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A small company has 2 Cisco Catalyst 3550, with 24 ports each, connecting their equipaments.

After some trouble, we found that two NICs had the same MAC address. After the initial reaction of blaming who bought such NICs, etc., I began to wonder: if the NICs belong to separate VLANs, would it still cause problems?

Could I connect each NIC to a different VLAN and work ok (except in the future, if someone forget that and connect them to the same VLAN again) ?

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    Name the vendor! – chrylis -cautiouslyoptimistic- Aug 28 '13 at 14:24
  • :) In Brazil we call it XingLing, but I just found that actually there is a company with that name, and that internationally it's called Shanzhai – woliveirajr Aug 28 '13 at 14:33
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    @woliveirajr, would you mind adding the first three bytes of the duplicate mac-addresses? a googlable record of the OUI may help someone in the future – Mike Pennington Aug 28 '13 at 16:47
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    For reference, a number of years back there was a bad BIOS image that when used on certain motherboards would corrupt the MAC of the on board Nvidia NICs, leaving them with 044b.8080.8003 or 044b.8080.8004 as the MAC (and no way to fix). Came across this troubleshooting a problem at a university where one of the departments had purchased a number of these for use in a computer lab. – YLearn Aug 29 '13 at 3:51
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    Those NICs began with 00E04C , but I can assure that they were not RealTek ones... now I'm curious on how is works when you set the MAC address by hand, but I'll ask another question about it (and won't be able to test any further on those NICs) – woliveirajr Aug 30 '13 at 16:03
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As long as the NICs are in completely different layer-2 broadcast domains, it shouldn't be a problem, but it is a headache waiting to happen when one of them is moved. If there are just two offending NICs, it's probably better to replace one.

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It's very common to have the same MAC address in multiple VLANs. Look at any Catalyst 6500/7600 router, for example: all its routed interfaces will have the same MAC address. Same goes for Brocade MLX/XMR.

All switches these days index their CAM on (MAC, VLAN) instead of merely (MAC); your Catalyst 3550 certainly does.

You should switch network card vendors if they're really identical. I once lost a crate of beer to a co-worker when I thought I had two NICs with the same MAC address and he replied with "Want to bet?" and it turned out I and by that point a few other co-workers too had read over a few bytes in the middle that differed.

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I like to add one thing that is. - Actual switches do have separated FDB tables per VLAN. - Older switches can have one common FDB table for all VLAN's. In the second scenario one MAC address can only be learned once.

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