Extending same VLAN across multiple distributions

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Apr 30th, 2012
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Just wondering what the current thoughts are on the following common problem  .......

  • Have 3 x L3 distribution pairs (#1, 2 and 3) with a number of fully redundant L2 access switches off each
  • Distributions are connected via a L2 core
  • VLAN X is root/root secondary on Distribution Pair 1. Need to extend the same VLAN X to L2 access switches hanging off Distribution Pair 2 and 3.
  • Single Mode fiber between distributions. Multimode fiber between access and distribution.

Do I simply connect a fiber from Distribution Pair 1 to both Distribution Pair 2 and 3 and extend down to access via trunk? This is obviously possible but not allowing for a clear demarcation and could cause instability from one network to extend to another.

Do I play it safe and put in a L2 aggregation switch (with fiber ports) beside each of Distributions 2 and 3 and in turn have dedicated (parallel) access switches for that VLAN X. This is a waste but would probably less of a risk than the first option.

Other options?



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danielmcollins Mon, 04/30/2012 - 14:02
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- do you have VTP running on the network?

- are there any overlapping VLAN's between distribution sites?

- what's the distance between distribution sites? Do you expect high latency?

- what's the diameter of the switched network?

Assuming no VTP, no overlap, small distance/low-latency between sites, relatively small # of hops between hosts that need to talk ...

You can safely connect each of the distribution sites together for VLAN X. You should manually prune all VLAN's except VLAN X on the new distribution trunks. Use "switchport trunk allowed vlan X".



Andrew Cink Mon, 04/30/2012 - 21:36
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This is a pretty tough one. There might be some options in Nexus that would let you do it.

Basically, if it's layer 3 between switches, you're pretty screwed.

You COULD go to a hybrid layer 3 model, where you trunk vlans across all of the switches as suggested above, and move the L3 IP configuration from the interface on the switch to a SVI, then trunk that SVI between switches to keep the layer 3 routing working. So basically you could have layer 3 connectivity using the IP addresses on trunked vlan Y, and then add vlans X and Z to the trunk to have the same layer 2 vlan in all 3 areas.



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