RSPAN and trunk pruning issue

[Andy White]

Hello,

I have enabled RSPAN between 2 switches where the source port is on one switch and the destination is to the RSPAN VLAN and the destination port in on the other switch over the trunk.  It is currently working but I can see that the other trunks on the source switch are also getting this RSPAN traffic as the trunks allow this RSPAN VLAN.  The thing is I am using VTP and pruning so I would of though the traffic would only go down the required trunk?

How can I block the RSPAN trafic going down the other trunk ports?

I did think about using the 'switchport trunk pruning' option on the trunks or using the allow VLAN option, but want to check with you guys first.

on the Source switch I'm using:

monitor session 2 source interface Gi2/0/1

monitor session 2 destination remote vlan 123

Destination switch:

monitor session 1 destination interface Gi0/21

monitor session 1 source remote vlan 123

and one of the trunk ports onthe source switch that is also receiving the span traffic is:

interface FastEthernet1/0/2

description Trunk to Cisco 3560 gig 0/1

switchport trunk encapsulation dot1q

switchport mode trunk

switchport nonegotiate

Thanks

[Gregory Snipes]

VTP pruning should be pruning your RSPAN traffic from unneeded trunks. This document clearly highlights that fact. Are you sure you do not have a more general VTP pruning problem?

[Andy White]

I have enabled VTY pruning only on the VTP server, but how can I tell if I have more of a general issue?

The doc says "If you enable VTP and VTP pruning, RSPAN traffic is pruned in the trunks to prevent the unwanted flooding of RSPAN traffic across the network for VLAN-IDs that are lower than 1005."

Thanks

[Gregory Snipes]

The most common issue with VTP pruning is trunks connected to a device in a different VTP domian or that does not support VTP. In this case vtp will pass all VLANs done every trunk allong the way, just incase they are needed by the trunk at the end. If this was the case you would see all of the VLANs in your database being forwarded down the trunk. To remedy this issue you need to use manual pruning on the non-VTP trunk.

If you could provide a "show interface trunk" from some of the switches it would be helpful in diagnosing any issues.

[Andy White]

Sure this is one big VTP domain, I just create the VLAN on the server and the clients all get it.

This is the switch where the destination RSPAN port is (wireshark).

Port        Mode             Encapsulation  Status        Native vlan

Gi0/2       on               802.1q         trunking      1

Gi0/23      on               802.1q         trunking      1

Gi0/24      on               802.1q         trunking      1

Port        Vlans allowed on trunk

Gi0/2       1-4094

Gi0/23      1-4094

Gi0/24      1-4094

Port        Vlans allowed and active in management domain

Gi0/2       1-6,9-15,100-101,123,300

Gi0/23      1-6,9-15,100-101,123,300

Gi0/24      1-6,9-15,100-101,123,300

Port        Vlans in spanning tree forwarding state and not pruned

Gi0/2       1,15

Gi0/23      none

Gi0/24      1-6,9-15,100,300

This is one of the trunks that seems to get the data too and isn't pruned, no ports are in the RSPAN VLAN.

Port        Mode             Encapsulation  Status        Native vlan

Gi0/1       on               802.1q         trunking      1

Port        Vlans allowed on trunk

Gi0/1               1-4094

Port                Vlans allowed and active in management domain

Gi0/1               1-6,9-15,100-101,123,300

Port                Vlans in spanning tree forwarding state and not pruned

Gi0/1               1-6,9-15,100-101,123,300

[Gregory Snipes]

The switch you have listed second only has one trunk on it. Can I assume that this is the trunk that is leading back to the spanning tree root? If so this is normal behavior. the pruning should only prune moving out from the root. it should not be pruning back the other way.

[Andy White]

That second switch is leading back to the root yes, well I have noticed this on the root, no VLAN 123 RSPAN, which could explain the issue?

spanning-tree vlan 1-6,9-15,100,300 priority 24576

The second switch in the earlier output seems to be the root.  I guess I need to change this, so my main root switch is the root for the RSPAN?

[Gregory Snipes]

Without knowing every detail of your network I can guarantee this will be a panacea, but rooting the vlan in a manner consistent with the rest of your network can only be a good thing here.

[Andy White]

Yeah thanks for your help.  I can get Cisco to jump on I guess.  I did manually prune the trunk and allow only the require VLANs and it obviously worked, but pruning should be doing this job.  I also fix the root issue witht he RSPAN VLAN, but didn't fix this issue.

Even though VTP pruning is on and these trunked switches don't have ports in the RSPAN VLAN maybe they think they do for some reason hense the VLAN is not pruned.

Thanks