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Vlan Question

Discussion in 'Other Cisco Equipment' started by ifican, Jul 31, 2007.

  1. ifican

    ifican Network Guru Member

    The only reason i can think of as to why i come up with this stuff is because i have a tendency to try and make things work outside of the box.

    The question is as follows: If you have 3 switches connected, we will call them left, right and middle. If you assign all ports on left to vlan 2, middle to vlan 3 and right to vlan 4 and you connect them together will they work. I say they will, (have yet to take the time to test).

    Ok now all of these switches are L3 and you ip each vlan (not the interface but the vlan itself) where all the switches reside in the same subnet but on different vlan facing ports. Will they route? Again i say yes they will, anyone care to venture a stab at this one?
  2. eric_stewart

    eric_stewart Super Moderator Staff Member Member

    Per your 1st paragraph (we'll call this scenario 1):
    There will be no inter-vlan communication. Each switch will be a separate VLAN island. If you connect the switches the link lights will be on but nobody will be home. VLAN 2's frames will ping pong around in its own broadcast domain (ie: switch ports on the left switch) and the other VLANs will exhibit the same behaviour.

    Per your 2nd paragraph (we'll call this scenario 2):
    Assuming you create a VLAN 2, VLAN 3 and VLAN 4 interface on every switch but put the VLANs in *different* IP subnets (I know, this wasn't exactly your example) there will still be no communication inter-VLAN until you setup trunks between the switches. Thus, since trunks are automatically in every VLAN, and you have VLAN 2, VLAN 3 and VLAN 4 interfaces configured on all 3 switches with (I suppose) unique IP addresses within each VLAN on each switch then you now have the *potential* for inter-VLAN communication. If these are true layer 3 switches then you will be able to ping between VLANs. If not, you will need a router trunked into one of the 3 switches configured to do inter-VLAN routing on its own VLAN interfaces.

    If you put all the VLANs in the same IP subnet *and* these are layer 3 switches *and* you have trunks between the switches then you should be able to ping between VLANs without a router.

  3. t4thfavor

    t4thfavor Network Guru Member

    I learn something new everyday, I had no idea, I thought vlans were only effective on one single switch, and that 3 different switches in 3 different vlans would talk if they were connected. That is pretty darn interesting.

    Before this I assumed that vlans were for segmenting one switch into multiple virtual switches, and that if you wanted to segment physical switches from each other you would just not connect them.
  4. ifican

    ifican Network Guru Member


    I agree with your response in theory and what is taught, though I am not sure about the first scenario i do know because i have done it by accident that you can essentially get 2 L3 switches to talk across mismatched vlans. I dont remember the exact cause, may have been a trunk going into 1 router but the other day when i was re-reading STP I had the thought. So a little later this morning i will build the above scenarios to test. Not sure exactly what the equipment will be but it will be all cisco running ios 12.x, I dont believe we have any L3 swithces lying around so i will have to use L2 and routers but we will see how that works out. Will be interesting indeed.
  5. ifican

    ifican Network Guru Member

    This is right and wrong depending on how you look at it. For those of use that have been around and work within the industry we never even remotely think of using equipment that does not vlan tag. If you are connecting vlan tagging capable switches together then you essential create broadcast domains over all switches that have ports configured in the numbered vlans in question. However if you are using swithces that do not vlan tag, then what you say above is correct as each switch only knows about its configured ports locally and does not care what ports are on the other side.

    My whole point to this thread was, i believe you can even get vlan tag'ed swithces to ignore / get confused if you put neighboring switchports in the same ip domain. We will see...........
  6. kspare

    kspare Computer Guy Staff Member Member

    Ok here's the thing.

    If you changed the default vlan on each switch to something different, and left it at that, then connected a cable from left and right to the middle switch, traffic would pass among all the switches. All you are doing is bridging the vlans. However it you turn on 802.11q trunking then everything eric said would apply. All a vlan does it segment a switch. You could split a switch into 2 vlans, but if you took a cable from a port in each vlan and connected the vlans, they would be able to talk to each other no problem.
  7. ifican

    ifican Network Guru Member

    This was my initial point, you could make / allow them to talk and i dont think trunking in itself will change that fact. I havent gotten to crazy or even wanted to but i have built out a switch put a router off of 3 different ports vlaned to 3 different vlans and they all spoke (were all assigned to the same subnet). Now for anyone not understanding vlans and wanting to learn what they are about, ignore this thread. This thread is just my morbid curiosity, it mostly stems from what can i expect a user or junior engineer to do that could potentially get me a call in the middle of the night. So by designing and configuring around that mind set i make my overall life easier. I have more crazy vlan tests to run but i will not get to them today.
  8. kspare

    kspare Computer Guy Staff Member Member

    Keep in mind, that changing the vlan on 3 switches, and then connecting them all together, it's completely useless and accomplishes nothing.
  9. ifican

    ifican Network Guru Member

    Agreed, its just more so if it will work. The thought popped up when i was considering what would happen if i designed a complex network and then had a junior engineer incorrectly interconnect switches or ports? Got me to think would certain things still work? As tested, it does and its just good to know.

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