How can I connect 1 Switch to 2 different networks ?

If you are using a crossover, you can just set both ports as access ports and make sure the vlan is created on both sides. As an example, lets say you have two different core networks, on is 1.1.0.0 and the other is 2.2.0.0. The first has 255 vlans, 1.1.0.0 - 1.1.255.255. The second has the same but 2.2.x.x. You want 1.1.100.0 available on the second core. Provided you don't already have a vlan with the same number (if so, just move it to a number that is unused), you can just run a crossover between the two and bridge the VLAN over. This way you don't need to turn up 1.1.100.x interfaces on the second core, but you'll simply have an extension over.

The only time you would need trunking is if you have more than one vlan you are doing that for. From your perspective, you don't seem to care about layer 3 interfaces, you just want layer 2 connectivity on an existing switch. I would run a copper crossover in whatever this vlan is (as an access port on both sides), and put the three access ports for your servers in that same vlan.

If I've missed a requirement, let me know and Ill do my best to respond quickly.

Ok, I forgot to mention that the VLAN I want to create in the Switch Main doesn't exist in Switch Old, because the whole Old network is in a default vlan.

You've got the point I don't have problems in layer 3. My problem is that a VLAN f.e. Vlan 7 in Main switch won't be configured in the rest of Old switches .

So... do i have to configure it? In all of the ports of all of the switches? Is there a way so that the switch understands that vlan 7 in the other switch is its default broadcast domain?

Thanks > >

I'm not sure I understood that part. A router somewhere has to own that vlan, and by own, I mean there has a be layer 3 interface somewhere for routing. You can turn up vlan 7 on the switch, trunk or crossover it over to the switch where ports are needed, and so long as VLAN 7 goes back to a router somewhere, and is advertised out, you should be ok. I know you seem to be focused on layer 2, but where will VLAN 7's router interface be? Do both your networks (old vs. new) have core routers or MSFCs? Are they separate or does one set of routers own layer 3 for both networks? Perhaps a small diagram would do best.

If you are saying that you want to turn up VLAN 7 on one switch, and that will be VLAN 1 on your old switches, I've never tried that and would not be surprised if that didnt work. Frames are tagged with VLAN, and there would most likely be a mismatch, but I suppose it could work...just never been there myself. Is there a reason you can't turn up a new subnet in the new network, put the servers there, and ACL it off to only be able to talk to old network devices and vice versa?

Ok so I'll have to try a different approach.

Switches on Main are connected to a cisco 3745 Fa0/1 Switches on Old are connected to cisco 3745 Fa0/0 , so I have on both sides different default broadcast domains, and that is my problem. They used to be connected by 2 routers because we where on a different building, but now we moved all to the same building, so layer 3 translations are done on the same 3745 router (different int.)

... so if tagget frames have to go to the 3745 to translate to default vlan ... other approach could better my experiment.

No layer 3 switches? If not, why not plan a migration and get all the vlans turned up on one of the 3745 interfaces? If you still have the

2nd router, turn up the same interfaces but one IP address up and get HSRP up for redundancy. It may be a decent amount of switches to go touch and move vlan access ports, but I would get to a single core as quickly as possible to save confusion, sustainability, and future growth. If you do have layer 3 switches, this could be made a lot easier by turning the vlans up on a set and use the router only for WAN/Internet. Lots of options here....but if you are in a bind on the 3 server thing, I'd consider putting them in the new network and use ACLs to protect the networks....or just leave them open if this is a high speed network.......

Thank you, I'll have to find an alternative, I'll try to convince to buy layer 3 switches. For my problem I'll put a small router dedicated to that network.