17.1.1 Overview

If a loop failure occurs in a Layer 2 network, MAC address learning becomes unstable, or normal communication cannot continue because of the load on the switch. Spanning Tree Protocols and the Ring Protocol are provided to avoid such states.Generally, the L2 loop detection functionality corrects loop failures in a non-redundant access network, but not in the core network in which these protocols are used.

When a loop failure is detected on a local switch, the L2 loop detection functionality deactivates the port on which the failure was detected to isolate the failure cause from the network. Isolation is necessary to prevent the loop failure from spreading throughout the entire network.

The following figure shows the basic pattern of a loop failure.

Figure 17-1: Basic pattern of a loop failure

Example loop failure patterns

1. The line is incorrectly connected in Switch C, resulting in a loop failure.

2. A loop failure has occurred because a line was incorrectly connected to Switch E below Switch C.

3. A loop failure has occurred because a line was incorrectly connected to a device lower than Switch D.

4. A loop failure has occurred across the core network due to incorrect connection of a line by a lower-level device.

As described above, the L2 loop detection functionality can detect loop failures in various locations, including those with incorrect connections to the local switch or to other switches.