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New Member

Hostflapping caused network crash

We experience numerous of the following error message resulting in a very unstable network that was only recoverable by rebooting the switches. Info suggests a STP issue, but I am not convinced because it failure started without any changes being made on the network.

%C4K_EBM-4-HOSTFLAPPING

2 REPLIES
Cisco Employee

Re: Hostflapping caused network crash

What ports were you seeing the flapping on(host ports or uplink ports to other switches)? The switch prints these messages if it sees a MAC address on 2 ports at the same time(most common reason is STP loop). However, I have seen other causes such as packets getting looped back etc. If you are seeing it on host ports, it is a good idea to enable portfast and BPDU guard so that the ports are shut down if they see a BPDU getting looped back.

New Member

Re: Hostflapping caused network crash

The issue shows a flap between the host interface and a channel trunk port connecting to a remote switch (fiber uplink).

The network consists of 3 switches connected in a loop, with trunk channels between each switch.

I had thought of the BPDU guard as a preventative measure, but it doesn't get to the root cause.

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