Cisco Catalyst 2960 Series Switch: Loop Detection Guard Configuration Guide PDF

Information About Loop Detection Guard

Configuring Loop Detection Guard Restrictions for Loop Detection Guard, on page 1 Information About Loop Detection Guard, on page 1 Enabling Loop Detection Guard and Error-Disabling the Required Port, on page 4 Additional References for Configuring Loop Detection Guard, on page 5 Feature History for Loop Detection Guard, on page 5 Restrictions for Loop Detection Guard Loop detection guard can be configured only on Layer 2 physical interfaces. Layer 3 ports and virtual interfaces, such as port channels, switch virtual interfaces (SVIs), and tunnels, are not supported.

Information About Loop Detection Guard A computer network can experience a network loop where there is more than one Layer 2 path between two endpoints. This is possible when there are multiple connections between two switches in a network or two ports on the same switch are connected to each other. The following figure shows a few examples of a network loop: Example 1: Switch SW A, which is within the network, is sending traffic to an unmanaged switch on one port and receiving traffic from the same unmanaged switch, on another port. On the unmanaged switch, the port receiving traffic is connected to the port sending traffic back to the SW A in the network, resulting in a network loop.

Example 2: This example shows a network loop involving four switches, two within the network (SW A and SW B) and two unmanaged switches (Un A and Un B). Traffic is moving in the following direction SW A to SW B to Un B to Un A and back to SW A, resulting in a network loop. Example 3: Two ports on the unmanaged switch are connected to each other, resulting in a network loop. Configuring Loop Detection Guard 1

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Information About Loop Detection Guard

Figure 1: Examples of Network Loop Between Managed and Unmanaged Switches While Spanning Tree Protocol (STP) is normally the protocol that is configured for this purpose (to prevent network loops), loop detection guard is suited to situations where there may be unmanaged switches in a network that do not understand STP, or where STP is not configured on the network. Loop detection guard is enabled at the interface level. To detect loops, the system sends loop-detect frames from the interface, at preconfigured intervals. When a loop is detected, the configured action is taken. Loop detection guard is disabled by default. When you enable the feature, you can configure one of these actions:

• Error-disable the port sending traffic. • Error-disable the port receiving traffic (default). • Display an error message and not disable any port. Configuring Loop Detection Guard Information About Loop Detection Guard Configuring Loop Detection Guard 2

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VLANs and Loop Detection Guard

When a port is error-disabled, no traffic is sent or received on that port. Interaction of Loop Detection Guard with Other Features The following sections provide information about how loop detection guard interacts with other feature: Spanning Tree Protocol and Loop Detection Guard When both loop detection guard and STP are enabled on a device, STP takes over monitoring the network for loops. In this case loop-detect packets are neither received nor processed in the network. VLANs and Loop Detection Guard We do not recommend configuring this feature on a switch that is connected to a hub for these reasons: The hub floods traffic to all of its interfaces.

If the switch in your network is receiving traffic from the same hub, but on a port in a different VLAN, you may be inadvertently error-disabling those destination ports. The figure below illustrates such a situation. The port in VLAN 1 is sending traffic to the hub. The switch is also receiving traffic from the same hub, but on a port in a different VLAN, that is, VLAN 10. If you configure loop detection guard (and you have configured the default action of error-disabling the destination port), then the port in VLAN 10 is blocked.

Configuring the option to display a message (instead of error-disabling a port) is not recommended either, because the system displays as many messages as the number of interfaces configured in the hub, resulting in a CPU overload. Figure 2: A Switch Connected to an Unmanged Network Hub Configuring Loop Detection Guard Interaction of Loop Detection Guard with Other Features Configuring Loop Detection Guard 3

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Enabling Loop Detection Guard and Error-Disabling the Required Port

EnablingLoopDetectionGuardandError-DisablingtheRequired Port The feature is disabled by default. Complete the following steps to enable loop detection guard and configure the action that you want the system to take when a loop is detected: Procedure Configuring Loop Detection Guard Enabling Loop Detection Guard and Error-Disabling the Required Port Command or Action Purpose Step 1 enable Example: Device> enable Enables privileged EXEC mode. Enter your password if prompted. Step 2 configure terminal Example: Device# configure terminal Enters global configuration mode. Step 3 interface {interface-id|subinterface-id|vlan-id} Example:

Device(config)# interface tengigabitethernet 1/0/20 Device(config-if)# Enters interface configuration mode. Specify only a physical interface to configure loop detection guard on the device. Layer 3 ports and virtual interfaces like PortChannels, switch virtual interfaces (SVIs), and tunnels are not supported. Step 4 [no] loopdetect Example: Device(config-if)# loopdetect Enables loop detection guard on the device. Loopdetect frames are sent from the configured interface. Use the loopdetect command without any keyword to enable loop detection guard. Use the no form of this command to disable this feature.

Note You can enable the feature on trunk ports, but a warning message is displayed, for the following reason: A trunk port carries traffic for several VLANs, simultaneously. A loop that is detected in one VLAN can result in the error-disabling of all VLAN traffic that is associated with the trunk port. Step 5 [no] loopdetect{time|action syslog|source-port} Example: Device(config-if)# loopdetect 7 Specifies the frequency at which loop-detect frames are sent and the action the system takes when a loop is detected. If you do not specify an action, the destination port is error-disabled by default. You can configure the following: Configuring Loop Detection Guard 4

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