4000 Router Series Layer 2 - LAN Switching Configuration Guide

• Version—Protocol version, which is always 0x0000.

• Length—Length of the frame. The value includes the inner header, but excludes the Ethernet header.

• Reserved—This field is reserved.

Frames for loop detection are encapsulated as TLV triplets.

Table 6 TLVs supported by loop detection

TLV Description Remarks

End of PDU End of a PDU. Optional.

Device ID Bridge MAC address of the sending device. Required.

Port ID ID of the PDU sending port. Optional.

Port Name Name of the PDU sending port. Optional.

System Name Device name. Optional.

Chassis ID Chassis ID of the sending port. Optional.

Slot ID Slot ID of the sending port. Optional.

Sub Slot ID Sub-slot ID of the sending port. Optional.

Loop detection uses the following important concepts.

Loop detection interval

Loop detection is a continuous process as the network changes. Loop detection frames are sent at a

specified interval (called a "loop detection interval") to check whether loops occur on ports and whether

loops are removed.

Loop protection actions

When the device detects a loop on a port, it generates a log but performs no action on the port by default.

You can configure the device to take one of the following actions:

• Block—Disables the port from learning MAC addresses and blocks inbound traffic to the port.

• No-learning—Disables the port from learning MAC addresses.

• Shutdown—Shuts down the port to disable it from receiving and sending any frames. The port is

always in the down state until manually brought up with the undo shutdown command.

Port status auto recovery

Port status auto recovery applies only to the block and no-learning loop protection actions. If the device

receives no loop detection frame three loop detection intervals after a loop is detected on a port, the

device automatically sets the port to the forwarding state, and notifies the user of the event.

NOTE:

Incorrect recovery can occur when loop detection frames are discarded to reduce the load. To avoid this,

use the shutdown action, or manually remove the loop.