802.1q tagging/untagging on Catalyst 2950

We have some problems using our Catalyst 2950 switches, problems that may be generated by an incomplete VLAN configuration: If we configure a trunk port, all the frames the switch sends on that trunk have no VLAN tag, no matter what VLAN they are coming from. It is possible to change the configuration, to obtain properly tagged frames (if the configuration is the problem)?

Thanks in advance for any information.

This is the configuration (partial):

vtp mode transparent ! spanning-tree mode pvst no spanning-tree optimize bpdu transmission spanning-tree extend system-id vlan 12-14 ! interface FastEthernet0/5 ! interface FastEthernet0/6 switchport mode trunk switchport nonegotiate ! interface Vlan1 ip address no ip route-cache

the result of show interface fa0/6 switchport: #sh interface fa0/6 switchport Name: Fa0/6 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: Off Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Capture Mode Disabled Capture VLANs Allowed: ALL

the result of show interface fa0/6 trunk: #sh interface fa0/6 trunk

Port Mode Encapsulation Status Native vlan Fa0/6 on 802.1q trunking 1

Port Vlans allowed on trunk Fa0/6 1-4094

Port Vlans allowed and active in management domain Fa0/6 1,12-14

Port Vlans in spanning tree forwarding state and not pruned Fa0/6 1,12-14

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Your problem is the 'vtp mode transparent'. The vlans you created on this switch will only be available on this switch. You will need to configure 'vtp mode server' in order to create vlans for use on other switches. Also just for general reference VLAN1 is always present and DOT1Q encapsulation does not tag frames on VLAN1 by default. Encapsulation is a confusing term here as 802.1Q tags frames and Cisco's ISL encapsulates the frame.


Reply to
Joel Salminen

Correct me if I am wrong (wouldn't be the first time), but would it be possible to use an 802.1q trunk such as the one described here without configuring this switch as a VTP server? It was my understanding that VTP simply consolidated the management of the VLANs by allowing you to create VLANs once and have them propagate automatically.

Reply to
Scott Lowe

what is or is not occuring that leads you to believe that there are no tagged frames being transmitted or received over the trunk interface.

As a previous poster has indicated any frame sent on VLAN 1 will be untagged.

Reply to

I guessing you responded in this thread to a message I posted in another thread....sorry if that's not the case.

My question was more directed toward the behavior of the switch after the addition of the "vlan dot1q tag native" command, which is required in order to allow VMware ESX Server port groups to interoperate with external VLANs. (VMware recommends to avoid the use of the native VLAN in the port groups to sidestep around this problem.) The URL below from Cisco implies that the addition of this command causes the switch to drop all untagged frames on 802.1q trunks.


was trying to determine if anyone had used this feature before and what other side effects, if any (like the dropping of untagged frames on

802.1q trunks) had been observed. I was also curious to know if using this command on 1 switch in the network would then, in turn (to avoid the dropping of untagged frames) require the use of this command on ALL switches in the network.

I hope this clarifies my questions.

Reply to
Scott Lowe

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