Configuring loopback detection· 1

Overview·· 1

Background· 1

Basic concepts in loopback detection· 1

How loopback detection works 3

Loopback detection configuration task list 3

Configuring loopback detection· 4

Enabling loopback detection· 4

Configuring the loopback detection action· 4

Configuring the loopback detection interval 5

Displaying and maintaining loopback detection· 5

Loopback detection configuration example· 5

 

Overview

Background

Incorrect network connections or configurations may create loops at Layer 2, causing related devices to repeatedly transmit broadcasts, multicasts, and unknown unicasts. This wastes the network resources and sometimes even paralyzes the networks. The loopback detection mechanism timely notifies you when loops occur, so that you can promptly check network connections and configurations and remove the loops by automatically shutting down the looped ports. The loopback detection mechanism notifies you of the network loop by printing logs and sending trap messages, and may shut down the looped port as configured.

 

NOTE: For more information about logs and trap messages, see Network Management and Monitoring Configuration Guide.

 

Basic concepts in loopback detection

Loopback detection frame

A switch detects loops by sending loopback detection frames and then checking whether these frames returns (not necessarily to the sending ports); if a port on the switch receives a loopback detection frame sent by the switch, the port is considered looped.

 

NOTE: Loopback detection is usually VLAN based; however, incorrect QinQ or VLAN mapping configurations may also cause loops (even though the VLAN information carried in loopback detection frames returned to the switch is changed, the switch still considers the receiving ports looped). For more information about QinQ and VLAN switching, see the chapters “QinQ configuration” and “VLAN Mapping configuration.”

 

Figure 1 Ethernet header of a loopback detection frame

 

Figure 1 shows the format of the Ethernet header of a loopback detection frame. The Ethernet header contains the following fields:

·           DMAC—Destination MAC address of the loopback detection frame, which is the multicast MAC address 010F-E200-0007. When a loopback detection-enabled switch receives a frame with this destination MAC address, it sends the frame to the CPU and broadcasts the frame in the VLAN from which the frame was originally received.

·           SMAC—Source MAC address of the loopback detection frame, which is the bridge MAC address of the sending switch.

·           TPID—Tag Protocol Identifier, type of the VLAN tag, with the value of 0x8100.

·           TCI—Tag Control Information, information of the VLAN tag, including the priority and VLAN ID.

·           Type— Protocol type, with the value of 0x8918.

Figure 2 Inner header of a loopback detection frame

 

Figure 2 shows the format of the inner header of a loopback detection frame. The inner header contains the following fields:

·           Code—Protocol sub-type, with the value of 0x0001, indicating the loopback detection protocol.

·           Version—Protocol version, with the value of 0x0000, which is reserved.

·           Length—Length of the loopback detection frame, including the inner header, but not the Ethernet header.

·           Reserved—This field is reserved.

Loopback detection frames are constructed in the form of TLV (type/length/value) triplets. Table 1 lists the TLVs (required or optional) supported by the loopback detection mechanism.

Table 1 TLVs supported by the loopback detection mechanism

TLV	Description	Remarks
End of PDU	TLV that indicates the end of a PDU	Optional
Device ID	TLV that indicates the bridge MAC address of the sending switch	Required
Port ID	TLV that indicates the ID of the PDU sending port	Optional
Port Name	TLV that indicates the name of the PDU sending port	Optional
System Name	TLV that indicates the switch name	Optional
Chassis ID	TLV that indicates the chassis ID of the sending port	Optional
Slot ID	TLV that indicates the slot ID of the sending port	Optional

 

Loopback detection interval

Loopback detection should be a continuous process. Loopback detection frames are sent at a specified interval (called a “loopback detection interval”) to check whether loops occur on ports and whether loops are removed.

How loopback detection works

Loopback detection actions

Loopback detection actions refer to the actions taken by the system when detecting loops. The following actions are available:

·           None—When detecting a looped port, the system takes no action on the port except printing log information and sending trap messages. If no loopback detection frames are received within three loopback detection intervals, the system determines that the loop is already removed, and again prints log information to notify the user.

·           Shutdown—When detecting a looped port, besides printing log information and sending trap messages, the system also shuts down the port to disable it from receiving and sending frames (including loopback detection frames).

 

CAUTION: A port shut down by the system during the loopback detection process can only be manually brought up by using the undo shutdown command.

 

Loop status auto recovery

Loop status auto recovery works as follows: After a device detects a loop on a port, the device continues to monitor the loopback detection frames. If no loopback detection frame is received within three times the loopback detection interval, the device concludes that the loop is removed and notifies the users of this event.

 

CAUTION: ·       Loop status auto recovery applies only when the loopback detection action is none. When the loopback detection action is shutdown, the device automatically shuts down looped ports and thus removes the loop. ·       When a network loop occurs, the device discards some of the frames to reduce the load. If the loopback detection frames are among the discarded frames, the loop status auto recovery function on the device will erroneously conclude that the loop has already been removed. To avoid this, set the loopback detection action to shutdown, or manually remove the loop when the device reports the occurrence of the loop if you set the loopback detection action to none.

 

Loopback detection configuration task list

Complete the following tasks to configure loopback detection:

 

Task	Remarks
Enabling loopback detection	Required
Configuring the loopback detection action	Optional
Configuring the loopback detection interval	Optional

 

Configuring loopback detection

Enabling loopback detection

You can enable the loopback detection function in system view or VLAN view. After you enable loopback detection for a VLAN, the system performs loopback detection on all the ports in the VLAN.

Enable loopback detection in system view

In system view, you can enable loopback detection for multiple VLANs or all VLANs at a time.

To enable loopback detection in system view:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enable loopback detection.	loopback-detection enable vlan { vlan-list | all }	Disabled by default.

 

Enable loopback detection in VLAN view

In VLAN view, you can enable loopback detection for the current VLAN only.

To enable loopback detection in VLAN view:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enter VLAN view.	vlan vlan-id	N/A
3.     Enable loopback detection.	loopback-detection enable	Disabled by default.

 

NOTE: H3C recommends that you not configure the port mirroring function on the member ports of a loopback detection-enabled VLAN. For more information about port mirroring, see Network Management and Monitoring Configuration Guide.

 

Configuring the loopback detection action

You can set the loopback detection action to none or shutdown as needed.

To set the loopback detection action:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Configure the loopback detection action.	loopback-detection action { none | shutdown }	By default, the loopback detection action is none.

 

Configuring the loopback detection interval

With loopback detection enabled, the switch sends loopback detection frames at a specified interval. The shorter this interval is, the faster the system can detect loops, but the more system resources will be used. You must consider both the system performance and loopback detection speed when choosing an appropriate interval.

To configure the loopback detection interval:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Configure the loopback detection interval.	loopback-detection interval-time interval	Defaulted to 30 seconds.

 

Displaying and maintaining loopback detection

 

Task	Command	Remarks
Display the status of loopback detection.	display loopback-detection [ | { begin | exclude | include } regular-expression ]	Available in any view

 

Loopback detection configuration example

 

NOTE: By default, Ethernet, VLAN, and aggregate interfaces are in DOWN state. Before configuring these interfaces, use the undo shutdown command to bring them up.

 

Network requirements

As shown in Figure 3,

·           Device A, Device B, and Device C form a ring-shaped network. The network administrator typically shuts down GigabitEthernet 4/0/1 of Device B to prevent loops in the network.

·           Configure loopback detection on Device A so that when a loop resulting from incorrect configuration occurs, Device A can automatically shut down the looped port and remind the user to check the network connections by printing log information and sending trap messages.

Figure 3 Network diagram

 

Configuring Device A

# Create VLAN 100 and then enable loopback detection on it.

<DeviceA> system-view

[DeviceA] vlan 100

[DeviceA-vlan100] loopback-detection enable

[DeviceA–vlan100] quit

# Configure GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2 as trunk ports and assign them to VLAN 100.

[DeviceA] interface GigabitEthernet 4/0/1

[DeviceA-GigabitEthernet4/0/1] port link-type trunk

[DeviceA-GigabitEthernet4/0/1] port trunk permit vlan 100

[DeviceA-GigabitEthernet4/0/1] quit

[DeviceA] interface GigabitEthernet 4/0/2

[DeviceA-GigabitEthernet4/0/2] port link-type trunk

[DeviceA-GigabitEthernet4/0/2] port trunk permit vlan 100

[DeviceA-GigabitEthernet4/0/2] quit

# Set the loopback detection action to shutdown.

[DeviceA] loopback-detection action shutdown

# Set the loopback detection interval to 35 seconds.

[DeviceA] loopback-detection interval-time 35

Configuring Device B

# Create VLAN 100.

<DeviceB> system-view

[DeviceB] vlan 100

[DeviceB–vlan100] quit

# Configure GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2 as trunk ports and assign them to VLAN 100.

[DeviceB] interface GigabitEthernet 4/0/1

[DeviceB-GigabitEthernet4/0/1] port link-type trunk

[DeviceB-GigabitEthernet4/0/1] port trunk permit vlan 100

[DeviceB-GigabitEthernet4/0/1] quit

[DeviceB] interface GigabitEthernet 4/0/2

[DeviceB-GigabitEthernet4/0/2] port link-type trunk

[DeviceB-GigabitEthernet4/0/2] port trunk permit vlan 100

[DeviceB-GigabitEthernet4/0/2] quit

# Shut down GigabitEthernet 4/0/1 to prevent loops.

[DeviceB] interface GigabitEthernet 4/0/1

[DeviceB-GigabitEthernet4/0/1] shutdown

[DeviceB-GigabitEthernet4/0/1] quit

Configuring Device C

# Create VLAN 100.

<DeviceC> system-view

[DeviceC] vlan 100

[DeviceC–vlan100] quit

# Configure GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2 as trunk ports and assign them to VLAN 100.

[DeviceB] interface GigabitEthernet 4/0/1

[DeviceB-GigabitEthernet4/0/1] port link-type trunk

[DeviceB-GigabitEthernet4/0/1] port trunk permit vlan 100

[DeviceB-GigabitEthernet4/0/1] quit

[DeviceB] interface GigabitEthernet 4/0/2

[DeviceB-GigabitEthernet4/0/2] port link-type trunk

[DeviceB-GigabitEthernet4/0/2] port trunk permit vlan 100

[DeviceB-GigabitEthernet4/0/2] quit

Verifying the configuration

After the configurations are completed, you can use the display loopback-detection command to check the status of loopback detection on each device.

# Display the loopback detection status on Device A.

[DeviceA] display loopback-detection

 Loopback-detection is running.

 Detection interval is 30 second(s).

 Action mode: Shutdown

 Loopback-detection is enabled on the following VLAN(s):

 100

 No loopback is detected on any interface.

The output shows that loopback detection is enabled on Device A, and no looped ports are detected.

# Display the loopback detection status on Device B.

[DeviceB] display loopback-detection

 Loopback-detection is not running.

# Display the loopback detection status on Device C.

[DeviceC] display loopback-detection

 Loopback-detection is not running.

The output shows that loopback detection is not enabled on Device B or Device C.

Assume that later on, GigabitEthernet 4/0/1 of Device B is brought up by the network administrator by mistake. Within a loopback detection interval, Device A will detect a loop on ports GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2. Consequently, it automatically shuts down the ports and prints the following log information:

[DeviceA]

%Feb 24 15:04:29:663 2010 DeviceA LPDT/4/LOOPED:Slot=4;

 Loopback exists on GigabitEthernet4/0/1.

%Feb 24 15:04:29:667 2009 DeviceA LPDT/4/LOOPED:Slot=1;

 Loopback exists on GigabitEthernet4/0/2.

%Feb 24 15:04:44:243 2010 DeviceA LPDT/4/RECOVERED:Slot=4;

 Loopback on GigabitEthernet4/0/1 recovered.

%Feb 24 15:04:44:248 2009 DeviceA LPDT/4/RECOVERED:Slot=1;

 Loopback on GigabitEthernet4/0/2 recovered.

When you see the log information above, use the display loopback-detection command again to display the loopback detection status on Device A.

# Display the loopback detection operating status on Device A.

[DeviceA] display loopback-detection

 Loopback-detection is running.

 Detection interval is 35 second(s).

 Action mode: Shutdown

 Loopback-detection is enabled on the following VLAN(s):

 100

 No loopback is detected on any interface.

The output shows that no loop is detected on GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2. The reason is that the loopback detection action is set to shutdown, in which case, the two ports are automatically shut down when a loop occurs on them. The shutdown action removes the loop. Use the display interface command to display the status information of GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2 on Device A:

# Display the status information of GigabitEthernet 4/0/1 on Device A.

[DeviceA] display interface gigabitethernet 4/0/1

 GigabitEthernet 4/0/1 current state: DOWN ( Loopback detection-protected )

...

# Display the status information of GigabitEthernet 4/0/2 on Device A.

[DeviceA] display interface gigabitethernet 4/0/2

 GigabitEthernet 4/0/2 current state: DOWN ( Loopback detection-protected )

...

The output above shows that GigabitEthernet 4/0/1 and GigabitEthernet 4/0/2 have already been shut down by the loopback detection module.