Identical MAC address on 2 VLANs on the same 3550

Unanswered Question

I created a VLAN 50 and 51 and gave them an IP address of and, respectively. So far, I can ping each interface without any problem, but I am concerned that it could potentially cause a problem somewhere down the line. To give you a better idea of how this is setup, this 3550 is at our colo and connects to our main office via metro E. We have 2 remote offices that connect to the internet and to the main ofice via the colo. Everything internal is on the 10.x.x.x network. Anyone think this would cause a problem? Should I change the MAC on one of the VLANs, or let it go? The bonus question for extra credit is, why did the 2 VLANs (50 and 51), that were created within minutes of each other, get assigned the exact same MAC address?

Thanks in advance for any and all help with this!

PS: C3550 Software (C3550-IPSERVICES-M), Version 12.2(25)SED

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c.lemaire Tue, 04/17/2007 - 04:36
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MAC address have only significance local to a VLAN so having the same MAC in two different VLANs isn't a problem. Catalyst 3550 have only one MAC address that's why the two vlan interfaces have the same MAC...

Thanks for clearing that up!

I just thought it was odd that they have the same MAC address. I would expect it to generate a MAC that isn't in use, or something. Do the 3550's all generate and use the same MAC address? That would be odd. I guess my next question is, how does the router decide what the MAC address of a newly created VLAN should be?

ankbhasi Thu, 04/19/2007 - 09:00
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  • Cisco Employee,

Hi Friend,

I doubt the 2 interfaces will have same mac address. Can you post the output of "sh interface vlan 50" and "sh interface vlan 51"?




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