Hi All,
I have the following setup in CML:
- SW1 is the Root Birdge
- I have VLANs 1-20
- All Switches have the same MST configuration and running in the same region.
When I shut down interface gi1/0/3 on SW3 the MST environment converges and uses Gi1/0/6 as the root port for some time until it falls back to Po1.
I cannot explain why as the internal cost is higher than using Po1.
Relevant configuration on SW3:
spanning-tree mode mst spanning-tree extend system-id spanning-tree mst configuration name CiscoSTD revision 1 interface GigabitEthernet1/0/2 switchport mode trunk channel-group 1 mode active interface GigabitEthernet1/0/3 switchport mode trunk channel-group 1 mode active interface GigabitEthernet1/0/6 switchport access vlan 20 switchport mode access spanning-tree portfast interface GigabitEthernet1/0/7 switchport access vlan 20 switchport mode access spanning-tree portfast interface Port-channel1 switchport mode trunk spanning-tree mst 0 cost 10000
Debug when I shut down Gi1/0/3:
SW3(config-if)#do show span mst 0
##### MST0 vlans mapped: 1-4094
Bridge address 5254.0002.8506 priority 32768 (32768 sysid 0)
Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
port Po1 path cost 0
Regional Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
internal cost 30000 rem hops 18
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 20000 128.1 P2p
Gi1/0/4 Desg FWD 20000 128.4 P2p
Gi1/0/5 Desg FWD 20000 128.5 P2p
Gi1/0/6 Altn BLK 20000 128.6 P2p
Gi1/0/7 Altn BLK 20000 128.7 P2p
Gi1/0/8 Desg FWD 20000 128.8 P2p
Po1 Root FWD 10000 128.2281 P2p
SW3(config-if)#int gi 1/0/3
SW3(config-if)#shut
SW3(config-if)#
*Jul 16 08:06:17.403: %LINK-5-CHANGED: Interface GigabitEthernet1/0/3, changed state to administratively down
*Jul 16 08:06:18.402: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/3, changed state to down
*Jul 16 08:06:19.764: MST[0]: Po1 state change forwarding -> blocking
*Jul 16 08:06:19.765: MST[0]: Gi1/0/1 state change forwarding -> blocking
*Jul 16 08:06:19.765: MST[0]: Gi1/0/4 state change forwarding -> blocking
*Jul 16 08:06:19.765: MST[0]: Gi1/0/5 state change forwarding -> blocking
*Jul 16 08:06:19.765: MST[0]: Gi1/0/8 state change forwarding -> blocking
*Jul 16 08:06:19.765: MST[0]: Gi1/0/6 state change blocking -> forwarding
*Jul 16 08:06:19.767: STP[0]: Generating TC trap for port GigabitEthernet1/0/6
SW3(config-if)#do show span mst 0
##### MST0 vlans mapped: 1-4094
Bridge address 5254.0002.8506 priority 32768 (32768 sysid 0)
Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
port Gi1/0/6 path cost 0
Regional Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
internal cost 50000 rem hops 17
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg BLK 20000 128.1 P2p
Gi1/0/4 Desg BLK 20000 128.4 P2p
Gi1/0/5 Desg BLK 20000 128.5 P2p
Gi1/0/6 Root FWD 20000 128.6 P2p
Gi1/0/7 Altn BLK 20000 128.7 P2p
Gi1/0/8 Desg BLK 20000 128.8 P2p
Po1 Desg BLK 10000 128.2281 P2p
SW3(config-if)#
*Jul 16 08:06:34.765: MST[0]: Gi1/0/1 state change blocking -> learning
*Jul 16 08:06:34.765: MST[0]: Gi1/0/4 state change blocking -> learning
*Jul 16 08:06:34.765: MST[0]: Gi1/0/5 state change blocking -> learning
*Jul 16 08:06:34.765: MST[0]: Gi1/0/8 state change blocking -> learning
*Jul 16 08:06:34.765: MST[0]: Po1 state change blocking -> learning
SW3(config-if)#do show span mst 0
##### MST0 vlans mapped: 1-4094
Bridge address 5254.0002.8506 priority 32768 (32768 sysid 0)
Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
port Gi1/0/6 path cost 0
Regional Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
internal cost 50000 rem hops 17
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg LRN 20000 128.1 P2p
Gi1/0/4 Desg LRN 20000 128.4 P2p
Gi1/0/5 Desg LRN 20000 128.5 P2p
Gi1/0/6 Root FWD 20000 128.6 P2p
Gi1/0/7 Altn BLK 20000 128.7 P2p
Gi1/0/8 Desg LRN 20000 128.8 P2p
Po1 Desg LRN 10000 128.2281 P2p
SW3(config-if)#
*Jul 16 08:06:49.765: MST[0]: Gi1/0/1 state change learning -> forwarding
*Jul 16 08:06:49.765: MST[0]: Gi1/0/4 state change learning -> forwarding
*Jul 16 08:06:49.765: MST[0]: Gi1/0/5 state change learning -> forwarding
*Jul 16 08:06:49.765: MST[0]: Gi1/0/8 state change learning -> forwarding
*Jul 16 08:06:49.765: MST[0]: Po1 state change learning -> forwarding
*Jul 16 08:06:49.767: STP[0]: Generating TC trap for port Port-channel1
*Jul 16 08:07:36.649: MST[0]: Gi1/0/6 state change forwarding -> blocking
*Jul 16 08:07:36.649: MST[0]: Gi1/0/1 state change forwarding -> blocking
*Jul 16 08:07:36.649: MST[0]: Gi1/0/4 state change forwarding -> blocking
*Jul 16 08:07:36.649: MST[0]: Gi1/0/5 state change forwarding -> blocking
*Jul 16 08:07:36.649: MST[0]: Gi1/0/8 state change forwarding -> blocking
*Jul 16 08:07:36.650: STP[0]: Generating TC trap for port Port-channel1
*Jul 16 08:07:51.651: MST[0]: Gi1/0/1 state change blocking -> learning
*Jul 16 08:07:51.651: MST[0]: Gi1/0/4 state change blocking -> learning
*Jul 16 08:07:51.651: MST[0]: Gi1/0/5 state change blocking -> learning
*Jul 16 08:07:51.651: MST[0]: Gi1/0/8 state change blocking -> learning
*Jul 16 08:08:06.652: MST[0]: Gi1/0/1 state change learning -> forwarding
*Jul 16 08:08:06.652: MST[0]: Gi1/0/4 state change learning -> forwarding
*Jul 16 08:08:06.652: MST[0]: Gi1/0/5 state change learning -> forwarding
*Jul 16 08:08:06.652: MST[0]: Gi1/0/8 state change learning -> forwarding
SW3(config-if)#
SW3(config-if)#do show span mst 0
##### MST0 vlans mapped: 1-4094
Bridge address 5254.0002.8506 priority 32768 (32768 sysid 0)
Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
port Po1 path cost 0
Regional Root address 5254.0009.3a34 priority 24576 (24576 sysid 0)
internal cost 30000 rem hops 18
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 20000 128.1 P2p
Gi1/0/4 Desg FWD 20000 128.4 P2p
Gi1/0/5 Desg FWD 20000 128.5 P2p
Gi1/0/6 Altn BLK 20000 128.6 P2p
Gi1/0/7 Altn BLK 20000 128.7 P2p
Gi1/0/8 Desg FWD 20000 128.8 P2p
Po1 Root FWD 10000 128.2281 P2p
SW3(config-if)#
At first I thought the issues was that the cost on po1 changed and MST had to do a recalculation, but I have manually configured it to 10.000 in the lab. And I still see the same issue.
I know it seems strange that we have SW3 and SW4 connected directly but IRL we have them connected to a production ring of siemens switches.
Can someone explain why the MST tree does not stay stable when I shut down one port in the LAG?
