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STP LoopDifficulty · easyHigh asset
Automated monitoring alert.
DIST-SW-03 (Building 3 distribution) is running at sustained 99% CPU since 11:02 UTC. Users in Bldg 3 are reporting that network drops for ~5 seconds every 10–20 seconds, then comes back. Other buildings look fine.
Latest config change on this switch was 9 days ago (port channel to ACCESS-SW-307 added).
Please triage.
— Network Monitoring
Evidence
DIST-SW-03 console + counters
# Last 20 lines of console
%STP-2-LOOPGUARD_BLOCK: Loop guard blocking port Gi1/0/24 on VLAN 10
%SPANTREE-7-RECV_TC: Received topology change BPDU on VLAN 10 (port Gi1/0/24)
%SPANTREE-7-RECV_TC: Received topology change BPDU on VLAN 10 (port Gi1/0/24)
%MAC-4-FLAP: Host 0011.2233.4455 flapped between Gi1/0/12 and Gi1/0/24 in VLAN 10
%MAC-4-FLAP: Host aabb.ccdd.eeff flapped between Gi1/0/9 and Gi1/0/24 in VLAN 10
... (TCN floods every ~3s) ...
# show processes cpu
CPU utilization for five seconds: 99%/45%; one minute: 96%; five minutes: 94%
PID Runtime(ms) Process 5Sec 1Min 5Min
116 3812440 Spanning Tree 58.4% 56.2% 55.8%
105 994320 HLFM-process 18.1% 17.4% 16.9%
# show interface Gi1/0/24
Gi1/0/24 is up, line protocol is up (connected)
Hardware is iGigabitEthernet, address is bcd1.7f00.1801
Last input never, output 00:00:00
Input queue: 982/2000/24/0 (size/max/drops/flushes); Total output drops: 0
... 60-second input rate 480000 bits/sec, 740 packets/sec ...
Affected asset
- Name
- DIST-SW-03 (Cisco Catalyst 9300)
- Type
- Distribution switch — Building 3 access layer
- Owner
- Network Operations · Building 3
- Level
- High