Why did switch become a root bridge?

dtekaccount10 · ‎12-29-2021

We have two Catalyst 4500-x switches connected to each other, LAN-CORE-A switch is a root for 270 vlan and LAN-CORE-B has a root port interface Te1/16 connected to LAN-CORE-A. Suddenly LAN-CORE-B became a root for vlan 270, how is it possible if we see in logs that LAN-CORE-B received bpdu from LAN-CORE-A? log output is following:

2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:31: RSTP(270): sending BPDU out Te1/10
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: VLAN0270 rx BPDU: config protocol = rstp, packet from TenGigabitEthernet1/16 , linktype SSTP , enctype 3, encsize 22
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: enc 01 00 0C CC CC CD 18 8B 9D A8 9A CF 00 32 AA AA 03 00 00 0C 01 0B
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: Data 000002023C610E188B9DA89AC000000000610E188B9DA89AC080100000140002000F00
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: VLAN0270 Te1/16:0000 02 02 3C 610E188B9DA89AC0 00000000 610E188B9DA89AC0 8010 0000 1400 0200 0F00
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: RSTP(270): Te1/16 repeated msg
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: RSTP(270): Te1/16 rcvd info remaining 6
2021-12-29 13:33:32 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: VLAN0270 rx BPDU: config protocol = rstp, packet from TenGigabitEthernet1/16 , linktype SSTP ,
2021-12-29 13:33:33 local7.debug 172.31.138.12 enctype 3, encsize 22
2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: enc 01 00 0C CC CC CD 18 8B 9D A8 9A CF 00 32 AA AA 03 00 00 0C 01 0B
2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: Data 000002023C610E188B9DA89AC000000000610E188B9DA89AC080100000140002000F00
2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: STP: VLAN0270 Te1/16:0000 02 02 3C 610E188B9DA89AC0 00000000 610E188B9DA89AC0 8010 0000 1400 0200 0F00
2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: RSTP(270): Te1/16 repeated msg
2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: RSTP(270): Te1/16 rcvd info remaining 6
2021-12-29 13:33:34 local7.debug 172.31.138.12 Dec 29 13:33:33: RSTP(270): sending BPDU out Te1/10
2021-12-29 13:33:36 local7.debug 172.31.138.12 Dec 29 13:33:35: RSTP(270): sending BPDU out Te1/10
2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:37: RSTP(270): sending BPDU out Te1/10
2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:38: RSTP(270): Te1/16 rcvd info expired
2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:38: RSTP(270): updt roles, information on root port Te1/16 expired
2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:38: RSTP(270): we become the root bridge

marce1000 · ‎12-29-2021

- Post the output of : Switch# show spanning-tree vlan 270

M.

-- Each morning when I wake up and look into the mirror I always say ' Why am I so brilliant ? '
When the mirror will then always repond to me with ' The only thing that exceeds your brilliance is your beauty! '

MHM Cisco World · ‎12-30-2021

the SW is add as new ?
do you config any Root Priority or UplinkFast ?

paul driver · ‎12-31-2021

@dtekaccount10 wrote:

We have two Catalyst 4500-x switches connected to each other, LAN-CORE-A switch is a root for 270 vlan and LAN-CORE-B has a root port interface Te1/16 connected to LAN-CORE-A. Suddenly LAN-CORE-B became a root for vlan 270,

2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: RSTP(270): Te1/16 repeated msg
2021-12-29 13:33:33 local7.debug 172.31.138.12 Dec 29 13:33:32: RSTP(270): Te1/16 rcvd info remaining 6

2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:38: RSTP(270): Te1/16 rcvd info expired
2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:38: RSTP(270): updt roles, information on root port Te1/16 expired
2021-12-29 13:33:38 local7.debug 172.31.138.12 Dec 29 13:33:38: RSTP(270): we become the root bridge

Looks like designated port Te1/16 switch A failed to send bpdus, Switch B root port Te1/16 didn't receive bpdu's so its timed out hence the stp convergence and switch B became root for that vlan

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Kind Regards
Paul

MHM Cisco World · ‎12-31-2021

Root port is expired, that mean that RP of SW-B don't see any more the BPDU from Root bridge?
so we must seek the topology change.