Spanning Tree Loop

[Dustin Flint]

I am having a spanning tree issue between 2 switches. Each port is setup as a basic trunk, and the 2 ports are connected over a microwave link. Spanning tree is setup as MST with 4 instances, and  I dont have this problem anywhere else. Lopguard default and portfast bpdugard are set on both switches.

There are multiple paths to each switch, but like I said, I do not have this issues anywhere where there are multiple paths. Both switches have been swapped out with new ones, and the IOS has been updated. Both switches are 2960s. What am I missing?

Switch1

Fa0/7 Desg BLK 200000 128.7 P2p Dispute

Switch2

Fa0/2 Desg BKN*200000 128.2 P2p *LOOP_Inc

[Georg Pauwen]

Hello,

are the other switches with multiple paths also 2960s connected with microwave links ?

Post the configs of both 2960s. Also, which IOS version are you running (so we can check for bugs) ?

[Dustin Flint]

Both switches are running Version 12.2(55)SE11. Below is the config of spanning tree on both switches(they match) and the switch ports.

spanning-tree mode mst
spanning-tree loopguard default
spanning-tree portfast bpduguard default
spanning-tree extend system-id
!
spanning-tree mst configuration
name SPAN
revision 10
instance 1 vlan xx
instance 2 vlan xx
instance 3 vlan xx
instance 4 vlan xx

Switch 1
interface FastEthernet0/7
description Microwave to Site2
switchport mode trunk

Switch 2

interface FastEthernet0/2
description Microwave to Site1
switchport mode trunk

[rasmus.elmholt]

The problem is not the configuration, It looks perfekt.
But both switches have designated ports towards each other and this indicate they dont have the same RootBridge.
And that is the reason SW1 is making a STP Dispute.

Please share the show spanning-tree on both switches.

[Dustin Flint]

I have verified there is something wrong with the link. Either cabling, or the microwave link itself. Will investigat further and update from there.

[paul driver]

Hello
Sw1 seems to suggest a config mismatch/error -  Do you have any aggregation on these interfaces and have you checked for any physical issues?

Sw2 is receiving bpdus when it shouldnt be, You need to understand why that is?
If you know you network then suggest check the trunk interfaces and establish what port SHOULD be DSG/ROOT/BLK etc... between these multiple paths.

res
Paul

[rasmus.elmholt]

Hi Paul

Don't you mean Sw2 isn't receiving BPDUs when it should be?

[rasmus.elmholt · Accepted solution]

Hi Dustin

To me it looks like you have unidirectional link.
SW1 is reseiving a BDPU with disputable information like a Designated port on SW1's own designated port.
And SW2 has moved into Loop_Inconsistent state because it does not see a BPDU from SW1.

Do some L1/L2 troubleshooting on the microware link.
Most likely you will se SW1 haveing a designated port towards SW2, but still have SW2 being its own Root Bridge in its own STP world.

Could you please share show span on both switches?

[Dustin Flint]

Yes, for some reason on MST 4 switch 2 is seeing itself as the root bridge. Which is weird because it sees MST3 correctly, and instances 3 and 4 have the same root bridge.

Switch 1
sh span

MST0
Spanning tree enabled protocol mstp
Root ID Priority 4202
Address 002a.6a95.1cc1
Cost 22250
Port 31 (FastEthernet0/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address e840.4082.8200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Desg FWD 200000 128.5 P2p
Fa0/6 Desg FWD 200000 128.6 P2p
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/9 Desg FWD 200000 128.9 P2p Edge
Fa0/13 Desg FWD 200000 128.13 P2p
Fa0/15 Desg FWD 200000 128.15 P2p
Fa0/16 Desg FWD 200000 128.16 P2p
Fa0/17 Desg FWD 200000 128.17 P2p

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------

Fa0/18 Desg FWD 200000 128.18 P2p
Fa0/19 Desg FWD 200000 128.19 P2p
Fa0/20 Desg FWD 200000 128.20 P2p
Fa0/22 Desg FWD 200000 128.22 P2p
Fa0/23 Desg FWD 200000 128.23 P2p
Fa0/25 Desg FWD 200000 128.25 P2p
Fa0/30 Desg FWD 200000 128.30 P2p
Fa0/31 Root FWD 200000 128.31 P2p Pre-STD-Rx
Fa0/40 Desg FWD 200000 128.40 P2p Edge



MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address 001b.d4e4.ec00
Cost 210000
Port 31 (FastEthernet0/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address e840.4082.8200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/6 Desg FWD 200000 128.6 P2p
Fa0/7 Desg BLK 200000 128.7 P2p Dispute
Fa0/9 Desg FWD 200000 128.9 P2p Edge
Fa0/15 Desg FWD 200000 128.15 P2p
Fa0/17 Desg FWD 200000 128.17 P2p
Fa0/18 Desg FWD 200000 128.18 P2p
Fa0/22 Desg FWD 200000 128.22 P2p
Fa0/23 Desg FWD 200000 128.23 P2p

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------

Fa0/30 Desg FWD 200000 128.30 P2p
Fa0/31 Root FWD 200000 128.31 P2p Pre-STD-Rx



MST2
Spanning tree enabled protocol mstp
Root ID Priority 24578
Address 001b.d4e4.ec00
Cost 210000
Port 31 (FastEthernet0/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address e840.4082.8200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/6 Desg FWD 200000 128.6 P2p
Fa0/7 Desg BLK 200000 128.7 P2p Dispute
Fa0/13 Desg FWD 200000 128.13 P2p
Fa0/16 Desg FWD 200000 128.16 P2p
Fa0/17 Desg FWD 200000 128.17 P2p
Fa0/19 Desg FWD 200000 128.19 P2p
Fa0/25 Desg FWD 200000 128.25 P2p
Fa0/31 Root FWD 200000 128.31 P2p Pre-STD-Rx



MST3
Spanning tree enabled protocol mstp
Root ID Priority 16387
Address 001b.d493.5600
Cost 200000
Port 31 (FastEthernet0/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32771 (priority 32768 sys-id-ext 3)
Address e840.4082.8200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Desg FWD 200000 128.5 P2p
Fa0/6 Desg FWD 200000 128.6 P2p
Fa0/7 Desg BLK 200000 128.7 P2p Dispute
Fa0/17 Desg FWD 200000 128.17 P2p
Fa0/20 Desg FWD 200000 128.20 P2p
Fa0/31 Root FWD 200000 128.31 P2p Pre-STD-Rx
Fa0/40 Desg FWD 200000 128.40 P2p Edge



MST4
Spanning tree enabled protocol mstp
Root ID Priority 16388
Address 001b.d493.5600
Cost 200000
Port 31 (FastEthernet0/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32772 (priority 32768 sys-id-ext 4)
Address e840.4082.8200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/6 Desg FWD 200000 128.6 P2p
Fa0/7 Desg BLK 200000 128.7 P2p Dispute
Fa0/17 Desg FWD 200000 128.17 P2p
Fa0/31 Root FWD 200000 128.31 P2p Pre-STD-Rx

Switch 2
sh span

MST0
Spanning tree enabled protocol mstp
Root ID Priority 4202
Address 002a.6a95.1cc1
Cost 22250
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 6416.8db7.a380
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 200000 128.1 P2p
Fa0/2 Desg BKN*200000 128.2 P2p *LOOP_Inc
Fa0/3 Desg FWD 200000 128.3 P2p Edge
Fa0/4 Desg FWD 200000 128.4 P2p
Fa0/5 Desg FWD 2000000 128.5 P2p
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/14 Desg FWD 200000 128.14 P2p
Fa0/16 Desg FWD 200000 128.16 P2p

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------

Fa0/17 Desg FWD 200000 128.17 P2p
Fa0/18 Desg FWD 200000 128.18 P2p
Fa0/19 Desg FWD 200000 128.19 P2p
Fa0/20 Desg FWD 200000 128.20 P2p
Fa0/21 Desg FWD 200000 128.21 P2p



MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address 001b.d4e4.ec00
Cost 810000
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 6416.8db7.a380
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 200000 128.1 P2p
Fa0/2 Desg FWD 200000 128.2 P2p *LOOP_Inc
Fa0/3 Desg FWD 200000 128.3 P2p Edge
Fa0/4 Desg FWD 200000 128.4 P2p
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/17 Desg FWD 200000 128.17 P2p
Fa0/18 Desg FWD 200000 128.18 P2p
Fa0/19 Desg FWD 200000 128.19 P2p

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------

Fa0/21 Desg FWD 200000 128.21 P2p



MST2
Spanning tree enabled protocol mstp
Root ID Priority 24578
Address 001b.d4e4.ec00
Cost 810000
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 6416.8db7.a380
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 200000 128.1 P2p
Fa0/2 Desg FWD 200000 128.2 P2p *LOOP_Inc
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/14 Desg FWD 200000 128.14 P2p
Fa0/16 Desg FWD 200000 128.16 P2p
Fa0/19 Desg FWD 200000 128.19 P2p
Fa0/20 Desg FWD 200000 128.20 P2p
Fa0/21 Desg FWD 200000 128.21 P2p



MST3
Spanning tree enabled protocol mstp
Root ID Priority 16387
Address 001b.d493.5600
Cost 800000
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32771 (priority 32768 sys-id-ext 3)
Address 6416.8db7.a380
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 200000 128.1 P2p
Fa0/2 Desg FWD 200000 128.2 P2p *LOOP_Inc
Fa0/5 Desg FWD 2000000 128.5 P2p
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/19 Desg FWD 200000 128.19 P2p
Fa0/21 Desg FWD 200000 128.21 P2p



MST4
Spanning tree enabled protocol mstp
Root ID Priority 32772
Address 6416.8db7.a380
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32772 (priority 32768 sys-id-ext 4)
Address 6416.8db7.a380
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 200000 128.1 P2p
Fa0/2 Desg FWD 200000 128.2 P2p *LOOP_Inc
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/19 Desg FWD 200000 128.19 P2p
Fa0/21 Desg FWD 200000 128.21 P2p

[rasmus.elmholt]

Hi
The problem is the same on MST1,MST2,MST3,MST4. Loop inconsistent on one side, and Dispute on the other.

What does your show span mst configuration show?
And are you sure there is L2 connectivity between SW1 and SW2 on the microwave link?
Could you share a show cdp nei <int> on the two switches on the ports connected together?

[Dustin Flint]

I have verified there is indeed a problem with the microwave link itself. Not sure if it is out of alignment, or malfunctioning equipment .Will have to get a tower crew. But essentially only 2% of traffic was getting through the link, so bpdus werent being properly exchanged.

[Dustin Flint]

Microwave link has been fixed and hence the problem is now fixed.

[rasmus.elmholt]

Great news. Thank you for the feedback.