Technical Tip: How to elect the Root Bridge by adjusting the priority

[ehamud]

Description	This article describes how to use any other FortiSwitch as a Root bridge. It can be done when the FortiSwitch is managed by FortiGate or by Standalone.
Scope	FortiSwitch.
Solution	Sometimes, it is necessary to elect another Switch as a Root Bridge, this particular case is most related to when there are Standalone FortiSwitches, but the procedure is the same even though FortiGate is used with FortiLink. This is a demonstration of how to do it, but when FortiGate is under the topology, the recommendation about the Root Bridge is always that the root will be the one connected directly to the FortiGate. Now the topology is:        This FortiSwitch S224EPTF18-----1 is the Root Bridge. It is possible to see the Burn MAC and its priority now is set to 24576 under the two instances 0 and 15. S224EPTF18-----1 # diagnose stp instance list | grep  -e Instance ID 0 -e Regional Root MAC e81cba853884 (CLI):MST Instance Information, primary-Channel: (CLI):Instance ID 0 (CST) (CLI):  Regional Root  MAC e81cba853884, Priority 24576, Path Cost 0 (CLI):Instance ID 15 (CLI):  Regional Root  MAC e81cba853884, Priority 24576, Path Cost 0 S224EPTF18004511 # get system status | grep Burn Burn in MAC: e8:1c:ba:85:38:84 The FortiSwitch S248EPTF19-----0 the Root Bridge also is reflected under this FortiSwitch e81cba853884, but it has as a Priority 28672 this number is higher versus 24576: S248EPTF19-----0 # diagnose stp instance list | grep  -e Priority -e Instance ID 0 -e Regional Root MAC (CLI):MST Instance Information, primary-Channel: (CLI):Instance ID 0 (CST) (CLI):  Config         Priority 28672 (CLI):  Root           MAC e81cba853884, Priority 24576, Path Cost 0, Remaining Hops 19 CLI):Instance ID 15 (CLI):  Config         Priority 28672 , VLANs 4094 (CLI):  Regional Root  MAC e81cba853884, Priority 24576, Path Cost 1, Root Port 4EPTF18004511-0 (CLI):  Port               Speed   Cost       Priority   Role         State        Flags S248EPTF19-----0 # get system status | grep Burn Burn in MAC: 04:d5:90:3c:26:b2 The last FortiSwitch S248EPTF19-----1 has the same behavior with priority 28672 and identifies its Root Bridge e81cba853884: S248EPTF19-----1 # diagnose stp instance list | grep  -e Priority -e Instance ID 0 -e Regional Root MAC (CLI):MST Instance Information, primary-Channel: (CLI):Instance ID 0 (CST) (CLI):  Config         Priority 28672 (CLI):  Root           MAC e81cba853884, Priority 24576, Path Cost 0, Remaining Hops 19 (CLI):  Regional Root  MAC e81cba853884, Priority 24576, Path Cost 1, Root Port 4EPTF18004511-0 (CLI):  Port               Speed   Cost       Priority   Role         State        HelloTime  Flags (CLI):Instance ID 15 (CLI):  Config         Priority 28672 , VLANs 4094 (CLI):  Regional Root  MAC e81cba853884, Priority 24576, Path Cost 1, Root Port 4EPTF18004511-0 (CLI):  Port               Speed   Cost       Priority   Role         State        Flags S248EPTF19-----1 # get system status | grep Burn Burn in MAC: 04:d5:90:10:a8:18 Now let's convert S248EPTF19-----1 to the Root Bridge giving a lower priority 16384 in comparison with S224EPTF18-----1 which has 24576: S248EPTF19-----1 # config switch stp instance S248EPTF19-----1 (instance) # edit 0 S248EPTF19-----1 (0) # set priority 0        0. 12288    12288. 16384    16384. 20480    20480. 24576    24576. 28672    28672. 32768    32768. 36864    36864. 4096     4096. 40960    40960. 45056    45056. 49152    49152. 53248    53248. 57344    57344. 61440    61440. 8192     8192. S248EPTF19-----1 (0) # set priority 16384 S248EPTF19-----1 (0) # end Let’s see the result: S248EPTF19-----1 # diagnose stp instance list MST Instance Information, primary-Channel: Instance ID 0 (CST)   Config         Priority 16384                  Bridge MAC 04d59010a818, MD5 Digest 742f04075e42ee2d6606ac1b87d85cb4   Root           MAC 04d59010a818, Priority 16384, Path Cost 0, Remaining Hops 20                  (This bridge is the root) But!! The instance 15 is still is under the command of e81cba853884 Switch as its Root Bridge: Instance ID 15   Config         Priority 28672 , VLANs 4094                  Bridge MAC 04d59010a818   Regional Root  MAC e81cba853884, Priority 24576, Path Cost 1, Root Port 4EPTF18004511-0 Now let’s see the topology again, the port 41 now is under Discarding State: It is possible to do the same with any instance, possible to include more independent Instances and change the priority as explain in this related document: Configuring STP settings

[laltuzar]

Great post! It is also possible to configure STP priority with custom commands on FortiGate:
https://community.fortinet.com/t5/FortiSwitch/Technical-Tip-Configure-STP-priority-using-custom-comm...