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Cake day: June 10th, 2023

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  • Certain types of broadcast traffic always get re-broadcast from of every port on a switch. So if you directly connect two ports, and you get some broadcast coming into the switch, that broadcast will loop forever across that loopback, and then get propagated repeatedly until it hits a broadcast boundary. It’s surprisingly difficult to prevent even with managed switches unless you are willing to hand manage every port and significantly restrict the kind of network services which can flow through it.

    Some devices can detect these loops and break them, but that can have other unintended impacts if your network is designed (some would argue poorly) around using dumb switches to multiply limited Ethernet drops at the edge.


  • Yup, the good old “loopback FU.”

    Routers do have some protections which can mitigate this, but the entire problem is broadcast flooding which can’t really be dealt with at later 2, or even at layer 3 within the same segment. Most places will have no broadcast forwarding between segments, but even if you detect unusual broadcast activity and ban that class of traffic, you break other things. A lot of times it is ARP floods, so it doesn’t happen when the network is static and converged until someone plugs a new laptop in, and then everyone assumes it’s that laptop.