VLAN problem and possible lack of understanding!

Posted on 2009-04-30
Last Modified: 2012-05-06
Hi All
I have a problem connecting VLans, it appears to result in a broadcast storm on one of the switches and all the servers are unable to connect:  Setup explained below:
I have an HP Switch 3400cl with a 3C_lan on id 1
I have an HP Switch 3400cl with 2 VLANs - 1 is 22 ports on 4D_Vlan id 1 and the is 2 ports on 3c_vlan on id 2
I have a Dell switch with 5E_Vlan on id 0, 4d_vlan on id 1 and 3C_vlan on id 2
If I connect to the 4D switch from the 5E switch I can link to all vlans.
If I connect to the 3C switch from the 5E switch I appear to get a broadcast storm at the 5E end and connection to the attached servers drops off.
Why is this happening?
I don't understand vlans very well so any help would be appreciated

Question by:Eric
    LVL 10

    Expert Comment

    1st. The VLAN ID is a very significant. It should be assigned uniquely to the entire building or campus area that shares the same Level 2 infrastructure.

    So when you connect the switches, assuming they trunk up fully, you are putting the same l2 area in up to three vlans, as far as I can tell. This probably results in traffic from one of the vlans being looped onto another, and back into the first, and spanning tree might not do anything because the vlan IDs are wrong in the BPDUs.

    Just a guess. You also need to know what the switch does with native vlans, that is untagged frames on a trunk. This might be causing an issue as well.

    I would start by assigning the vlan IDs consistently. I would also not use vlan ID 1 or 0, as these are often used for the native vlan. So give the 3c vlan  an id of 3, the 4d vlan an id of 4 and the 5e vlan an id of 5.

    I would then see how the interswitch connections are forming trunks. Is it an automatic thing or did you configure them.

    That should be enough to start.

    I would start with making each floor

    Author Comment

    Hi IanBoyo

    The 3C Vlan is the default vlan do I need to create a dummy vlan to enable changing C3 Vlan to id 2? - if I start changing id numbers does it change dynamically or will I need to restart the switches.  Will I lose any traffic in the process if they are changed dynamically?

    Thanks for the info so far

    LVL 10

    Accepted Solution

    You would definitely lose traffic as they changed.

    Author Closing Comment

    Thank you for your help with this, I have chosen a different way to get round my problem.

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