Active VLAN on switch?

[afsharki2]

Hello everyone,

What is the quickest way to tell if a VLAN is being activley used on a switch....such as traffic from that VLAN going through the switch. 

the vlan i speak of is vlan 76.

So, I have a VLAN 76 on a few of my access switches that is added to a trunk link somewhere upstream on one of my interfaces to some switch.  When looking into the routing tables: no traffic from that vlan is even suppose to be going there.  How can i find out if that vlan 76 is being activley used on that switch, or having traffic from that vlan 76 passing through the switch?

thank you

[malshara]

hi ,

#show interface trunk

#show spanning-tree vlan 76

the first command will show you if vlan pruned or not on the trunk interface ,and the second command will show you if the trunk forwarding traffic from vlan 76

Regards

[afsharki2]

thanks Malshara,

This is what I got from the 2 commands. under 'sh in trunk' the VLAN was not listed in the 'STP forward state' with the other vlans.  So does this all mean that no packets from vlan 76 are passing this switch?

#sh spanning-tree vlan 76

Spanning tree instance(s) for vlan does not exist.

#sh in trunk

--------------------------------------------------------------------------------
Port          Vlans allowed on trunk
--------------------------------------------------------------------------------

Po333         76

--------------------------------------------------------------------------------

[Jon Marshall]

On your switch do -

1) "sh vlan brief"

do you see that vlan ?

2) "sh mac address-table"

do you see any mac addresses in that vlan ?

Jon

[afsharki2]

Hey Jon,

When you do a 'sh vlan br' ..there is nothing listed for vlan 76.  I should've mentioned this in my initial post but the actual VLAN isn't defined on this switch...it's just allowed on one of the trunk links and i'm trying to figure out why. So that's why i want to figure out whether packets bound in/out for this VLAN are traversing this switch. 

[Jon Marshall]

If it isn't in the vlan database it won't be passing any traffic.

Doesn't matter that it is included on the trunk links.

You can verify by looking at the mac address table and you should see no entries for that vlan.

Jon

[afsharki2]

Thanks Jon!

So, essentially, it's almost worthless to have those vlans added to trunk link?

[Jon Marshall · Accepted solution]

It does nothing as a far as I can see.

I would say that this is standard behaviour ie. the vlan must be in the local switches vlan database and I haven't come across any switches that behave differently but I haven't used all switch platforms.

So it really is worth verifying by looking at the mac address table just in case your switches behave differently.

I suspect though it is just a misconfiguration or perhaps the vlan did used to exist but it was deleted and they forgot to remove if off the trunk links.

Jon

[afsharki2]

Yeah, the mac-address table shows nothing because the VLAN isn't defined on that switch.  I think it was for testing purposes from one of my predecessors. 

Thanks for the help, Jon!!

[Ganesh Hariharan]

Hello everyone,


What is the quickest way to tell if a VLAN is being activley used on a switch....such as traffic from that VLAN going through the switch. 


the vlan i speak of is vlan 76.


So, I have a VLAN 76 on a few of my access switches that is added to a trunk link somewhere upstream on one of my interfaces to some switch.  When looking into the routing tables: no traffic from that vlan is even suppose to be going there.  How can i find out if that vlan 76 is being activley used on that switch, or having traffic from that vlan 76 passing through the switch?


thank you

Hi,

Show vlan brief can give you an idea about the lan configuration on switch lan database along with ports configured in that vlan.

If those ports connected devices are active and sending traffic then lan is active in your switch.

Hope it Helps.

-GI

[afsharki2]

Hey Ganesh,

If you look below, i posted my answer for this question.  I should've mentioned it originally.