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Resolution Guide - EX - Troubleshoot Spanning Tree Protocol (STP)

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Article ID: KB22774 KB Last Updated: 20 Feb 2020Version: 5.0
Summary:

This article aims to assist with Spanning Tree Protocol (STP) troubleshooting in EX Series Ethernet switches that are configured as Layer 2 switches, by providing a step-by-step approach.

Note: Spanning Tree Protocol was introduced as 802.1D to have a loop-free network.

 

Symptoms:

Symptoms:

  • A loop has formed in the network.
  • Topology changes have increased.
  • Layer 2 traffic is disrupted.
  • Interface goes into BLOCK state causing traffic to be dropped.
  • Traffic is flooding.

 

Solution:

Perform the following steps to troubleshoot:

[Map out network topology]

Step 1. Map out the network topology and identify the following components. For an example topology with definitions, refer to KB22832 - Sample Spanning Tree Network Topology with EX switches.

[Spanning-Tree Protocol Flavor Check]

Step 2.  Verify that all the switches connected in the network are running the same Spanning-Tree Protocol flavor (i.e RSTP, MSTP, STP, or VSTP).

Are all the switches connected in the network running the same Spanning-Tree Protocol flavor?

[Root Bridge Interface Check]

Step 3.  Starting on the Root Bridge in your network, are all the interfaces on this Root switch in the Forwarding (FWD) state?

  • To verify, run the command:   show spanning-tree interface.

    • In STP, all connected ports on a Root Bridge should be in FWD state.

    • For more information on the output of the above command, refer to show spanning-tree interface


  • Yes - Continue to Step 4

  • No - Call your technical support representative.  This is a basic requirement.

[Topology Change Check] 

Step 4.  On this switch, do you see the 'Number of topology changes' increasing?

  • To verify, run the command:   show spanning-tree bridge detail.


  • Yes - Continue to Step 5

  • No - Jump to Step 7 (to do Basic Loop Check)

Step 5.  On this switch, verify if the non-edge interfaces have changed state. 
show log messages
show interfaces  extensive
  • Do you see that a non-edge interface has changed state?

    • No - Continue to Step 6

    • Yes - Troubleshoot the interface that has flapped, and the STP should stabilize:

      • Check for any loose cables.
      • Check for any outage.
      • Check if any configuration change has been made.

Step 6.  Since the cause of the topology change was not identified in Step 5 on the current switch, we need to move to the next node in the Spanning Tree to trace the source of the topology change.

  • The next node in the Spanning Tree can be determined from the value of the 'Topology change Initiator' field; it is the node from which the current switch received the topology change. 

  • Run the command: show spanning-tree bridge detail.

Switch@juniper>show spanning-tree bridge

STP bridge parameters
Context ID : 0
Enabled protocol : RSTP <--
Root ID : 32768.28:c0:da:3d:50:40
Hello time : 2 seconds
Maximum age : 20 seconds
Forward delay : 15 seconds
Message age : 0
Number of topology changes : 2
Time since last topology change : 3218 seconds 
Topology change initiator : ge-0/0/4.0   <-------------
Topology change last recvd. from : 2c:6b:f5:8b:23:04
Local parameters
Bridge ID : 32768.28:c0:da:3d:50:40
Extended system ID : 0
Internal instance ID : 0

a.  Connect to the switch off of the interface identified in the 'Topology Change initiator' output. 

b.  Go to Step 4 and repeat Steps 4-5 (Repeat Steps 4-5 on the switch from which the topology change is received.)        

[Basic Loop Check]

Step 7.  Analyze the traffic interface activity for unusually heavy traffic.

  • To do this, run the following command, and compare the output with the baseline output when traffic is normal.

SwitchA> monitor interface traffic

  • For example, below is the output of SwitchA with normal network activity:

Interface    Link  Input packets        (pps)     Output packets        (pps)
 ge-0/0/0    Down              0          (0)                0          (0)
 ge-0/0/1    Down              0          (0)                0          (0)
 ge-0/0/2    Down              0          (0)                0          (0)
 ge-0/0/3    Down              0          (0)                0          (0)
 ge-0/0/4    Down              0          (0)                0          (0)
 ge-0/0/5    Down              0          (0)                0          (0)
 ge-0/0/6    Down              0          (0)                0          (0)
 ge-0/0/7    Down              0          (0)                0          (0)
 ge-0/0/8    Down              0          (0)                0          (0)
 ge-0/0/9    Down              0          (0)                0          (0)
 ge-0/0/10     Up             37          (0)             5570          (0)
 ge-0/0/11   Down              0          (0)                0          (0)
 ge-0/0/12   Down              0          (0)                0          (0)
 ge-0/0/13   Down              0          (0)                0          (0)
 ge-0/0/14   Down              0          (0)                0          (0)
 ge-0/0/15   Down              0          (0)                0          (0)
 ge-0/0/16   Down              0          (0)                0          (0)
 ge-0/0/17   Down              0          (0)                0          (0)
 ge-0/0/18   Down              0          (0)                0          (0)
 ge-0/0/19   Down              0          (0)                0          (0)
 ge-0/0/20     Up             88          (0)             1646          (0)
 ge-0/0/21   Down              0          (0)                0          (0)
 ge-0/0/22   Down              0          (0)                0          (0)
 ge-0/0/23     Up            122          (0)             1639          (0)
 vcp-0       Down              0                             0
 vcp-1       Down              0                             0
 ae0           Up            125          (0)             7216          (0)
 ae1         Down              0          (0)                0          (0)
 ae2         Down              0          (0)                0          (0)
 ae3         Down              0          (0)                0          (0)
 ae4         Down              0          (0)                0          (0)
 ae5         Down              0          (0)                0          (0)

Bytes=b, Clear=c, Delta=d, Packets=p, Quit=q or ESC, Rate=r, Up=^U, Down=^D
  • Below is the example output of SwitchA when it is experiencing abnormal activity:  

(Note: The packet rates below may be normal for another switch.  The key is to create a baseline and capture the packet rates during normal periods, so you can compare packet rates when experiencing problems.)

Interface    Link  Input packets        (pps)     Output packets        (pps)
 ge-0/0/0    Down              0          (0)                0          (0)
 ge-0/0/1    Down              0          (0)                0          (0)
 ge-0/0/2    Down              0          (0)                0          (0)
 ge-0/0/3    Down              0          (0)                0          (0)
 ge-0/0/4    Down              0          (0)                0          (0)
 ge-0/0/5    Down              0          (0)                0          (0)
 ge-0/0/6    Down              0          (0)                0          (0)
 ge-0/0/7    Down              0          (0)                0          (0)
 ge-0/0/8    Down              0          (0)                0          (0)
 ge-0/0/9    Down              0          (0)                0          (0)
 ge-0/0/10     Up             37          (0)             5593          (1)
 ge-0/0/11   Down              0          (0)                0          (0)
 ge-0/0/12   Down              0          (0)                0          (0)
 ge-0/0/13   Down              0          (0)                0          (0)
 ge-0/0/14   Down              0          (0)                0          (0)
 ge-0/0/15   Down              0          (0)                0          (0)
 ge-0/0/16   Down              0          (0)                0          (0)
 ge-0/0/17   Down              0          (0)                0          (0)
 ge-0/0/18   Down              0          (0)                0          (0)
 ge-0/0/19   Down              0          (0)                0          (0)
 ge-0/0/20     Up             88          (0)         35162079    (1116047)
 ge-0/0/21   Down              0          (0)                0          (0)
 ge-0/0/22   Down              0          (0)                0          (0)
 ge-0/0/23     Up       35160729    (1116048)             1640          (0)
 vcp-0       Down              0                             0
 vcp-1       Down              0                             0
 ae0           Up            125          (0)         35167672    (1116048)
 ae1         Down              0          (0)                0          (0)
 ae2         Down              0          (0)                0          (0)
 ae3         Down              0          (0)                0          (0)
 ae4         Down              0          (0)                0          (0)
 ae5         Down              0          (0)                0          (0)

Bytes=b, Clear=c, Delta=d, Packets=p, Quit=q or ESC, Rate=r, Up=^U, Down=^D
  • Is this switch encountering unusually heavy traffic activity on the interfaces?

    • Yes - Continue to Step 8

    • No - End of workflow. The basic STP checks (a Topology Change Check and a Basic Loop Check) are good. If you are still encountering a Spanning Tree Protocol issue, jump to Step 9

 Step 8.   Is Spanning-Tree Protocol enabled? For more information on how to check, refer to KB22775 - Verify the flavor of the Spanning Tree Protocol (STP) running on the EX switch.

  • No - Enabling Spanning-Tree Protocol will mitigate loops, so if possible, enable Spanning-Tree Protocol on the devices in the network segment where the loop is observed.  For more information on how to do this, refer to KB22775 - Verify the flavor of the Spanning Tree Protocol (STP) running on the EX switch.

  • Yes - Run the following commands to help troubleshoot this switch around the abnormal traffic flow. If the cause cannot be identified, continue to Step 9.

show system processes
show interfaces  extensive
show log messages

[Data Collection for further troubleshooting]

Step 9.  If the above steps do not resolve the problem, then collect the following information and open a case with your technical support representative:

  • Network Topology/Diagram labeling spanning-tree roles of each device

  • Logs on the EX device:

show spanning-tree bridge
show spanning-tree bridge detail
show spanning-tree interface
show spanning-tree interface detail
show spanning-tree statistics interface 
show spanning-tree statistics interface  detail

show ethernet-switching interfaces
show ethernet-switching table

show interface
show interface extensive

show log messages
Request Support Information
request support information all-member | no-more

 

Modification History:

2020-02-20: Article reviewed for accuracy; no changes required.

 

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