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AR Router Troubleshooting Guide

This Product Documentation provides guidance for maintaining AR Enterprise Router, covering common information collection and fault diagnostic commands, typical fault troubleshooting guide, and troubleshooting.
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Pinging a Directly Connected Device Fails Because of an Incorrect ARP Entry

Pinging a Directly Connected Device Fails Because of an Incorrect ARP Entry

Fault Symptom

As shown in Figure 14-18, Router A and Router B are directly connected. Router A replaced another device that was previously connected to Router B. After the network adjustment, Router A cannot ping Router B, and the OSPF neighbor status on Router A is Exchange. After Router A is replaced by the original device, the fault is rectified.

Figure 14-18 Network diagram of directly connected devices

Fault Analysis

  1. The original device could ping Router B, indicating that the link between the two devices functions properly. Router A and Router B are directly connected, so the fault is not caused by routing problems. The fault may be caused by errors in ARP learning.

  2. Run the display arp all command on Router A to check the ARP table.
    <RouterA> display arp all
    IP ADDRESS      MAC ADDRESS     EXPIRE(M) TYPE        INTERFACE   VPN-INSTANCE
                                              VLAN/CEVLAN PVC
    ------------------------------------------------------------------------------
    1.1.1.1  0025-9e80-2494            I -         GE1/0/0
    1.1.1.2  0025-9e80-248e  18        D-0         GE1/0/0                
    ------------------------------------------------------------------------------
    Total:2        Dynamic:1       Static:0    Interface:1
    
    The preceding command output shows that Router A has learned the ARP entry of Router B.
  3. Run the display arp all command on Router B to check the ARP table.
    <RouterB> display arp all
    IP ADDRESS      MAC ADDRESS     EXPIRE(M) TYPE        INTERFACE   VPN-INSTANCE
                                              VLAN/CEVLAN PVC
    ------------------------------------------------------------------------------
    1.1.1.2  0025-9e80-248e            I -         GE1/0/0
    1.1.1.1  0016-ecb9-0eb2  18        s         GE1/0/0                
    ------------------------------------------------------------------------------
    Total:2        Dynamic:0       Static:1    Interface:1
    
    This ARP table shows that the IP address 1.1.1.1 maps the MAC address 0016-ecb9-0eb2. The ARP entry type is S, indicating a static ARP entry. According to the ARP table on Router A, however, 0016-ecb9-0eb2 is not the MAC address actually mapped to 1.1.1.1.

    This static ARP entry was configured before the network adjustment. The ARP entry was not updated after the network adjustment and therefore, Router A cannot ping Router B.

Procedure

  1. Run the system-view command on Router B to enter the system view.
  2. Run the undo arp static ip-address mac-address command to delete the static ARP entry.

    NOTE:

    After the static ARP entry is deleted, Router A can ping Router B. A new static ARP entry needs to be configured to prevent ARP attacks.

  3. Run the arp static ip-address mac-address command to configure a new static ARP entry.

    Router A can ping Router B. Run the display ospf peer command to check the status of the OSPF neighbor. The OSPF neighbor is in Full state.

    <RouterA> display ospf peer
             OSPF Process 1 with Router ID 11.11.11.105                             
                     Neighbors                                                      
                                                                                    
     Area 0.0.0.0 interface 1.1.1.1(GigabitEthernet1/0/0)'s neighbors                           
     Router ID: 2.1.1.1               Address: 1.1.1.2                                   
       State: Full  Mode:Nbr is  Master  Priority: 1                                
       DR: 1.1.1.2  BDR: 2.1.1.1  MTU: 0                                            
       Dead timer due in 30  sec                                                    
       Retrans timer interval: 5                                                    
       Neighbor is up for 00:28:17                                                  
       Authentication Sequence: [ 0 ]                                               
    

Summary

If a static ARP entry is configured on a device, this entry must be modified after the MAC address changes. If Router B is a non-Huawei device and you cannot log in to it to check the configuration, configure the mirroring function to analyze packets transmitted between Router A and Router B, and then ping Router B from Router A. Check whether the destination MAC addresses of the packets are correct.

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Updated: 2019-08-09

Document ID: EDOC1000079719

Views: 495754

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