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Troubleshooting Guide

CloudEngine 16800, 12800, 12800E, 8800, 7800, 6800, and 5800 Series Switches

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Linux Server Connected to the CE12808 Fails to Ping the Virtual Gateway IP Address After Restart

Linux Server Connected to the CE12808 Fails to Ping the Virtual Gateway IP Address After Restart

Fault Symptom and Networking

1.1 Network Topology

Two CE12808 switches run the Virtual Router Redundancy Protocol (VRRP) and function as active and standby gateways. The gateway switches connect to two access switches, each of which is connected to a Linux server, as shown in the following figure.



1.2 Fault Symptom

After Linux virtual machine (VM) 1 with IP address 10.69.3.66 restarts, it cannot ping the virtual gateway address 10.69.3.254 on the CE12808-A. The ping succeeds after 20 minutes.

Alarms

A MAC address flapping alarm is generated:

hwMflpVlanLoopAlarm_active(l):CID=0x807f0477-alarmID=0x095e0012;MAC flapping detected, VlanId = 315, MacAddress = 0050-569f-1439, Original-Port = Eth-Trunk13, Flapping port = Eth-Trunk23, -. Please check the network to which the interface learning a flapping MAC address is connected.

Root Cause

After Linux VM 1 restarts, the outbound interface in the ARP entry matching VM 1 on the CE12808 is Eth-Trunk 23, but not Eth-Trunk 13. As a result, ping reply packets are forwarded to an incorrect interface. After 20 minutes, the ARP entry ages out and a new ARP entry is learned, so the ping succeeds.

After Linux VM 1 restarts, VM 2 sends IPv6 multicast packets with the MAC address of VM 1 as the source MAC address. The IPv6 multicast packets trigger a MAC address flapping, which in turn triggers an ARP update. As a result, the switches learn the ARP entries with incorrect interfaces and the ping fails.

Fault Identification

  1. After VM 1 restarts, the outbound interface at 10.69.3.66 on CE12808-A becomes Eth-Trunk 23. Normally, the outbound interface should be Eth-Trunk 13.

    <CE12808-A> display arp interface Vlanif 315
    ARP timeout:1200s
    ARP Entry Types: D - Dynamic, S - Static, I - Interface           
    
    IP ADDRESS  MAC ADDRESS   EXPIRE(M) TYPE    INTERFACE VPN-INSTANCE
                                        VLAN/CEVLAN                   
    ------------------------------------------------------------------
    10.69.3.66 0050-569f-1439 18        D-0     Eth-Trunk23     VRF 
                                        315/-

  2. On CE12808-A, hundreds of MAC address flappings occur within 12s, the flapped MAC address is 0050-569f-1439, and flapped interfaces are Eth-Trunk 13 and Eth-Trunk 23. (The following display is used as an example only. The display depends on the device configuration.)

    <CE12808-A> display mac-address flapping
    Mac-address Flpping Configurations :        
    -----------------------------------------------------------------------------
    Flapping detection          : Enable                              
    Aging  time(s)              : 300                                 
    Quit-vlan Recover time(m)   : --                                  
    Exclude VLAN-list           : --                                  
    -----------------------------------------------------------------------------
    S : start time   E : end time   (D) : error down
    -----------------------------------------------------------------------------
    Time                  VLAN MAC-Address   Original-Port Move-Ports     MoveNum
    -----------------------------------------------------------------------------
    S:2014-01-08 17:58:13 315 0050-569F-1439 Eth-Trunk13   Eth-Trunk11    268
    E:2014-01-08 17:58:25                                  Eth-Trunk23

  3. Check MAC address entries on CE12808-A. The outbound interface corresponding to the MAC address 0050-569f-1439 is Eth-Trunk 13.

    <CE12808-A> display mac-address
    ------------------------------------------------------------------
    MAC Address    VLAN/VSI                  Learned-From     Type    
    ------------------------------------------------------------------
    0050-569f-1439 315/-                     Eth-Trunk13      dynamic

  4. Perform inbound packet mirroring on Eth-Trunk 23 of CE12808-A. Eth-Trunk 23 receives IPv6 packets with unexpected MAC address.

    Due to punishment mechanism for MAC address flapping, ARP entries are not updated in real time when MAC address flappings occur multiple times on an interface.

Solution

Disable the IPv6 service on VM 2, and restart VM 1. No MAC address flapping occurs on the CE12808-A, and VM 1 can ping the gateway IP address successfully.

Summary

  • When many MAC address flappings occur on the network, first check whether physical loops exist and then check whether another interface sends packets with the same source MAC address.
  • The MAC address-triggered ARP update mechanism on CE switches helps update ARP entries quickly upon Layer 2 network changes. The MAC address flapping punishment mechanism is introduced to prevent excessive resource consumption (disable MAC address-triggered ARP update upon a large number of MAC address flappings). Generally, frequent MAC address flappings indicate a network flapping.
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Updated: 2020-01-07

Document ID: EDOC1000060766

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