Cisco 6503's trunk interfaces prune VLANs

[tinhnho123]

Hello Guys,

I have a Cisco 6503, there are 16 ports on module 3 and they've been set as trunk ports for Layer 2 switches' uplinks for quite long time. Recently I've noticed these trunk ports' VLANs are pruned. 

Below is configuration of one of interfaces on 6503:

GigabitEthernet3/1

   switchport
   switchport trunk encapsulation dot1q
   switchport trunk allowed vlan 1,10,25,36,45
   switchport mode trunk
   no ip address
   udld port aggressive

My-6503-01#sh int g3/1 trunk

Port Mode Encapsulation Status Native vlan
Gi3/1 on 802.1q trunking 1

Port Vlans allowed on trunk
Gi3/1 1,10,25,36,45

Port Vlans allowed and active in management domain
Gi3/1 1,10,25,36,45

Port Vlans in spanning tree forwarding state and not pruned
Gi3/1 10,25
My-6503-01#

I also set 'switchport trunk pruning vlan none' under int g3/1 but the result is the same. Any ideas? Thanks.

[Juan Ospina]

Hi tinhnho123,

Your issue maybe with VTP pruning.

[tinhnho123]

How do I fix it?

[Juan Ospina]

Take a look at this...see if it helps.

http://packetlife.net/media/library/20/VLANs.pdf

[Ganesh Hariharan]

Hello,

I believe VTP pruning is enabled and configured on your VTP servers on your network Because VTP pruning can be learned by VTP clients, meaning once you enable VTP pruning on VTP servers, each VTP client automatically enables VTP pruning.

Also with pruning behaviour , switches automatically do the job based on inactivity of host on respective switch port vlans.

Hope it helps..

-GI

[Rolf Fischer]

Hi,

is there a particular problem you need to solve or are you just curious about the output of the show interface trunk command you've posted?

As Ganesh already said, VTP pruning is a domain-wide parameter (at least in version 1 and 2) and it enables the VTP clients and servers in the domain to send so-called Join messages, which contain a list of their locally active ("needed") VLANs. So the receiver of a Join message knows which VLANs should be pruned on that trunk in order to block unnecessary flooding (broadcast, multicast, unknown unicast) for those VLANs, or -to look at it the other way round- which VLANs must not be pruned. That's exactly what you want to achieve when you enable VTP pruning.

Make sure that you don't have any switches in VTP transparent mode in redundant topologies when you use VTP pruning. A transparent switch acts as a "relay" for VTP messages and this will lead to very undesired effects. We see this kind of error every once in a while so I thought it's worth mentioning here.

I also set 'switchport trunk pruning vlan none' under int g3/1 but the result is the same.

By default, only VLAN 1 is pruning-uneligible, that means flooding traffic in VLAN 1 is never pruned by VTP. You can configure other VLANs as pruning-uneligible too on a per-trunk basis with the switchport trunk pruning vlan command, which effectively disables VTP pruning for this VLANs on this interface (enables flooding for those VLANs to the neighbor switch, even if the neighbor  doesn't request this VLAN in its Join messages). Btw, disabling VTP pruning on a per-trunk basis is another solution to avoid the problems I mentioned above when you need to have transparent switches in your domain for whatever reasons.

The show interface [<interface>] pruning command often provides a more straightforward output than show interface trunk; unfortunately I don't have access to a lab at the moment so I can't tell if it takes into account the interface-specific configuration or not.

HTH
Rolf