VLAN trunk problem

Unanswered Question
Aug 2nd, 2008

I have three switches connected in series, I see the following on the middle switch and there is no traffic flow...

I have tried turning off pruning but I cannot get traffic to flow over interface g6/0/1...can anyone help?

LDA-LNK-AccSwch02>sh int g6/0/1 tru

Port Mode Encapsulation Status Native vlan

Gi6/0/1 on 802.1q trunking 1

Port Vlans allowed on trunk

Gi6/0/1 50

Port Vlans allowed and active in management domain

Gi6/0/1 none

Port Vlans in spanning tree forwarding state and not pruned

Gi6/0/1 none


I have this problem too.
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sundar.palaniappan Sat, 08/02/2008 - 13:24

Does VLAN 50 show as active in the 'show vlan' output?

Is VLAN 50 allowed on the trunk port on the other switch?

What model switch is this? Some of the low end catalyst switches have a limitation on how many VLANs they can support.

lamav Sat, 08/02/2008 - 13:31

Sundar makes a good point that is oftentimes missed.

If the vlan is not configured on the switch, traffic from that vlan will not traverse that trunk, even though it's allowed.

So, as SUndar said, lets see the o/p of a "sh vlan"


davegibelli Sat, 08/02/2008 - 13:36


All three 3750 switches have vlan 50 active, there is a maximum of 5 vlans on any switch

50 BLG-Data active Fa1/0/11

50 BLG-Data active

50 BLG-Data active

davegibelli Sat, 08/02/2008 - 13:41


int vl 50--------------no svi------int vl 50


sw2 can ping only

sw3 can ping only

sundar.palaniappan Sat, 08/02/2008 - 13:52

Can you provide the following output from all 3 switches?

show int trunk

show vlan

show vtp status

davegibelli Sun, 08/03/2008 - 05:47


I have provided the output for you...I am going crazy with this as I just cannot see waht is wrong...starting to suspect ios bug!

Giuseppe Larosa Sun, 08/03/2008 - 03:11

Hello Dave,

take care to verify if vlan 50 exists at Layer 2,


conf t

vlan 50


then redo sh vlan id 50

Because from your output is clear that vlan 50 doesn't exist at layer 2 (may be you created an interface vlan 50 but that is another object that exists at L3 )

Port Vlans allowed on trunk

>>>>Gi6/0/1 50

Port Vlans allowed and active in management domain

>>>Gi6/0/1 none

Port Vlans in spanning tree forwarding state and not pruned

>>>Gi6/0/1 none

only vlans in STP forwarding state really travels on the trunk

Hope to help


r.waterman Sun, 08/03/2008 - 06:53


out of interest can you also display the results from:

sh span vlan 50 sum

I suspect that the trunk interfaces from SW2 to SW1 to SW3 may be in a blocking state, but the above command will identify the stp status of those trunk links.

davegibelli Sun, 08/03/2008 - 11:42

Well I suspected an IOS bug....have a look at the output of sh span vl 50 sum for SW2 in the attachment!!!

I removed VLAN 50 on SW1 (VTP server) and immediately added it back and everything works as it should

Yet another IOS bug obviously :)

Giuseppe Larosa Mon, 08/04/2008 - 09:25

hello Dave:

interesting issue.

note that there is a mismatch on the type of STP you are running on your switches:

SW3>sh span vl 50 sum

Switch is in pvst mode

SW1>sh spann vl 50 sum

Switch is in rapid-pvst mode

SW2>sh span vlan 50 sum

Switch is in pvst mode

I would suggest to have the same type of STP on all switches even if RPVST is backward compatible

Hope to help



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