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VLANS from 3COM to CISCO via Trunk Port

Steven Wells
Steven Wells used Ask the Experts™
I am having a bit of a problem

Currently I have a 3COM core switch hosting a number of VLANs. These are for various subnets for printers, workstations, servers etc.

All of these vlans are trunked through to a cisco switch (various) using 3COM's trunk port and vlan tagging.

Last night I created two new VLANs on the 3COM switch, and configured my ESX server (which is plugged into one of the cisco switch'es trunking ports) to use this VLAN.

Nothing worked. I was unable to see in ESX that the network adapter observed this new VLAN with its assocaiated IP addreses and I was unable to ping the new VLAN's interface IP address from the cisco switch.

It's almost as if the 3COM switch isn't tagging any of the trunk traffic with this new VLAN, even though the trunk is configured to pass all vlans.

I have gone crazy looking for a reason. All existing VLANs are being passed through the trunk ports into the cisco switches without problem. It's almost like it's not thinking it needs to. The 3COM model is a 5500 series, so there are not too many options. We are using Link Agreegation between the 3COM and the CISCO but that shouldn't make any difference..

Any ideas?
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Have you configured the virtual switch inside of ESX with a trunk link as well?  have you vlan tagged the virtual machines?
Steven WellsSystems Administrator


Hi bill,
Yes the ESX switch has been tagged and VM's have been configured with the new vlans.
All VM's can ping each other on the new VLAN but they can't ping the VLAN Interface IP address on the 3COM.

All of the ESX hosts connect to 4 seperate Cisco switches. Each with the same config
I can connect to other pre-existing vlans that are on the trunk, just not the new ones.
And you're sure the Cisco is allowing all VLANS through the trunk? Can you paste any config?
Steven WellsSystems Administrator


Hi Yip. All trunks have been allowed through. WIll post some config shortly (away from office).

I even tried setting the native VLAN for the ports that the esx is plugged in and that didn't work either.

Steven WellsSystems Administrator


Here is my cisco config

interface Port-channel1
 description Connected to 3750 Core Switch
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 2-4094
 switchport mode trunk

--Port ESX connected to.

interface GigabitEthernet0/3
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 100
 switchport trunk allowed vlan 2-4094
 switchport mode trunk
 speed 1000
 spanning-tree portfast

3 COM switch port

vlan 212
 description DMZ VLAN 212
---Link to Cisco Switch Trunk

interface GigabitEthernet1/0/24
 stp edged-port enable
 port link-type trunk
 port trunk permit vlan all
 broadcast-suppression pps 3000
 undo jumboframe enable
 port link-aggregation group 4
 apply qos-profile default

I might try converting to Access Ports rather than trunk ports to test.
It seems like it could be a problem with the trunk port protocol between 3com and Cisco
Are the Cisco switches on your network set to vtp mode client or transparent?  If you type "sh vlan brief" on the Cisco's, do you see the new VLAN?  You may need to create the VLAN's on the Cisco switches if they are not accepting new VLAN ID's.

See this - http://en.wikipedia.org/wiki/VLAN_Trunking_Protocol
Steven WellsSystems Administrator


Great. I was confused as this wasn't shown in the show run command.
It's all working now.