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Multiple Instance Spanning-Tree Operation
Loop Protection
Loop Protection
In cases where spanning tree cannot be used to prevent loops at the edge of
the network, loop protection may provide a suitable alternative. Loop
protection operates in two modes:
■Untagged—The default mode. This mode can be used to find loops in
untagged downlinks.
■Tagged VLAN—Finds loops on tagged VLANs. This mode can be used to
detect loops in tagged-only uplinks where STP cannot be enabled.
The cases where loop protection might be chosen ahead of spanning tree to
detect and prevent loops are as follows:
â– On ports with client authentication. When spanning tree is enabled
on a switch that use 802.1X, Web authentication, and MAC authentication,
loops may go undetected. For example, spanning tree packets that are
looped back to an edge port will not be processed because they have a
different broadcast/multicast MAC address from the client-authenticated
MAC address. To ensure that client-authenticated edge ports get blocked
when loops occur, you should enable loop protection on those ports.
â– On ports connected to unmanaged devices. Spanning tree cannot
detect the formation of loops where there is an unmanaged device on the
network that does not process spanning tree packets and simply drops
them. Loop protection has no such limitation, and can be used to prevent
loops on unmanaged switches.
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