noticing port-flapping problems over multiple switches

RobertL
RobertL Posts: 5  Freshman Member
First Comment
in Switch

hello everyone,

we have 1 XGS1930-28HP and 3 XMG1915-18EP and we started noticing people getting kicked off the AP's. we thought at first it was a 802.1x authentication issue as it seemed the handshake would fail, but now we are seeing rapid up/logs in the nebula event logs, for specific ports, over 3 different switches.

the flapping seems to be almost random, both on which switch, port and time it occurs, and the effects can last for hours. the results are also very inconsistent, some people are able to connect, some are not. some people have a decent experience with network speeds over 100Mb/s, some are not even able to get 10Mb/s at the same time on the same AP, the AP's are NWA130BE btw.

all of them are running the latest firmware, RSTP is enabled with the root bridge being the main switch with priority of 16k. CPU and memory are below 30% usage, temps are fine and fans are reported green, POE is set up for 802.3at and is below 50w on each.

any idea on how to solve this?

EVENT LOG Secondary switch:
2025-08-29 10:28:25NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:28:24DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link up 2.5G/F
2025-08-29 10:28:18NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:28:17DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link down
2025-08-29 10:27:42NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:27:41DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link up 2.5G/F
2025-08-29 10:27:38DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link down
2025-08-29 10:27:38NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:27:35DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link up 2.5G/F
2025-08-29 10:27:35NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:27:31NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:27:30DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link down
2025-08-29 10:27:20DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link up 2.5G/F
2025-08-29 10:27:20NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:27:01NoticeItvitae-SER-BG-02systemSystemPethPse Port 2 - Port2 OnOff Trap, Port Detection Status is Delivering Power
2025-08-29 10:27:00InformationItvitae-SER-BG-02systemSystemSave system configuration
2025-08-29 10:26:58NoticeItvitae-SER-BG-02systemSystemPethPse Port 2 - Port2 OnOff Trap, Port Detection Status is Disabled
2025-08-29 10:26:57DebugItvitae-SER-BG-02interfaceInterfacePort 2 - Port2 link down
2025-08-29 10:26:57NoticeItvitae-SER-BG-02stpSwitchSTP topology changes
2025-08-29 10:13:41InformationItvitae-SER-BG-02systemSystemSave system configuration

Event log Main switch:
2025-08-29 10:11:09NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:11:08DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link up 1G/F
2025-08-29 10:11:03NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:11:02DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link down
2025-08-29 10:10:26NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:10:25DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link up 1G/F
2025-08-29 10:10:24NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:10:23DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link down
2025-08-29 10:10:20NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:10:19DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link up 1G/F
2025-08-29 10:10:17NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:10:16DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link down
2025-08-29 10:10:06NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:10:05DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link up 1G/F
2025-08-29 10:09:30NoticeItvitae-MER-BG-01stpSwitchSTP topology changes
2025-08-29 10:09:29DebugItvitae-MER-BG-01interfaceInterfacePort 5 - Port5 link down

Accepted Solution

  • RobertL
    RobertL Posts: 5  Freshman Member
    First Comment
    Answer ✓

    so, we found the loop.
    a rogue switch was set up, without our knowledge.
    which connected our secondary switch, back to our primary switch.
    so it went primary, secondary, rogue, primary, LOOP.
    and when we find out who placed that there, they will have a big problem

All Replies

  • Zyxel_Melen
    Zyxel_Melen Posts: 3,713  Zyxel Employee
    Zyxel Certified Network Engineer Level 1 - Switch Zyxel Certified Network Administrator - Switch Zyxel Certified Network Administrator - Nebula Zyxel Certified Sales Associate

    Hi @RobertL

    Please help to provide your topology and the tech-support file from each of your switches with us.

    You may send the tech-support file to me via private message.

    https://community.zyxel.com/en/discussion/21237/switch-how-to-collect-the-tech-support-info-on-v4-80-firmware-version-and-newer
    Zyxel Melen


  • RobertL
    RobertL Posts: 5  Freshman Member
    First Comment
    edited August 29

    UPDATE:
    logs show a loop on the main switch's uplink port to our router.
    no clue how that is even possible, but what i find even weirder is that STP isnt putting the port in blocking state and that loop guard is enabled sitewide, despite the loop being detected. and even stranger is that bpduguard is disabled sitewide.

    so, should i enable bpduguard? since it is the uplink port from our router, if that port goes down the entire network will not be able to communicate with the outside internet, so thats not great either, but having this loop create intermittent issues is even worse , especially for troubleshooting pruposes, so?

  • RobertL
    RobertL Posts: 5  Freshman Member
    First Comment

    UPDATE 2:

    2025-08-29 14:03:31InformationItvitae-MER-BG-01interfaceInterfaceBroadcast storm detected on port 27 - Port27.

    2025-08-29 13:43:56InformationItvitae-MER-BG-01interfaceInterfaceBroadcast storm detected on port 27 - Port27.

    so we do have loop issues, but that port is the uplink port to one of our secondary switches.
    and STP correctly placed the port into blocking state, so that works. so now we need to find where the actual loop is and fix it

  • RobertL
    RobertL Posts: 5  Freshman Member
    First Comment
    Answer ✓

    so, we found the loop.
    a rogue switch was set up, without our knowledge.
    which connected our secondary switch, back to our primary switch.
    so it went primary, secondary, rogue, primary, LOOP.
    and when we find out who placed that there, they will have a big problem

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