Maipu Administration Guide

Configure Local Loopback Detection

Network Requirements

Device1 and Device2 form a loop through two links, and all L2 Ethernet interfaces in the loop is in one VLAN.
On Device1, loopback detection has been enabled.
After Device1 detects a loop, it closes the interconnected L2 Ethernet interfaces to eliminate the loop.

Network Topology

Networking for Configuring the Local Loopback Detection Function

Figure 10-2 Networking for Configuring the Local Loopback Detection Function

Configuration Steps

Step 1: Configure VLANs, and configure the link type of the L2 Ethernet interface.

#On Device1, create VLAN2.

Device1#configure terminal
Device1(config)#vlan 2
Device1(config-vlan2)#exit

#On Device1, configure the link type of L2 Ethernet interface gigabitethernet0/1 and gigabitethernet0/2 to Trunk and allow the services of VLAN2 to pass.

Device1(config)# interface gigabitethernet 0/1-0/2
Device1(config-if-range)#switchport mode trunk
Device1(config-if-range)#switchport trunk allowed vlan add 2
Device1(config-if-range)#exit

#On Device2, create VLAN2.

Device2#configure terminal
Device2(config)#vlan 2
Device2(config-vlan2)#exit

#Configure the link type of L2 Ethernet interface gigabitethernet0/1 and gigabitethernet0/2 to Trunk, allow the services of VLAN2 to pass, and close the spanning tree.

Device2(config)# interface gigabitethernet 0/1-0/2
Device2(config-if-range)#switchport mode trunk
Device2(config-if-range)#no switchport trunk allowed vlan 1
Device2(config-if-range)#switchport trunk allowed vlan add 2
Device2(config-if-range)#no spanning-tree enable
Device2(config-if-range)#exit

Step 2: Enable the loopback detection function.

#On Device1, enable the loopback detection function globally.

Device1(config)#loopback-detection enable

#On Device1, query the loopback detection status.

Device1#show loopback-detection  
------------------------------------------------------
 Global loopback-detection : ENABLE 
------------------------------------------------------   	  	   	 
 Interface     Loopback Time(s) State        Control 
------------------------------------------------------   	  	   	 
 gi0/1         DISABLE  30      NORMAL       TRUE    
 gi0/2         DISABLE  30      NORMAL       TRUE

#The L2 Ethernet interface gigabitethernet0/1 on Device1 enables the loopback detection function.

Device1(config)#interface gigabitethernet 0/1
Device1(config-if-gigabitethernet0/1)#loopback-detection enable
Device1(config-if-gigabitethernet0/1)#exit

Step 3: Check the result.

#On Device1, query the loopback detection status.
After a loop is detected:

Device1#show loopback-detection  
--------------------------------------------------------
 Global loopback-detection : ENABLE 
-------------------------------------------------------- 	   	  	   	 
 Interface      Loopback Time(s) State        Control 
--------------------------------------------------------	  	   	 
 gi0/1          ENABLE   30      ERR-DISABLE  TRUE    
 gi0/2          DISABLE  30      NORMAL       TRUE

#A loop has been detected on Device1. The L2 Ethernet interface gigabitethernet0/1 is closed, and the device outputs the following prompt information:

Jul 30 2014 03:29:59: %LOOP-BACK-DETECTED : loop-back send tag packet in vlan2 on gigabitethernet0/1, detected in vlan2 from gigabitethernet0/2
Jul 30 2014 03:29:59: %LINK-INTERFACE_DOWN-4: interface gigabitethernet0/1, changed state to down
Jul 30 2014 03:29:59: %LINEPROTO-5-UPDOWN : gigabitethernet0/1 link-status changed to err- disable

#On Device1, query the status of L2 Ethernet interface gigabitethernet0/1, and you will find that the link status of the L2 Ethernet interface gigabitethernet0/1 is changed to Down.

Device1#show interface gigabitethernet 0/1
gigabitethernet0/1 configuration information 
        Description      :  
        Status           : Enabled 
        Link             : Down (Err-disabled) 
        Set Speed        : Auto 
        Act Speed        : Unknown 
        Set Duplex       : Auto 
        Act Duplex       : Unknown 
        Set Flow Control : Off 
        Act Flow Control : Off 
        Mdix             : Auto 
        Mtu              : 1824 
        Port mode        : LAN 
        Port ability     : 10M HD,10M FD,100M HD,100M FD,1000M FD 
        Link Delay       : No Delay 
        Storm Control    : Unicast Disabled 
        Storm Control    : Broadcast Disabled 
        Storm Control    : Multicast Disabled 
        Storm Action     : None
        Port Type        : Nni 
        Pvid             : 1 
        Set Medium       : Copper 
        Act Medium       : Copper 
        Mac Address      : 0000.0000.008b

When gigabitethernet 0/1 or gigabitethernet 0/2 of Device1 is L3 Ethernet interface, there is no loop in the networking environment.