Break-fix: port-security err-disabled
- Troubleshooting
- Intermediate
- 8 min
- 4 steps
- Cisco IOS-style
SW1.Eth0 has port-security with maximum 1 / violation shutdown. A second host hangs off a hub on the same port — the port is err-disabled. Diagnose and fix it.
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Scenario
Port-security caps how many MAC addresses are allowed through an access port. In shutdown mode a violation hard-trips the port into err-disabled — it stays dead until you cycle shutdown / no shutdown (or errdisable recovery fires).
Topology
2 × switch, 3 × router · Cisco IOS-style
SW1— switchHUB— switchH1— routerH2— routerH3— router
What you will do
Ping from H1 to H3 (10.0.0.3) first — fails. H2 would also be affected.
On SW1 run
show port-security interface Eth0to see what happened.Bring SW1.Eth0 back: shutdown, then no shutdown.
Ping H3 from H1 again — should now succeed because H1's MAC is on the secure list.
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