131Figure 36 Inner frame header for loop detectionThe inner frame header for loop detection contains the following fields:• Code—Protocol sub-type, which is 0x0001, indicating the loop detection protocol.• 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 10 TLVs supported by loop detectionTLV Description RemarksEnd 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 intervalLoop detection is a continuous process as the network changes. Loop detection frames are sent at aspecified interval (called a "loop detection interval") to check whether loops occur on ports and whetherloops are removed.Loop protection actionsWhen the switch detects a loop on a port, it generates a log but performs no action on the port by default.You can configure the switch 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 switchautomatically brings up the port after the detection timer expires. You can use theshutdown-interval command to set the detection timer. For more information, see FundamentalsCommand Reference.
