marshall
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It is a fairly common design but if you don't need to span the vlans to all switches then you probably shouldn't but equally it's probably not going to harm if you do with the amount of vlans you have in your network.

You only really need to do this if you need L2 adjacency between devices on different floors and it is rare that you do need this.

Wireless has it's own requirements so i wouldn't like to say one way or the other on that but you can always post a specific question about those vlans in the Wireless forums.

Wireless aside, it really comes down to -

a) can you limit specific vlans to specific switches in which case you can simply only allow those vlans on the trunks

b) if you are running VTP you would then need to use transparent on your switches and only have the vlans that were actually in use on the switch in the vlan database

c) how much work you want to do in setting it up.

To be honest i have run similar setups in the past (with dual connectivity for redundancy) and had all vlans on all switches but if i was doing it now i would limit the vlans if possible.

It becomes more important if you have a pair of core switches running HSRP but you aren't.

So i guess after all the short answer is if you need the same vlans on multiple floors then it is a good idea but that is rarely the case.


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