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Two sites connected by layer 2 on 1 cable, but two different network both with same vlan ID

Hello Community!
In our company we have 2 phisically different networks. Network 1 and Network 2
Both uses the default VLAN1 vlan id.
I have access to Network 1. This Network 1 uses VLAN1 in too many networking items, and since it is not well documented, I wouldn't be happy to bother it. And the time window is too narrow to do it even on weekend.
I don't have access to Network 2 right now, and after asking in words what should I expect, they told me they don't know what they have, but because of past incidents, I think that they are using only VLAN1.

Now two buildings were connected with an optical cable (ISP only allows to use 1 pair of cable).
So I need to send the two VLAN1 network on the same cable between two Cisco SG250 switch.
I have 2 additional Cisco SG250 switch for the job.

So is there some way to add a new vlan for Network 2 that just works on Network 1?
And is converted back on both sides?
Is it even possible to solve on layer 2 level?

Thanks in advance!

4 Replies 4

Dennis Mink

you can assign the fibre link to VLAN 1 so it spans across the two networks.


have you looked at subnets?


are networks1 and 2 different subnets, because if they are, spanning vlan 1 across the two networks is pointless and you will need a ip boundary between the two,


let us know

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If the L2VLAN / IP address overlapping, they can not me moved to New VLAN and New Address Space.


you have only Option left with NAT behind another IP Address spool between site  - i can think of now.


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Well because I can't buy new routers, and after considering about the problem, we have come to the solution, that it is not possible to send it.

So it stays that I need to change the vlan number on one of the networks.

Thanks for the help!

Joseph W. Doherty
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A bit involved, but if you have a SM fiber cable you can partition it optically. I.e. you can use CWDM or DWDM transceivers, one per L2 network. You would also need an OADM at each end. (BTW, besides allowing you to have two L2 connections, each would have it's "full" bandwidth.)
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