Spanning-tree and unused vlan 1

s.couland · ‎04-26-2016

Hello,

I have a simple question because I did not find any reply to it:

I have configured a redondant L2 topology with 1 core switch and many access switches with some L2 physical loops.

Spanning-tree Priority 4096 has been set for all vlans in core. Default 32768 is set on all access switchs.

Vlan 1 is not used, all SVI have been shutdown for vlan 1 on all switches. All uplinks are configured with trunk and 999 as native vlan (vlan 999 does not exist).

There are no vlan filters for trunks.

On all switches, I can see that the switch is root bridge for vlan 1. Why ?

Why I still have flapping messages in vlan 1 on all switches ? (Means STP loop)

I can see the loop is broken by STP (port BLK) for all vlans EXCEPT vlan 1 ! Why ?

Thanks & regards.

devils_advocate · ‎04-26-2016

Shutting down the Vlan 1 SVI will not remove the L2 Vlan itself, you can't remove Vlan 1 as its the default.

The SVI is simply the Layer 3 interface for that Vlan, it has no bearing on the Spanning Tree.

The best practice is not to use Vlan 1 for user traffic which you have achieved by shutting down the SVI and ensuring no user ports are a member of this vlan.

Changing the native Vlan is also fine but several protocols still use Vlan 1 (CDP, VTP DTP etc) so there is no way to remove it from the topology as such.

Assuming your Core has 4096 for STP priority for all Vlans (including Vlan 1) and you have simply changed the native vlan on all your trunks, it should have a topology the same as all the other vlans.

Can you post the results of the following from the Core switch and an access switch:

#show span vlan 1

s.couland · ‎04-26-2016

I totally agree that it should have a topology the same as all other vlans but unfortunately that is not the case for vlan 1 !

Here are the results:

core

VLAN0001
Spanning tree enabled protocol rstp
Root ID    Priority    4097
             Address     001a.a146.4800
             This bridge is the root
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority    4097   (priority 4096 sys-id-ext 1)
             Address     001a.a146.4800
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/13              Desg FWD 19        128.15   P2p
Fa0/14              Desg FWD 19        128.16   P2p
Fa0/15              Desg FWD 19        128.17   P2p
Fa0/22              Desg FWD 19        128.24   P2p
Po1                 Desg FWD 3         128.56   P2p

That is a normal situation. Core is root bridge.

One of the access switchs (same on other access switchs):

VLAN0001                     <------------ not used
Spanning tree enabled protocol rstp
Root ID    Priority    49153
             Address     0041.d265.d380
             This bridge is the root                        <-------------------- ?????
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority    49153 (priority 49152 sys-id-ext 1)
             Address     0041.d265.d380
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300 sec
UplinkFast enabled but inactive in rapid-pvst mode

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Desg FWD 3004      128.25   P2p
Gi0/2               Desg FWD 3004      128.26   P2p

VLAN0030                     <-------------- USERS vlan
Spanning tree enabled protocol rstp
Root ID    Priority    4126
             Address     001a.a146.4800                        <------------ correct: core
             Cost        3007
             Port        25 (GigabitEthernet0/1)
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority    49182 (priority 49152 sys-id-ext 30)
             Address     0041.d265.d380
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300 sec
UplinkFast enabled but inactive in rapid-pvst mode

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Root FWD 3004      128.25   P2p
Gi0/2               Desg FWD 3004      128.26   P2p

Carlos Villagran · ‎04-26-2016

Hi!

My theory is,

No BPDUs in vlan 1 are being received by the access switches (since they are isolated) therefore every switch claims itself as root.

Best regards!

JC