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vlan help

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I have a host connected to a 2950 access layer swicth in building A. The enitre access switch is vlan 100 (first floor). The switch uplinks to a 3550 in the basement on a port dedicated to vlan 100. The 3550 routes between vlans (the different floors) in the building okay and also has a trunk over to a similar 3550 in building B (the trunk includes all vlans.) Here's my problem, vlan 100 in building A can talk to any other vlan in building A, but it can only talk to vlan 100 in building B. It is like this for all vlans in either building-- They can talk to any vlan in the same building, but only the same vlan in the other building. I think this has something to do with the packet structure and the way the headers are aligned, but can't quite seem to figure it out.

I have traced the packet from building A vlan100 and found that it does reach any host (on any vlan) I choose in building B. However, when the packet is coming back from building B's 3550 switch it gets to building A's 3550 and then is dropped. What am I missing here? Please help!

Hall of Fame Super Blue

Re: vlan help

Hi Matt

Are the vlans in building A and building B the same vlans ?

Does the 3550 in building B also route between vlans ?

Sounds like it could be a routing issue in that if you talk to vlan 100 from building A to vlan 100 in building B it is just switched down the trunk link whereas any other vlan would need to be routed.


Community Member

Re: vlan help


Yes the buildings have the same vlans, as so:

Building A 1st Floor vlan 100

Building A 2nd Floor vlan 200

Building A 3rd Floor vlan 300

Building B 1st Floor vlan 100

Building B 2nd Floor vlan 200

Building B 3rd Floor vlan 300

Both 3550s have ip rotuing enabled and route locally just fine. The weird thing is, routing on both sides works okay for a trunked packet to an extent (according to my analyzer the echo requests get all the way to the destination after being routed on the other side to a different destination vlan, but the echo replies are dropped after coming back to the local 3550 where they need to re-routed to the original vlan). Strange. As far as I see it, as long as both 3550s are routing and know the same vlans all should work fine. Right???

Hall of Fame Super Blue

Re: vlan help


Are you running HSRP between your two 3550's.

Do the vlans use the same IP addressing in both buildings ?

If it isn't the same addressing it still sounds like routing.

If it is then when you send a packet from vlan 100 in building A to vlan 200 in building B the packet will be routed on the 3550 switch in building A and then switched across the trunk link to vlan 200 in building B. The return traffic will be routed on the 3550 in building B and then switched to building A. This doesn't sound from your explanation what is happening.

Could you post configs of both 3550's.

Pick this one up tomorrow morning.


Community Member

Re: vlan help

Hi, Can you paste ur config of one of Edge switch i means floor switch in bulding A & Edge switch in building B also both 3550 config so that i can check the config. See it cud be prob of routing, VTP domain & trunking also. So it wud be better if u can send the config. I suggest you to put building B switch as vtp client & let it take vlan from 3550 in building A this is first suggestion. Second you can create one vtp domain & put both the 3550 in vtp server mode & create HSRP between them it wud provide u redundancy as well as easy intervlan routing..anyways first let me try to solve this problem.



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