MST Path Selection with Port Priority
SW1～SW4 spanning-tree mst configuration name MST revision 1 instance 1 vlan 1-100 instance 2 vlan 101-200 spanning-tree mode mst SW1 spanning-tree mst 1 priority 0 SW2 spanning-tree mst 1 priority 4096 I noticed that INE initial config shut down the link between SW1 and SW4, SW2 and SW3,
at this point. SW1～SW4 int range e0/2-3 shut SW1#sh span mst 1 ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0100 priority 1 (0 sysid 1) Root this switch for MST1 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Et0/0 Desg FWD 2000000 128.1 Shr Et0/1 Desg FWD 2000000 128.2 Shr Et1/0 Desg FWD 2000000 128.5 Shr Et1/1 Desg FWD 2000000 128.6 Shr Et1/2 Desg FWD 2000000 128.7 Shr Et1/3 Desg FWD 2000000 128.8 Shr SW2#sh span mst 1 ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0200 priority 4097 (4096 sysid 1) Root address aabb.cc00.0100 priority 1 (0 sysid 1) port Et1/0 cost 2000000 rem hops 19 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Et0/0 Desg FWD 2000000 128.1 Shr Et0/1 Desg FWD 2000000 128.2 Shr Et1/0 Root FWD 2000000 128.5 Shr Et1/1 Altn BLK 2000000 128.6 Shr Et1/2 Desg FWD 2000000 128.7 Shr Et1/3 Desg FWD 2000000 128.8 Shr SW3#sh span mst 1 ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0300 priority 32769 (32768 sysid 1) Root address aabb.cc00.0100 priority 1 (0 sysid 1) port Et0/0 cost 2000000 rem hops 19 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Et0/0 Root FWD 2000000 128.1 Shr Et0/1 Altn BLK 2000000 128.2 Shr Et1/0 Desg FWD 2000000 128.5 Shr Et1/1 Desg FWD 2000000 128.6 Shr Et1/2 Desg FWD 2000000 128.7 Shr Et1/3 Desg FWD 2000000 128.8 Shr SW4#sh span mst 1 ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0400 priority 32769 (32768 sysid 1) Root address aabb.cc00.0100 priority 1 (0 sysid 1) port Et0/0 cost 4000000 rem hops 18 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Et0/0 Root FWD 2000000 128.1 Shr Et0/1 Altn BLK 2000000 128.2 Shr Et1/0 Altn BLK 2000000 128.5 Shr Et1/1 Altn BLK 2000000 128.6 Shr Et1/2 Desg FWD 2000000 128.7 Shr Et1/3 Desg FWD 2000000 128.8 Shr SW4#sh span mst 1 detail ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0400 priority 32769 (32768 sysid 1) Root address aabb.cc00.0100 priority 1 (0 sysid 1) port Et0/0 cost 4000000 rem hops 18 Ethernet0/0 of MST1 is root forwarding Port info port id 128.1 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0200 priority 4097 port id 128.1 Timers: message expires in 4 sec, forward delay 0, forward transitions 1 Bpdus (MRecords) sent 59, received 34174 Ethernet0/1 of MST1 is alternate blocking Port info port id 128.2 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0200 priority 4097 port id 128.2 Timers: message expires in 4 sec, forward delay 0, forward transitions 0 Bpdus (MRecords) sent 44, received 34171 Ethernet1/0 of MST1 is alternate blocking Port info port id 128.5 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0300 priority 32769 port id 128.5 Timers: message expires in 5 sec, forward delay 0, forward transitions 0 Bpdus (MRecords) sent 41, received 34175 Ethernet1/1 of MST1 is alternate blocking Port info port id 128.6 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0300 priority 32769 port id 128.6 Timers: message expires in 5 sec, forward delay 0, forward transitions 0 Bpdus (MRecords) sent 24, received 34175 Ethernet1/2 of MST1 is designated forwarding Port info port id 128.7 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 4000000 Designated bridge address aabb.cc00.0400 priority 32769 port id 128.7 Timers: message expires in 0 sec, forward delay 0, forward transitions 3 Bpdus (MRecords) sent 34177, received 0 Ethernet1/3 of MST1 is designated forwarding Port info port id 128.8 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 4000000 Designated bridge address aabb.cc00.0400 priority 32769 port id 128.8 Timers: message expires in 0 sec, forward delay 0, forward transitions 3 Bpdus (MRecords) sent 34177, received 0 SW4 has two links towards SW2. The end-to-end cost of 4,000,000 is equal cost. Port ID is created the port priority and port number. The port priority of both interfaces e0/0 and e0/1 on SW2 are default value, 128. Because e0/0 has lower port number, it was elected to Root Port.
Change port priority value of interface e0/1 on SW2 into 0. SW2(config)#int e0/1 SW2(config-if)#spanning-tree mst 1 port-priority 0 SW4#sh span mst 1 ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0400 priority 32769 (32768 sysid 1) Root address aabb.cc00.0100 priority 1 (0 sysid 1) port Et0/1 cost 4000000 rem hops 18 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Et0/0 Altn BLK 2000000 128.1 Shr Et0/1 Root FWD 2000000 128.2 Shr Et1/0 Altn BLK 2000000 128.5 Shr Et1/1 Altn BLK 2000000 128.6 Shr Et1/2 Desg BLK 2000000 128.7 Shr Et1/3 Desg BLK 2000000 128.8 Shr Root Port on SW4 was changed interface e0/1. SW4#sh span mst 1 detail ##### MST1 vlans mapped: 1-100 Bridge address aabb.cc00.0400 priority 32769 (32768 sysid 1) Root address aabb.cc00.0100 priority 1 (0 sysid 1) port Et0/1 cost 4000000 rem hops 18 Ethernet0/0 of MST1 is alternate blocking Port info port id 128.1 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0200 priority 4097 port id 128.1 Timers: message expires in 4 sec, forward delay 0, forward transitions 1 Bpdus (MRecords) sent 59, received 34344 Ethernet0/1 of MST1 is root forwarding Port info port id 128.2 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0200 priority 4097 port id 0.2 ~~~ Timers: message expires in 4 sec, forward delay 0, forward transitions 1 Bpdus (MRecords) sent 62, received 34340 Ethernet1/0 of MST1 is alternate blocking Port info port id 128.5 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0300 priority 32769 port id 128.5 Timers: message expires in 5 sec, forward delay 0, forward transitions 0 Bpdus (MRecords) sent 41, received 34345 Ethernet1/1 of MST1 is alternate blocking Port info port id 128.6 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 2000000 Designated bridge address aabb.cc00.0300 priority 32769 port id 128.6 Timers: message expires in 5 sec, forward delay 0, forward transitions 0 Bpdus (MRecords) sent 24, received 34345 Ethernet1/2 of MST1 is designated forwarding Port info port id 128.7 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 4000000 Designated bridge address aabb.cc00.0400 priority 32769 port id 128.7 Timers: message expires in 0 sec, forward delay 0, forward transitions 4 Bpdus (MRecords) sent 34348, received 0 Ethernet1/3 of MST1 is designated forwarding Port info port id 128.8 priority 128 cost 2000000 Designated root address aabb.cc00.0100 priority 1 cost 4000000 Designated bridge address aabb.cc00.0400 priority 32769 port id 128.8 Timers: message expires in 0 sec, forward delay 0, forward transitions 4 Bpdus (MRecords) sent 34348, received 0
The end-to-end costs remain at 4,000,000,but Port ID on e0/1 is 0.2, it is lower value than interface e0/0 because the port priority of e0/1 changed. So e0/1 was elected as root port.
