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Services Are Interrupted When the ARP Entry Learning Is Faulty in the Ping Process

Publication Date:  2013-01-07 Views:  23 Downloads:  0
Issue Description

As shown in the following topology, the services between the BTS and the BSC are interrupted. The on-site Huawei technical support personnel check the live network and find that the NE80E ping operation is attached to the BTS. The ping operation fails. There are NE80Es and CX200s on the link between the BTS and the BSC. The CX200 is a Layer 2 device, implementing Layer 2 forwarding.

Handling Process

The on-site Huawei technical support personnel first identify the problem. The NE80E pings the address 172.20.2.24 of the BTS. The ping operation alternates between Up and Down.

They check the routing table on the NE80E. The routes destined for 172.20.2.24 are correct and the corresponding outbound interface numbers are correct.

They check the ARP table on the NE80E and the MAC address table on the CX200.

To help locate the problem, a VLANIF interface can be configured on the CX200 that is directly connected to the BTS. The ARP table can be checked on the VLANIF interface. They find that the outbound interface number corresponding to 172.20.2.24 in the ARP table is abnormal. The corresponding outbound interface number should have been Eth 0/0/14, but Eth 0/0/10 is displayed in the ARP entry.

<TEC-CE-CX200B-IAA0011-1>display  arp dynamic

IP ADDRESS      MAC ADDRESS  EXPIRE(M) TYPE INTERFACE  VPN-INSTANCE   VLAN

---------------------------------------------------------------------------

172.20.2.112    0025-9e06-14ff  7      D-0  Eth0/0/10                   205

172.20.2.3      001a-704e-de17  11     D-0  Eth0/0/10                   205

172.20.2.24     001a-704e-de17  18     D-0  Eth0/0/10                   205

172.20.2.124    001d-09c5-246d  20     D-0  Eth0/0/14                   205

They check the ARP entry on the NE80E and find that the MAC address corresponding to 172.20.2.24 is flapping in the ARP entry.

<TEC-PE-NE80E-A2>display  arp interface  Vlanif  205 | include 172.20.2.24

IP ADDRESS  MAC ADDRESS   EXPIRE(M) TYPE        INTERFACE   VPN-INSTANCE                                                      VLAN/CEVLAN PVC                                                                          

--------------------------------------------------------------------------                                                     

172.20.2.24   001a-704e-de17  20        DF2   GE2/0/0        2G_IN_PLANE_B                                                 

--------------------------------------------------------------------------                                                   

Total:110       Dynamic:109     Static:0    Interface:1

<TEC-PE-NE80E-A2>display  arp interface  Vlanif 205 | include 172.20.2.2

IP ADDRESS MAC ADDRESS     EXPIRE(M) TYPE        INTERFACE   VPN-INSTANCE                                                     

VLAN/CEVLAN PVC                                                                          

---------------------------------------------------------------------------                                                

172.20.2.20   0018-82e5-e1e7  14        DF    GE2/0/0        2G_IN_PLANE_B                                                 

172.20.2.24   001a-a0fd-a726  18        DF2    GE2/0/0        2G_IN_PLANE_B                                                 

172.20.2.254  0018-8259-c65d  20        DF2    GE2/0/0        2G_IN_PLANE_B                                                                                       

---------------------------------------------------------------------------                                                 

Total:111       Dynamic:110     Static:0    Interface:1

The preceding symptom shows that the MAC addressed of the multiple BTSs attached to the NE80E may conflict or there are error ARP packets, which causes the ARP entries on the CX200 and the NE80E to be updated incorrectly. When the ARP table is correct and the MAC address corresponds to 001a-a0fd-a726, the ping operation succeeds. When the ARP table is incorrect and the MAC address corresponds to 001a-704e-de17, the ping operation fails.

The Huawei wireless engineers consider that the MAC addresses of all BTSs are different, so the address conflict does not exist. They continue to search the source of the abnormal ARP entry and check each hop. Finally, they find the source of the abnormal MAC address.

Ethernet 0/0/5 on CX200B-IAA0025-1 keeps learning the MAC address corresponding to 001a-704e-de17, which proves that the incorrect ARP packet is sent from the device attached to CX200B-IAA0025-1.

<SKB-CE-CX200B-IAA0025-1>display  mac-address 001a-704e-de17  v  205

MAC Address    VLAN ID           Port                      Type       Lsp  

--------------------------------------------------------------------------

001a-704e-de17 205               Ethernet0/0/5             dynamic    0/0

Total matching items displayed = 1

Finally they confirm that the device attached to Eth 0/0/5 on CX200-IAA0025 continuously sends error ARP packets, causing the MAC entry on the CX200 and the ARP entry on the NE80E to be incorrect. When the ARP table is correct, traffic can reach the BTS, so the NE80E can ping the BTS. When the ARP table is incorrect, the NE80E fails to ping the BTS.

Root Cause

Anomalous ARP packets make ARP entry learning become faulty.

Solution

To ensure that the services recover on time, shut down Eth 0/0/5 on CX200-IAA0025 to prevent incorrect ARP packets entering the interface. After the services recover, disable the device that sends the incorrect ARP packets.

Suggestions
The device attached to Eth 0/0/5 on CX200-IAA0025 continuously sends error ARP packets, but the NE80E can learn the ARP entries as expected and cannot decide whether the MAC addresses in the ARP entries are correct or not. Therefore, blocking the source of sending error ARP packets is the preventative measure.

END