Configuring loop detection· 1

Overview·· 1

Loop detection mechanism·· 1

Loop detection interval 2

Loop protection actions 2

Loop detection configuration task list 3

Enabling loop detection· 3

Configuring the loop protection action· 3

Configuring the global loop protection action· 4

Configuring the loop protection action on a port 4

Setting the loop detection interval 4

Displaying and maintaining loop detection· 4

Loop detection configuration example· 5

Network requirements 5

Configuration procedure· 5

Verifying the configuration· 7

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.

Loop detection usually works within a VLAN. If a detection frame is returned with a different VLAN tag than it was sent out with, an inter-VLAN loop has occurred. To remove the loop, examine the QinQ configuration for incorrect settings. For more information about QinQ, see "Configuring QinQ."

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 1 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. Alternatively, you can configure the switch to shut 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.

Loop detection configuration task list

 

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

 

Enabling loop detection

You can enable loop detection globally or on specific ports. The global configuration applies to all ports in the specified VLAN. The per-port configuration applies to the individual port only when the port belongs to the specified VLAN.

After you enable loop detection for a VLAN, do not configure it as the source VLAN for remote port mirroring. Otherwise, loop detection might not operate properly. For more information about remote port mirroring, see Network Management and Mirroring Configuration Guide.

To enable 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.

 

To enable 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.

 

Configuring the loop protection action

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

Configuring 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.

 

Configuring the loop protection action on a port

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enter Layer 2 Ethernet or aggregate interface view.	·       Enter Layer 2 Ethernet interface view: interface interface-type interface-number ·       Enter Layer 2 aggregate interface view: interface bridge-aggregation interface-number	Use either command.
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.

 

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

 

Loop detection configuration example

By default, Ethernet, VLAN, and aggregate interfaces are down. To configure such an interface, bring the interface up by executing the undo shutdown command.

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 2 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 GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 as trunk ports, and assign them to VLAN 100.

[DeviceA] interface GigabitEthernet 3/0/1

[DeviceA-GigabitEthernet3/0/1] port link-type trunk

[DeviceA-GigabitEthernet3/0/1] port trunk permit vlan 100

[DeviceA-GigabitEthernet3/0/1] quit

[DeviceA] interface GigabitEthernet 3/0/2

[DeviceA-GigabitEthernet3/0/2] port link-type trunk

[DeviceA-GigabitEthernet3/0/2] port trunk permit vlan 100

[DeviceA-GigabitEthernet3/0/2] 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 GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 as trunk ports, and assign them to VLAN 100.

[DeviceB] interface GigabitEthernet 3/0/1

[DeviceB-GigabitEthernet3/0/1] port link-type trunk

[DeviceB-GigabitEthernet3/0/1] port trunk permit vlan 100

[DeviceB-GigabitEthernet3/0/1] quit

[DeviceB] interface GigabitEthernet 3/0/2

[DeviceB-GigabitEthernet3/0/2] port link-type trunk

[DeviceB-GigabitEthernet3/0/2] port trunk permit vlan 100

[DeviceB-GigabitEthernet3/0/2] quit

3.      Configure Device C:

# Create VLAN 100.

<DeviceC> system-view

[DeviceC] vlan 100

[DeviceC–vlan100] quit

# Configure GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 as trunk ports, and assign them to VLAN 100.

[DeviceC] interface GigabitEthernet 3/0/1

[DeviceC-GigabitEthernet3/0/1] port link-type trunk

[DeviceC-GigabitEthernet3/0/1] port trunk permit vlan 100

[DeviceC-GigabitEthernet3/0/1] quit

[DeviceC] interface GigabitEthernet 3/0/2

[DeviceC-GigabitEthernet3/0/2] port link-type trunk

[DeviceC-GigabitEthernet3/0/2] port trunk permit vlan 100

[DeviceC-GigabitEthernet3/0/2] quit

Verifying the configuration

After the configurations are complete, Device A detects loops on ports GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 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 GigabitEthernet3/0/1.

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

 Loopback exists on GigabitEthernet3/0/2.

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

 Loopback on GigabitEthernet3/0/1 recovered.

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

 Loopback on GigabitEthernet3/0/2 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 GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 according to the shutdown action. Use the display interface command to display the status of GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 on Device A.

# Display the status of GigabitEthernet 3/0/1 on Device A.

[DeviceA] display interface GigabitEthernet 3/0/1

GigabitEthernet3/0/1 current state: DOWN (Loop detection down)

...

# Display the status of GigabitEthernet 3/0/2 on Device A.

[DeviceA] display interface GigabitEthernet 3/0/2

GigabitEthernet3/0/2 current state: DOWN (Loop detection down)

...

The output shows that GigabitEthernet 3/0/1 and GigabitEthernet 3/0/2 are already shut down by the loop detection module.