Troubleshooting Forwarding Loops

In a Spanning Tree Protocol (STP)-enabled network withdual core switches, a loop occurs in the originally stable network. It isobserved that the port status LEDs keep blinking at the same frequency. Run theshow interface counters command oninterconnected interfaces. It is shown that the counters are very large, mostof which are of increased broadcast packets.

Possible Causes

1) STP is not configured on certainswitches, which causes errors in STP algorithm.

2) STP standards run on switches areincompatible, which causes troubles in STP.

3) The BPDU filter is incorrectlyconfigured on switches, which causes STP packets to be filtered out.

4) Errors occur when switches processBPDU packets, which causes the failure of forwarding BPDU packets.

Troubleshooting Procedure

Overview：

Step 1: Check the STP configurationof the switches.

Step 2: Check the STP compatibilitybetween the switches.

Step 3: Check the STP configurationof the interfaces.

Step 4: Check the transmission ofBPDU packets by the switches.

Step 5: Collect fault information and submit cases toRuijie Service Portal.

Step1: Check the STP configuration of the switches

1. Certain switches do not support STP or cannot forwardBPDU packets when not enabled with STP. Check whether the switches involved inSTP are enabled with STP (including whether the devices are configured with thesame STP mode, such as STP/MSTP/RSTP) and can normally process BPDU packets. Checkwhether all related devices are enabled with STP.

Verification:

Ruijie(config)#show spanning-tree
StpVersion : MSTP
SysStpStatus : ENABLED
MaxAge : 20
HelloTime : 2
ForwardDelay : 15
BridgeMaxAge : 20
BridgeHelloTime : 2
BridgeForwardDelay : 15
MaxHops: 20
TxHoldCount : 3
PathCostMethod : Long
BPDUGuard : Disabled
BPDUFilter : Disabled
LoopGuardDef  : Disabled

###### mst 0 vlans map : ALL

BridgeAddr : 001a.a916.11b6
Priority: 32768
TimeSinceTopologyChange : 0d:0h:5m:26s
TopologyChanges : 0
DesignatedRoot : 32768.001a.a916.11b6
RootCost : 0
RootPort : 0
CistRegionRoot : 32768.001a.a916.11b6
CistPathCost : 0

2. If a switch does not support STP, check whether theswitch transparently transmits BPDU packets. Conduct experiment if necessary.

Step2: Check the STP compatibility between the switches

If some of the switches in a network run non-standardSTP, you need to check whether the local loop is caused by STP incompatibility.Based on the network deployment, check the status of STP and the STP-enabledinterfaces. Ensure that the blocking and forwarding of STP between switches iscompliant with the deployment requirements, so as to avoid the discarding of BPDUpackets due to the local loop.

Run the showspanning-tree, show spanning-treesummary and show spanning interfacexx commands to check the roles of the switches and interfaces in STP. Runthe following commands to display the information:

1. show spanning-treeinterface

ruijie#show spanning-tree

StpVersion : MSTP             <---Current STP mode

SysStpStatus : ENABLED

MaxAge : 20

HelloTime : 2

ForwardDelay : 15

BridgeMaxAge : 20

BridgeHelloTime : 2

BridgeForwardDelay : 15

MaxHops: 20

TxHoldCount : 3

PathCostMethod : Long

BPDUGuard : Disabled

BPDUFilter : Disabled

###### mst 0 vlans map : 1, 3-4094        <----VLAN1, 3 to 4094 are associated with Instance0.

BridgeAddr : 00d0.f822.3caa              <-------MAC address of the localswitch.

Priority: 32768

TimeSinceTopologyChange : 0d:3h:43m:21s

TopologyChanges : 3

DesignatedRoot : 8000.00d0.f822.3caa   <---MAC address of the root bridge in alldomains, so-called the CIST (Common and Internal Spanning Tree)root

RootCost : 0

RootPort : 0     <----RPselected in Instance0 of the local switch. If the root bridge is the localswitch, this value is 0.

CistRegionRoot : 8000.00d0.f822.3caa    <-----MAC address of the root bridge ofinstance0 in this domain.

CistPathCost : 0

###### mst 1 vlans map : 2

BridgeAddr : 00d0.f822.3caa            <------MAC address of the localswitch

Priority: 0

TimeSinceTopologyChange : 0d:3h:42m:40s

TopologyChanges : 3

DesignatedRoot : 0001.00d0.f822.3caa   <-----MAC address of the root bridgeselected by Instance1.

RootCost : 0

RootPort : 0                          <-----RPselected by of the Local switch in Instance1. If the root bridge is the localswitch, this value is 0.

Note: Instance 0 and other instances are different.Instance 0 plays a unique role in MSTP, which is involved in the topologicalcalculation of both this domain and the entire switching network.

2. show spanning-tree interface

ruijie#sh spanning-tree int gi 0/24

PortAdminPortFast : Disabled

PortOperPortFast : Disabled

PortAdminLinkType : auto

PortOperLinkType : point-to-point

PortBPDUGuard : disable

PortBPDUFilter : disable

###### MST 0 vlans mapped :1, 3-4094

PortState : forwarding                     <----Forwarding state ofthe current interface in Instance0.

PortPriority : 128

PortDesignatedRoot : 8000.00d0.f822.3caa   <----General root bridge (not a domainroot bridge) of Instance0 learned by this interface.

PortDesignatedCost : 0

PortDesignatedBridge :8000.00d0.f822.3caa  <---Bridge ID. If this interface is a DR,the value is the bridge ID of this interface. If this interface is RP,Alternate, or Backup, this value indicates the bridge ID of the uplink bridgein this instance.

PortDesignatedPort : 8018                 <---Interface ID. If theinterface is a DP, the value is the ID of this interface. If the interface isRP, Alternate, or Backup, this value indicates the bridge ID of the uplinkinterface in this instance.

This value consists of 16 bits; of which the firstfour bits indicate the priority 16 of the uplink interface and the last 12 bitsindicate the index of the uplink interface. They are in the hexadecimal format.Run the show interface command todisplay the index of an interface. In the example, this value indicates thatthe priority of the uplink interface is 128 (decimal) and the index of theuplink interface is 24 (decimal).

PortForwardTransitions : 2

PortAdminPathCost : 200000

PortOperPathCost : 200000

PortRole :designatedPort  <-------Role of thisinterface in instance0.

###### MST 1 vlans mapped :2

PortState : forwarding  <--------Forwarding state of thisinterface in Instance1.

PortPriority : 128

PortDesignatedRoot : 0001.00d0.f822.3caa  <--------Root bridge (regional root) ofthis interface learned in Instance1.

PortDesignatedCost : 0

PortDesignatedBridge :0001.00d0.f822.3caa    <--------Uplink bridge ID.

PortDesignatedPort : 8018

PortForwardTransitions : 1

PortAdminPathCost : 200000

PortOperPathCost : 200000

PortRole :designatedPort   <------Role of thisinterface inInstance1.

Step3: Check the STP configuration of the interfaces.

1. Interfaces involved in STP must be enabled with it; otherwise, a loopcannot be blocked.

2. If the loop occurs in the access switches, check whether the BPDU filteris enabled. Run the show interface countercommand to display the traffic passing through an interface:

Ruijie#show interfacecounter    //Check whether the input oroutput traffic is heavy on the interface and most increased packets arebroadcast packets.

Ruijie#showrunning-config interface xx

3. If BPDU filter is configured, you need to disable the BPDU function onthis interface. The command for disabling the BPDU filter is as follows:

Use the fa 0/1 interfacefor example:

Ruijie(config)#int f0/1

Ruijie(config-FastEthernet 0/1)#spanning-treebpdufilter disabled

4. Check whether relatedswitches can identify or normally forward BPDU packets. If you cannot confirm it,perform offline experiment for verification. If the fault is caused by STP incompatibility,you can modify the configuration or replace switches.

Step4: Check the transmission of BPDU packets by theswitches.

1. If switches involved in STP cannot receive and sendBPDU packets normally, STA errors may occur. This will cause a temporary loopwhich may deteriorate the network environment. There are various possiblecauses. One cause is that BPDU packets are discarded because excessive packetsare sent to the CPU and BPDU packets have a lower priority in the CPP queue.The debugging command can be used for troubleshooting.

2.Check whether timeout occurs in BPDU packet transmission on an interface byenabling debugging.

Run thedebug mstp rx command to checkinformation including whether a non-root bridge receives BPDU packets normallyfrom the RP, whether the packets are legitimate, and whether other interfacesreceive BPDU packets.

Checkwhether the peer switch regularly sends BPDU packets. Run the debug mstp tx command to check the timewhen the local switch sends BPDU packets, and the source interface sending thepackets.

3. Ifthe root bridge does not receive BPDU packets regularly, the fault may becaused by timeout of BPDU packets. In this case, check whether the CPU usage ofall switches involved in STP is too high. Being attacked by massive packets maycause high CPU usages and then abnormal packet reception and sending. You canrun the following commands to display the BPDU packets in the CPP queue tocheck whether the CPU usage is too high and whether the CPU has Drop packetswith the BPDU attribute.

Ruijie#show cpu

Ruijie#show cpu-protect mboard

4. High CPU usage may lead to the loss of BPDUpackets. Check whether the high CPU usage is in normal range. If it is normal,adjust the priority of BPDU packets and the BPDU bandwidth value in the CPPqueue on the switch. The commands are as follows:

ruijie(config)#cpu-protect type bpdu pps 400

ruijie(config)#cpu-protect type bpdu pri 7

Debugging operations cause risks (Theworst case is to restart the switch for recovery). Perform debugging only afterinforming customers of the risks and get them accepted. It is recommendeddebugging at low-traffic periods (Be more cautious when dealing with coreswitches). If packet capturing is also required for troubleshooting, rememberto collect information by debugging and packet capturing at the same time..

Step5:Collect fault information and submit cases to RuijieService Portal.

If the fault is not rectifiedthrough the preceding steps, collect the following information together withthe analysis information obtained in steps 1 to 4. Submit cases to RuijieService Portal for further support.

Collect the network topology information on allSTP-enabled devices.

show runnig-config

show spanning-tree

show spanning-tree summary

show spanning-tressinterface xx (Switch interface)

showversion

showversion slot  //Chassis switch slot