Troubleshooting Tip: STP Convergence Between FortiSwitch and Cisco Switches (MSTP and RPVST+)
| Description | This article provides troubleshooting steps for Spanning Tree Protocol (STP) convergence issues between FortiSwitch and Cisco switches. |
| Scope | This applies to FortiSwitch devices. |
| Solution | Problem: STP convergence issues can occur when connecting FortiSwitches configured with Multiple Spanning Tree Protocol (MSTP) to Cisco switches running Rapid Per-VLAN Spanning Tree Plus (RPVST+). Example Scenario: A common scenario involves two FortiSwitches connected to a Cisco switch. Specifically: Cisco switch ports FastEthernet0/1 and FastEthernet0/5 are connected to port 1 on each FortiSwitch. A Link Aggregation Group (LAG1) connects the two FortiSwitches. Below are the configurations on the Cisco and FortiSwitch side: Cisco: spanning-tree mode rapid-pvst spanning-tree extend system-id spanning-tree vlan 1,10,20 priority 0 ! vlan internal allocation policy ascending ! interface FastEthernet0/1 switchport trunk encapsulation dot1q switchport trunk allowed vlan 10,20 switchport mode trunk ! interface FastEthernet0/5 switchport trunk encapsulation dot1q switchport trunk allowed vlan 10,20 switchport mode trunk FortiSwitch: show switch interface LAG-1 config switch interface edit "LAG-1" set allowed-vlans 10,20 set edge-port disabled set snmp-index 31 next end show switch interface port1 config switch interface edit "port1" set allowed-vlans 10,20 set edge-port disabled set rpvst-port enabled set snmp-index 1 next end The above configurations often lead to STP convergence issues because of the incompatibility between MSTP (on the FortiSwitches) and RPVST+ (on the Cisco switch). Solution: To achieve proper STP convergence, the following approaches can be taken, depending on which device should be the root bridge: Case 1: Cisco Switch as Root Bridge: If the Cisco switch should be the root bridge, its VLAN 1 STP priority must be higher than the priorities of other VLANs on the Cisco switch. The following configurations should be applied to the Cisco switch: Below should be the configurations on Cisco and FortiSwitch: Cisco: Switch#show running-config | include spanning spanning-tree mode rapid-pvst spanning-tree extend system-id spanning-tree vlan 1 priority 4096 spanning-tree vlan 10,20 priority 0 Switch#show running-config interface fastEthernet 0/1 Building configuration... Current configuration : 133 bytes ! interface FastEthernet0/1 switchport trunk encapsulation dot1q switchport trunk allowed vlan 1,10,20 switchport mode trunk end Switch#show running-config interface fastEthernet 0/5 Building configuration... Current configuration : 133 bytes ! interface FastEthernet0/5 switchport trunk encapsulation dot1q switchport trunk allowed vlan 1,10,20 switchport mode trunk end
Cisco convergence outputs:
Switch#show spanning-tree VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 4097 Address 0014.a859.c880 <------- Root Bridge MAC-address. 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 0014.a859.c880 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/1 Desg LRN 19 128.1 Shr Fa0/5 Desg BLK 19 128.5 P2p VLAN0010 Spanning tree enabled protocol rstp Root ID Priority 10 Address 0014.a859.c880 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 10 (priority 0 sys-id-ext 10) Address 0014.a859.c880 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/1 Desg FWD 19 128.1 Shr Fa0/5 Desg FWD 19 128.5 P2p FortiSwitch convergence outputs: SW1 # diagnose stp instance list MST Instance Information, primary-Channel: Instance ID 0 (CST) Config Priority 24576 Bridge MAC 80802c0a7364, MD5 Digest ac36177f50283cd4b83821d8ab26de62 Root MAC 0014a859c880, Priority 4097, Path Cost 200000, Remaining Hops 20 <-- Root Bridge MAC. Regional Root MAC 80802c0a7364, Priority 24576, Path Cost 0, Root Port port1 Active Times Forward Time 15, Max Age 20, Remaining Hops 20 TCN Events Triggered 5 (0d 0h 8m 15s ago), Received 20 (0d 0h 0m 58s ago) Port Speed Cost Priority Role State HelloTime Flags ________________ ______ _________ _________ ___________ __________ _________ _______________ port1 100M 200000 128 ROOT FORWARDING 2 EN port2 - 200000000 128 DISABLED DISCARDING 2 ED port3 - 200000000 128 DISABLED DISCARDING 2 ED port4 - 200000000 128 DISABLED DISCARDING 2 ED port5 - 200000000 128 DISABLED DISCARDING 2 ED port6 - 200000000 128 DISABLED DISCARDING 2 ED port7 - 200000000 128 DISABLED DISCARDING 2 ED port8 - 200000000 128 DISABLED DISCARDING 2 ED port9 - 200000000 128 DISABLED DISCARDING 2 ED port10 - 200000000 128 DISABLED DISCARDING 2 ED port13 - 200000000 128 DISABLED DISCARDING 2 ED port14 - 200000000 128 DISABLED DISCARDING 2 ED port15 - 200000000 128 DISABLED DISCARDING 2 ED port16 - 200000000 128 DISABLED DISCARDING 2 ED port17 - 200000000 128 DISABLED DISCARDING 2 ED port18 - 200000000 128 DISABLED DISCARDING 2 ED port19 - 200000000 128 DISABLED DISCARDING 2 ED port20 - 200000000 128 DISABLED DISCARDING 2 ED port21 - 200000000 128 DISABLED DISCARDING 2 ED port22 - 200000000 128 DISABLED DISCARDING 2 ED port23 - 200000000 128 DISABLED DISCARDING 2 ED port24 - 200000000 128 DISABLED DISCARDING 2 ED port25 - 200000000 128 DISABLED DISCARDING 2 ED port26 - 200000000 128 DISABLED DISCARDING 2 ED port27 - 200000000 128 DISABLED DISCARDING 2 ED port28 - 200000000 128 DISABLED DISCARDING 2 ED internal 1G 20000 128 DESIGNATED FORWARDING 2 ED LAG-1 100M 200000 128 DESIGNATED FORWARDING 2 EN Flags: EN(STP enable), ED(Edge), LP(Loop Protection Triggered) RG(Root Guard Triggered), BG(BPDU Guard Triggered), IC(PVST Port Inconsistent) MV(PVST Port Vlan Mismatch) SW2 # diagnose stp instance list MST Instance Information, primary-Channel: Instance ID 0 (CST) Config Priority 28672 Bridge MAC 704ca57937ac, MD5 Digest ac36177f50283cd4b83821d8ab26de62 Root MAC 0014a859c880, Priority 4097, Path Cost 200000, Remaining Hops 20 <-- Root Bridge MAC. Regional Root MAC 704ca57937ac, Priority 28672, Path Cost 0, Root Port port1 Active Times Forward Time 15, Max Age 20, Remaining Hops 20 TCN Events Triggered 4 (0d 0h 9m 14s ago), Received 42 (0d 0h 0m 57s ago) Port Speed Cost Priority Role State HelloTime Flags ________________ ______ _________ _________ ___________ __________ _________ _______________ port1 100M 200000 128 ROOT FORWARDING 2 EN port2 - 200000000 128 DISABLED DISCARDING 2 ED port3 - 200000000 128 DISABLED DISCARDING 2 ED port4 - 200000000 128 DISABLED DISCARDING 2 ED port5 - 200000000 128 DISABLED DISCARDING 2 ED port6 - 200000000 128 DISABLED DISCARDING 2 ED port7 - 200000000 128 DISABLED DISCARDING 2 ED port8 - 200000000 128 DISABLED DISCARDING 2 ED port9 - 200000000 128 DISABLED DISCARDING 2 ED port10 - 200000000 128 DISABLED DISCARDING 2 ED port13 - 200000000 128 DISABLED DISCARDING 2 ED port14 - 200000000 128 DISABLED DISCARDING 2 port15 - 200000000 128 DISABLED DISCARDING 2 ED port16 - 200000000 128 DISABLED DISCARDING 2 ED port17 - 200000000 128 DISABLED DISCARDING 2 ED port18 - 200000000 128 DISABLED DISCARDING 2 ED port19 - 200000000 128 DISABLED DISCARDING 2 ED port20 - 200000000 128 DISABLED DISCARDING 2 ED port21 - 200000000 128 DISABLED DISCARDING 2 ED port22 - 200000000 128 DISABLED DISCARDING 2 ED port23 - 200000000 128 DISABLED DISCARDING 2 ED port24 - 200000000 128 DISABLED DISCARDING 2 ED port25 - 200000000 128 DISABLED DISCARDING 2 ED port26 - 200000000 128 DISABLED DISCARDING 2 ED port27 - 200000000 128 DISABLED DISCARDING 2 ED port28 - 200000000 128 DISABLED DISCARDING 2 ED internal 1G 20000 128 DESIGNATED FORWARDING 2 ED Lag-1 100M 200000 128 ALTERNATIVE DISCARDING 2 EN Flags: EN(STP enable), ED(Edge), LP(Loop Protection Triggered) RG(Root Guard Triggered), BG(BPDU Guard Triggered), IC(PVST Port Inconsistent) MV(PVST Port Vlan Mismatch) Case 2: FortiSwitch as Root Bridge. If a FortiSwitch should be the root bridge, the Cisco switch's VLAN 1 priority must be lower than the priorities of other VLANs on the Cisco switch. Additionally, the FortiSwitch's STP priority should be lower (lower numerical value) than the Cisco Switch's priority. Cisco Switch Configurations: Switch#show running-config | include spanning spanning-tree mode rapid-pvst spanning-tree extend system-id spanning-tree vlan 1 priority 4096 spanning-tree vlan 10,20 priority 8192 FortiSwitch Configurations: config switch stp instance edit "0" set priority 0 end Convergence on FortiSwitch: Switch-1: diagnose stp instance list MST Instance Information, primary-Channel: Instance ID 0 (CST) Config Priority 0 Bridge MAC 704ca57937ac, MD5 Digest ac36177f50283cd4b83821d8ab26de62 Root MAC 704ca57937ac, Priority 0, Path Cost 0, Remaining Hops 20 <- Root bridge mac-address 704ca57937ac. (This bridge is the root) Regional Root MAC 704ca57937ac, Priority 0, Path Cost 0 (This bridge is the regional root) Active Times Forward Time 15, Max Age 20, Remaining Hops 20 TCN Events Triggered 12 (0d 0h 0m 1s ago), Received 139 (0d 0h 0m 39s ago) Port Speed Cost Priority Role State HelloTime Flags ________________ ______ _________ _________ ___________ __________ _________ _______________ port1 100M 200000 128 DESIGNATED FORWARDING 2 EN port2 - 200000000 128 DISABLED DISCARDING 2 ED port3 - 200000000 128 DISABLED DISCARDING 2 ED port4 - 200000000 128 DISABLED DISCARDING 2 ED port5 - 200000000 128 DISABLED DISCARDING 2 ED port6 - 200000000 128 DISABLED DISCARDING 2 ED port7 - 200000000 128 DISABLED DISCARDING 2 ED port8 - 200000000 128 DISABLED DISCARDING 2 ED port9 - 200000000 128 DISABLED DISCARDING 2 ED port10 - 200000000 128 DISABLED DISCARDING 2 ED port13 - 200000000 128 DISABLED DISCARDING 2 ED port14 - 200000000 128 DISABLED DISCARDING 2 port15 - 200000000 128 DISABLED DISCARDING 2 ED port16 - 200000000 128 DISABLED DISCARDING 2 ED port17 - 200000000 128 DISABLED DISCARDING 2 ED port18 - 200000000 128 DISABLED DISCARDING 2 ED port19 - 200000000 128 DISABLED DISCARDING 2 ED port20 - 200000000 128 DISABLED DISCARDING 2 ED port21 - 200000000 128 DISABLED DISCARDING 2 ED port22 - 200000000 128 DISABLED DISCARDING 2 ED port23 - 200000000 128 DISABLED DISCARDING 2 ED port24 - 200000000 128 DISABLED DISCARDING 2 ED port25 - 200000000 128 DISABLED DISCARDING 2 ED port26 - 200000000 128 DISABLED DISCARDING 2 ED port27 - 200000000 128 DISABLED DISCARDING 2 ED port28 - 200000000 128 DISABLED DISCARDING 2 ED internal 1G 20000 128 DESIGNATED FORWARDING 2 ED Lag-1 100M 200000 128 DESIGNATED FORWARDING 2 EN Flags: EN(STP enable), ED(Edge), LP(Loop Protection Triggered) RG(Root Guard Triggered), BG(BPDU Guard Triggered), IC(PVST Port Inconsistent) MV(PVST Port Vlan Mismatch) Switch-2: diagnose stp instance list MST Instance Information, primary-Channel: Instance ID 0 (CST) Config Priority 0 Bridge MAC 80802c0a7364, MD5 Digest ac36177f50283cd4b83821d8ab26de62 Root MAC 704ca57937ac, Priority 0, Path Cost 200000, Remaining Hops 20 <---- Root bridge mac-address 704ca57937ac Regional Root MAC 80802c0a7364, Priority 0, Path Cost 0, Root Port LAG-1 Active Times Forward Time 15, Max Age 20, Remaining Hops 20 TCN Events Triggered 10 (0d 0h 19m 9s ago), Received 59 (0d 0h 1m 5s ago) Port Speed Cost Priority Role State HelloTime Flags ________________ ______ _________ _________ ___________ __________ _________ _______________ port1 100M 200000 128 ALTERNATIVE DISCARDING 2 EN port2 - 200000000 128 DISABLED DISCARDING 2 ED port3 - 200000000 128 DISABLED DISCARDING 2 ED port4 - 200000000 128 DISABLED DISCARDING 2 ED port5 - 200000000 128 DISABLED DISCARDING 2 ED port6 - 200000000 128 DISABLED DISCARDING 2 ED port7 - 200000000 128 DISABLED DISCARDING 2 ED port8 - 200000000 128 DISABLED DISCARDING 2 ED port9 - 200000000 128 DISABLED DISCARDING 2 ED port10 - 200000000 128 DISABLED DISCARDING 2 ED port13 - 200000000 128 DISABLED DISCARDING 2 ED port14 - 200000000 128 DISABLED DISCARDING 2 ED port15 - 200000000 128 DISABLED DISCARDING 2 ED port16 - 200000000 128 DISABLED DISCARDING 2 ED port17 - 200000000 128 DISABLED DISCARDING 2 ED port18 - 200000000 128 DISABLED DISCARDING 2 ED port19 - 200000000 128 DISABLED DISCARDING 2 ED port20 - 200000000 128 DISABLED DISCARDING 2 ED port21 - 200000000 128 DISABLED DISCARDING 2 ED port22 - 200000000 128 DISABLED DISCARDING 2 ED port23 - 200000000 128 DISABLED DISCARDING 2 ED port24 - 200000000 128 DISABLED DISCARDING 2 ED port25 - 200000000 128 DISABLED DISCARDING 2 ED port26 - 200000000 128 DISABLED DISCARDING 2 ED port27 - 200000000 128 DISABLED DISCARDING 2 ED port28 - 200000000 128 DISABLED DISCARDING 2 ED internal 1G 20000 128 DESIGNATED FORWARDING 2 ED LAG-1 100M 200000 128 ROOT FORWARDING 2 EN Flags: EN(STP enable), ED(Edge), LP(Loop Protection Triggered) RG(Root Guard Triggered), BG(BPDU Guard Triggered), IC(PVST Port Inconsistent) MV(PVST Port Vlan Mismatch) Cisco Convergence: Switch#show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 0 Address 704c.a579.37ac <---- Root bridge mac-address 704ca57937ac. Cost 19 Port 1 (FastEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 4097 (priority 4096 sys-id-ext 1) Address 0014.a859.c880 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/1 Root FWD 19 128.1 Shr Fa0/5 Desg FWD 19 128.5 P2p Switch# Switch# Switch#show spanning-tree vlan 10 VLAN0010 Spanning tree enabled protocol rstp Root ID Priority 0 Address 704c.a579.37ac Cost 19 Port 1 (FastEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 8202 (priority 8192 sys-id-ext 10) Address 0014.a859.c880 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/1 Root FWD 19 128.1 Shr Fa0/5 Desg FWD 19 128.5 P2p Switch#show spanning-tree vlan 20 VLAN0020 Spanning tree enabled protocol rstp Root ID Priority 0 Address 704c.a579.37ac Cost 19 Port 1 (FastEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 8212 (priority 8192 sys-id-ext 20) Address 0014.a859.c880 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/1 Root FWD 19 128.1 Shr Fa0/5 Desg FWD 19 128.5 P2p |
