Contents

Configuring loop detection· 1

Overview·· 1

Loop detection mechanism·· 1

Loop detection interval 2

Loop protection actions· 2

Port status auto recovery· 2

Loop detection configuration task list 3

Enabling loop detection· 3

Enabling loop detection globally· 3

Enabling loop detection on a port 3

Setting the loop protection action· 3

Setting the global loop protection action· 4

Setting the loop protection action on a Layer 2 Ethernet interface· 4

Setting the loop protection action on a Layer 2 aggregate interface· 4

Setting the loop detection interval 4

Displaying and maintaining loop detection· 5

Loop detection configuration example· 5

Network requirements· 5

Configuration procedure· 5

Verifying the configuration· 6

Configuring loop detection

Overview

Incorrect network connections or configurations can create Layer 2 loops, which results in repeated transmission of broadcasts, multicasts, or unknown unicasts, waste network resources, and sometimes even paralyze networks. The loop detection mechanism immediately generates a log when a loop occurs so that you are promptly notified to adjust network connections and configurations. You can even configure loop detection to shut down the looped port. Logs are maintained in the information center. For more information, see Network Management and Monitoring Configuration Guide.

Loop detection mechanism

The device detects loops by sending detection frames and then checking whether these frames return to any port on the device. If they do, the device considers that the port is on a looped link.

Figure 1 Ethernet frame header for loop detection

 

The Ethernet frame header for loop detection contains the following fields:

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

·          SMAC—Source MAC address of the frame, which is the bridge MAC address of the sending device.

·          TPID—Type of the VLAN tag, with the value of 0x8100.

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

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

Figure 2 Inner frame header for loop detection

 

The 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 1 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.

Port status auto recovery

When the device configured with the block or no-learning loop action detects a loop on a port, it performs the action and waits three loop detection intervals. If the device does not receive a loop detection frame within three loop detection intervals, it performs the following tasks:

·          Automatically sets the port to the forwarding state.

·          Notifies the user of the event.

When the device configured with the shutdown action detects a loop on a port, the following events occur:

1.        The device automatically shuts down the port.

2.        The device automatically sets the port to the forwarding state after the detection timer configured by using the shutdown-interval command expires. For more information about the shutdown-interval command, see Fundamentals Command Reference.

3.        The device shuts down the port again if a loop is still detected on the port when the detection timer expires.

This process is repeated until the loop is removed.

 

	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.

 

Loop detection configuration task list

Tasks at a glance
(Required.) Enabling loop detection
(Optional.) Setting the loop protection action
(Optional.) Setting the loop detection interval

 

Enabling loop detection

You can enable loop detection globally or on a per-port basis. When a port receives a detection frame in any VLAN, the loop protection action is triggered on that port, regardless of whether loop detection is enabled on it.

Enabling loop detection globally

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

 

Enabling loop detection on a port

Step	Command	Remarks
1.       Enter system view.	system-view	N/A
2.       Enter Layer 2 Ethernet interface view or Layer 2 aggregate interface view.	interface interface-type interface-number	N/A
3.       Enable loop detection on the port.	loopback-detection enable vlan { vlan-list | all }	Disabled by default.

 

Setting the loop protection action

You can configure the loop protection action globally or on specific ports. The global setting applies to all ports. The per-port setting applies to the individual ports. The per-port setting takes precedence over the global setting.

Setting the global loop protection action

Step	Command	Remarks
1.       Enter system view.	system-view	N/A
2.       Configure the global loop protection action.	loopback-detection global action shutdown	By default, the device generates a log but performs no action on the port on which a loop is detected.

 

Setting the loop protection action on a Layer 2 Ethernet interface

Step	Command	Remarks
1.       Enter system view.	system-view	N/A
2.       Enter Layer 2 Ethernet interface view.	interface interface-type interface-number	N/A
3.       Configure the loop protection action on the interface.	loopback-detection action { block | no-learning | shutdown }	By default, the device generates a log but performs no action on the port on which a loop is detected.

 

Setting the loop protection action on a Layer 2 aggregate interface

Step	Command	Remarks
1.       Enter system view.	system-view	N/A
2.       Enter Layer 2 aggregate interface view.	interface bridge-aggregation interface-number	N/A
3.       Configure the loop protection action on the interface.	loopback-detection action shutdown	By default, the device generates a log but performs no action on the port on which a loop is detected.

 

Setting the loop detection interval

With loop detection enabled, the device sends loop detection frames at a specified interval. A shorter interval offers more sensitive detection but consumes more resources. Consider the system performance and loop detection speed when you set the loop detection interval.

To set the loop detection interval:

 

Step	Command	Remarks
1.       Enter system view.	system-view	N/A
2.       Set the loop detection interval.	loopback-detection interval-time interval	The default setting is 30 seconds.

 

Displaying and maintaining loop detection

Execute display commands in any view.

 

Task	Command
Display the loop detection configuration and status.	display loopback-detection

 

Loop detection configuration example

Network requirements

As shown in Figure 3, configure loop detection on Device A, so that Device A generates a log as a notification and automatically shuts down the port on which a loop is detected.

Figure 3 Network diagram

 

Configuration procedure

1.        Configure Device A:

# Create VLAN 100, and globally enable loop detection for the VLAN.

<DeviceA> system-view

[DeviceA] vlan 100

[DeviceA-vlan100] quit

[DeviceA] loopback-detection global enable vlan 100

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

[DeviceA] interface fortygige 1/0/1

[DeviceA-FortyGigE1/0/1] port link-type trunk

[DeviceA-FortyGigE1/0/1] port trunk permit vlan 100

[DeviceA-FortyGigE1/0/1] quit

[DeviceA] interface fortygige 1/0/2

[DeviceA-FortyGigE1/0/2] port link-type trunk

[DeviceA-FortyGigE1/0/2] port trunk permit vlan 100

[DeviceA-FortyGigE1/0/2] quit

# Configure the global loop protection action as shutdown.

[DeviceA] loopback-detection global action shutdown

# Set the loop detection interval to 35 seconds.

[DeviceA] loopback-detection interval-time 35

2.        Configure Device B:

# Create VLAN 100.

<DeviceB> system-view

[DeviceB] vlan 100

[DeviceB–vlan100] quit

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

[DeviceB] interface fortygige 1/0/1

[DeviceB-FortyGigE1/0/1] port link-type trunk

[DeviceB-FortyGigE1/0/1] port trunk permit vlan 100

[DeviceB-FortyGigE1/0/1] quit

[DeviceB] interface fortygige 1/0/2

[DeviceB-FortyGigE1/0/2] port link-type trunk

[DeviceB-FortyGigE1/0/2] port trunk permit vlan 100

[DeviceB-FortyGigE1/0/2] quit

3.        Configure Device C:

# Create VLAN 100.

<DeviceC> system-view

[DeviceC] vlan 100

[DeviceC–vlan100] quit

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

[DeviceC] interface fortygige 1/0/1

[DeviceC-FortyGigE1/0/1] port link-type trunk

[DeviceC-FortyGigE1/0/1] port trunk permit vlan 100

[DeviceC-FortyGigE1/0/1] quit

[DeviceC] interface fortygige 1/0/2

[DeviceC-FortyGigE1/0/2] port link-type trunk

[DeviceC-FortyGigE1/0/2] port trunk permit vlan 100

[DeviceC-FortyGigE1/0/2] quit

Verifying the configuration

After the configurations are complete, Device A detects loops on ports FortyGigE 1/0/1 and FortyGigE 1/0/2 within a loop detection interval. Consequently, Device A automatically shuts down the ports and generates the following log messages:

[DeviceA]

%Feb 24 15:04:29:663 2011 DeviceA LPDT/4/LOOPED:Slot=1;

 Loopback exists on FortyGigE 1/0/1.

%Feb 24 15:04:29:667 2011 DeviceA LPDT/4/LOOPED:Slot=1;

 Loopback exists on FortyGigE 1/0/2.

%Feb 24 15:04:44:243 2011 DeviceA LPDT/4/RECOVERED:Slot=1;

 Loopback on FortyGigE 1/0/1 recovered.

%Feb 24 15:04:44:248 2011 DeviceA LPDT/4/RECOVERED:Slot=1;

 Loopback on FortyGigE 1/0/2 recovered.

Use the display loopback-detection command to display the loop detection configuration and status on Device A.

# Display the loop detection configuration and status on Device A.

[DeviceA] display loopback-detection

Loop detection is enabled.

Loop detection interval is 35 second(s).

No loopback is detected.

The output shows that the device has removed the loops from FortyGigE 1/0/1 and FortyGigE 1/0/2 according to the shutdown action. Use the display interface command to display the status of FortyGigE 1/0/1 and FortyGigE 1/0/2 on Device A.

# Display the status of FortyGigE 1/0/1 on Device A.

[DeviceA] display interface fortygige 1/0/1

FortyGigE 1/0/1 current state: DOWN (Loop detection down)

...

# Display the status of FortyGigE 1/0/2 on Device A.

[DeviceA] display interface fortygige 1/0/2

FortyGigE 1/0/2 current state: DOWN (Loop detection down)

...

The output shows that FortyGigE 1/0/1 and FortyGigE 1/0/2 are already shut down by the loop detection module.