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Community Member

STP forwarding and pruning problem

Hi,

Vlan 23 and 28 are trunked between Cisco 4500 and Cat 3750. On the Cisco 4500, these 2 vlans are available on the "Vlans in spanning tree forwarding state and not pruned" on sh int trunk. However, on the Cat 3750 these vlans does not appear on "Vlans in spanning tree forwarding state and not pruned" on sh int trunk (see below).

A sh vtp status shows that the vtp pruning mode is enabled. A device on vlan 28 are connected on the Cat 3750 but is not accessable. There are a few devides on vlan 23 on the Cat3730 which are accessable eventhough it wasn't in "Vlans in spanning tree forwarding state and not pruned".

Even with pruning mode enabled, with vlan 23 and 28 defined, these 2 vlans should be in "Vlans in spanning tree forwarding state and not pruned" .

Can someone explain why this is hapenning? Any help is much appreciated.

TIA.

PF

CISCO 4500#sh vlan

VLAN Name Status Ports

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

1 default active Gi2/2

23 vlan_webserver active Gi4/3

25 vlan_server_client active Gi1/1

28 vlan_mgmt active Gi4/18

CISCO 4500#sh int trunk

Port Mode Encapsulation Status Native vlan

Gi2/1 desirable n-isl trunking 1

Port Vlans allowed on trunk

Gi2/1 1,23,25-28

Port Vlans allowed and active in management domain

Gi2/1 1,23,25-26,28

Port Vlans in spanning tree forwarding state and not pruned

Gi2/1 1,23,25-26,28

Catalyst 3750#sh vlan

VLAN Name Status Ports

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

23 vlan_webserver active Fa2/0/17, Fa2/0/18, Fa2/0/19, Fa2/0/21

25 vlan_server_client active Fa1/0/1

26 vlan_server active

28 vlan_mgmt active Fa1/0/28

Catalyst 3750#sh int trunk

Port Mode Encapsulation Status Native vlan

Gi1/0/1 desirable n-isl trunking 1

Port Vlans allowed on trunk

Gi1/0/1 1,23,25-26,28

Port Vlans allowed and active in management domain

Gi1/0/1 1,23,25-26,28

Port Vlans in spanning tree forwarding state and not pruned

Gi1/0/1 1,25

16 REPLIES
Hall of Fame Super Bronze

Re: STP forwarding and pruning problem

Try hardcoding the trunking mode and trunking type on both interfaces

switchport trunk encapsulation dot1q

switchport mode trunk

HTH,

__

Edison.

Community Member

Re: STP forwarding and pruning problem

Edison,

The above commands were hard coded on the interfaces and the result is the same.

With vtp pruning active, a sh int gi1/0/1 pruning on the 3750 shows that the vlan 23 and 28 were pruned for lack of request by neighbour (please see below).

Do you know the reason for this?

TIA.

PF

Cat3750#sh int gi 1/0/1 pru

Port Vlans pruned for lack of request by neighbor

Gi1/0/1 23,26,28

Port Vlan traffic requested of neighbor

Gi1/0/1 1,23,25-26,28

Hall of Fame Super Bronze

Re: STP forwarding and pruning problem

That's odd, try disabling VTP pruning and enabling VTP pruning again. What version of IOS are you running on those switches ?

Community Member

Re: STP forwarding and pruning problem

Edison,

I have disable and enable vTP pruning a number of times before but the same results.

The version of IOS is Version 12.2(25)SEE2. Does the information on sh VTP status matters between the 2 switches (3750 and 4500)?

TIA.

PF

Hall of Fame Super Bronze

Re: STP forwarding and pruning problem

Let's see the show vtp status from both devices.

Also, you may want to upgrade the 3750 switch to SEE4.

Community Member

Re: STP forwarding and pruning problem

Edison,

Below is the sh vtp status for the switches. Please note that I have vtp pruning disabled on the cat3750 switch so that vlan 28 works on the switch.

Thanks.

PF

Cat4500#sh vtp status

VTP Version : 2

Configuration Revision : 0

Maximum VLANs supported locally : 1005

Number of existing VLANs : 9

VTP Operating Mode : Transparent

VTP Domain Name :

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Enabled

MD5 digest : 0xBC 0xF4 0x28 0xE1 0xBD 0xE5 0xD7 0x93

Configuration last modified by 172.31.1.1 at 0-0-00 00:00:00

cat3750#sh vtp status

VTP Version : 2

Configuration Revision : 29

Maximum VLANs supported locally : 1005

Number of existing VLANs : 10

VTP Operating Mode : Server

VTP Domain Name : bos

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generat

Hall of Fame Super Bronze

Re: STP forwarding and pruning problem

Well, yes - we have a problem.

Not sure how you had VTP Pruning enabled since transparent does not support VTP Pruning as it's not part of a VTP domain.

If you want pruning to work, you need to have a server/client model. I see the 3750 has more Vlans than the 4500, be careful when making these changes as the 3750 is going to wipe the 4500 Vlan information (it has a highest configuration revision).

Make the 4500 a client, make sure the VTP domain is the same and your VTP pruning should work as advertised.

HTH,

__

Edison.

Community Member

Re: STP forwarding and pruning problem

Edison,

The 3750 was on tranparent mode before. I change it to server mode as I need to play with the vtp pruning.

Will pruning work if they are on the SAME domain but both on transparent mode?

We do not want any vlan changes to go across switches so in this case vtp pruning has to be disabled on the Cat 3750 switches. Correct?

TIA.

PF

Hall of Fame Super Bronze

Re: STP forwarding and pruning problem

VTP Pruning works when a VTP domain is being used in a switched environment. VTP Transparent essentially disables VTP. If you want to prune between switches, you need to manually prune the Vlans on the inter-switch links.

Change both back to transparent and repost that output along with show vlan output.

Community Member

Re: STP forwarding and pruning problem

Edison,

Attached is the output requested.

Thanks.

PF

Cisco Employee

Re: STP forwarding and pruning problem

In addition to Edison's post, the best way to work it out is to assign a switch as VTP server and other as the VTP client. As mentioned above the 3750 has the highest configiration number, I would suggest to put both the switches to a default VTP config revision and then work it out. The steps to set the config revision to 0 are :

1. Change the domain name of the switch to some bogus name and put the desired domain name back.

2. change the VTP mode to transparent mode and put it back to the Server or client mode as desired.

Also, as a best practice shut the trunk ports for a while till the time you are doing the VTP configs and bring them back once you have done the VTP configs.

HTH,

-amit singh

Community Member

Re: STP forwarding and pruning problem

Amit,

Thanks for the information. Will make a note of this.

Thanks.

PF

Re: STP forwarding and pruning problem

Could you do a show spanning-tree vlan n for VLANs 28 on each switch please? Does the 4500 have anything on VLAN 28 apart form the trunk? Also, which switch is the Spanning-Tree root for that VLAN? Are you running PVST+ or MST?

Kevin Dorrell

Luxembourg

Community Member

Re: STP forwarding and pruning problem

Kevin,

We are running PVST. Here is the sh spanning-tree vlan 28.

Thanks.

PF

cat 4500#sh spanning-tree vlan 28

VLAN0028

Spanning tree enabled protocol ieee

Root ID Priority 28

Address 0011.20b7.6240

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 28 (priority 0 sys-id-ext 28)

Address 0011.20b7.6240

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

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

Gi2/1 Desg FWD 4 128.65 P2p

Gi2/3 Desg FWD 4 128.67 P2p

Gi4/1 Desg FWD 4 128.193 P2p

Gi4/18 Desg FWD 4 128.210 P2p

cat 4500#

cat 3750#sh spAnning-tree vlan 28

VLAN0028

Spanning tree enabled protocol ieee

Root ID Priority 28

Address 0011.20b7.6240

Cost 4

Port 1 (GigabitEthernet1/0/1)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32796 (priority 32768 sys-id-ext 28)

Address 000f.2490.8c00

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

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

Gi1/0/1 Root FWD 4 128.1 P2p

Re: STP forwarding and pruning problem

Is it all working now? I notice from one of your postings that the 3750 is no longer pruning VLAN 28 from the trunk.

In you original posting, you said that if you connect a device to VLAN 28 on the 3750, then it is not accessible. When you did this show spanning-tree, there were no devices connected to the 3750 on VLAN 28. The only port in the Spanning-Tree on that VLAN was G1/0/1, which I guess is your uplink to the 4500.

What is the current status of this problem?

Kevin Dorrell

Luxembourg

Community Member

Re: STP forwarding and pruning problem

Kevin,

Sorry for not mentioning that it is working now. Now both the switches are in transparent mode as desired. I was able to made it work by disabling pruning on the 3750 but I do not quite understand then. Hence the posting on the website. Edison helped me understand what my problem was.

However, I still am confused as to why devices on vlan 23 works on the 3750 eventhough it did not appear on "Vlans in spanning tree forwarding state and not pruned" in sh trunk. Like Vlan 28, vlan 23 should not have worked either but it did.

There was a device on port fa2/0/28 connected to 3750 on vlan 28 (Wasn't copied properly).

Thanks.

PF

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