MAC flapping / broadcast storm (spanning tree loop) Networking Origin: Network OSI layer 2 · Data Link Last updated 5 Nov 2026

Networking: %SW_MATM-4-MACFLAP_NOTIF: Host is flapping between ports / broadcast storm

A layer-2 loop is making frames circle the network, saturating links and making the whole VLAN crawl.

Exact message you may see Host is flapping between portbroadcast stormSpanning tree loop detected
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Seen on: Cisco Networking

Meaning

Someone plugged a switch or cable back into the same network, or spanning tree is off or misconfigured. Find the flapping MAC, unplug the loop, and enable STP protections: BPDU guard on edge ports, root guard, storm control and loop guard.

Common causes

  • Cable plugged into two ports of the same network
  • Spanning tree disabled or inconsistent
  • Unmanaged switch added by a user
  • Missing BPDU guard on access ports
  • Faulty switch or NIC flooding

⚡ Quick fix

  1. Find it: show mac address-table | include <mac>
  2. Unplug the offending cable and enable BPDU guard + storm control
  3. Verify the root bridge with show spanning-tree

Detailed fix by platform

Cisco

  1. text
    show spanning-tree summary
show interfaces | include broadcasts
show logging | include MACFLAP

How to diagnose

  1. Log — MAC flapping?
  2. STP — Enabled, correct root?
  3. Edge ports — BPDU guard?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

arp -a shows (incomplete) — Networking: `arp -n` shows (incomplete) / Destination Host Unreachable — no ARP reply from the next hop (Networking)arp -a shows (incompl…Cannot assign requested address (EADDRNOTAVAIL) — Networking: connect: Cannot assign requested address — ephemeral port range exhausted by TIME_WAIT connections (Networking)Cannot assign request…Duplex mismatch: late collisions / CRC errors — Networking: Duplex mismatch — late collisions, runts and CRC errors on the switch port (Networking)Duplex mismatch: late…Many sockets in CLOSE_WAIT — Networking: Thousands of sockets in CLOSE_WAIT — the application is not closing its side of the connection (Networking)Many sockets in CLOSE…Native VLAN mismatch / wrong VLAN, no connectivity — Networking: %CDP-4-NATIVE_VLAN_MISMATCH / device on the wrong VLAN can't reach the gateway (Networking)Native VLAN mismatch …Port err-disabled (psecure-violation) — Networking: %PM-4-ERR_DISABLE: psecure-violation error detected on Gi0/5, putting Gi0/5 in err-disable state (Networking)Port err-disabled (ps…Port forwarding not working (CGNAT / double NAT) — Networking: Port forwarding configured but the port stays closed — carrier-grade NAT (CGNAT) or double NAT (Networking)Port forwarding not w…MAC flapp…

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