vlan trunking help pls

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Apr 30th, 2008
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Hi everyone,

I meant to put this post here.

I need expert help with this scenario. Switch A is CAT 3560E PoE and switch B is CAT 3750 PoE. Connectivity between these swithces are 10gb fiber and is set to trunking so vlan1 can be access on both switches. I have a firewall dzm interface that i would like to bring in to switch A port # 48 and create a vlan 2 dmz there. Then on switch B, port #48, i configure it as a vlan2. Then when I connect my laptop, i could get an IP address from DHCP on the firewall dmz interface. So, looks like trunking between switch A and B carries all vlans. So, i have a Cisco CAT 3500XL that i would like to connect to port #48 on switch B so that i could add more workstations here. However, when i connect CAT 3500xl to port #48 of CAT 3570, port number #48 is disabled. Then, I set it up as trunk port but i am no longer have access to vlan2.

Here is the worse part, while trying to troubleshoot this, suddenly all VoIP phones and connectivities were down. I do not know if me adding the 3500xl would cause this problem but it seems we had a brownout while i was troubleshooting.

Please help since this is something i've never done before.


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VictorAKur Wed, 04/30/2008 - 13:36
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Port 48 could be disabled because it has 'bpdu guard' configured on it - every time you try to connect a switch to it, it would receive a bpdu on it and go down. If you configure the port 48 as a trunk and configure the port on the 3500XL as a trunk and VLAN 2 on your 3500XL, and then connect your laptop to the port on 3500XL assigned to VLAN2 you should be OK.

could you tell what VTP mode your switches are in? As in server, client, transparent. And what is the revision number on your 3500XL and the rest of the switches?


speedingwolfids Thu, 05/01/2008 - 09:11
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Hi Victor,

Thank you for your respond. yesterday, i configured port 48 as trunk and 3500xl as trunk and able to access vlan2 2. VTP on switches are server.

Would all switches go down because i put the 3500xl to the port without configure it as trunk?


bvsnarayana03 Thu, 05/01/2008 - 12:45
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Probably victor wants to say that you had a higher revision No. for VTP on 3500XL switch, which might have overwriten the vlans on existing switch with its own vlans.

Ideally one should check for the revision no. on the switch bfore introducing it in to the topology. The revision No on the switch can be reset by making it transparent & changing the domain name. Once the revision no. is reset, you can change the mode & domain name to hte desired as per topology.

nawas Thu, 05/01/2008 - 13:00
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3500XL are very notorious to create this type of STP loop and clearing versios or set to transparent is the only option to fix this problem. We recently replaced more than 100 3500XL with 3560s and they are way better.

speedingwolfids Thu, 05/01/2008 - 13:06
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Thank you all for your responds. So, the bottom line is that introduce a new switch to the network could bring down the entire network? Also, there is no domain name in the VTP and all switches are servers.


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