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Timely Updating of the MAC Address Table of an Intermediate Layer 2 Device on a Networking Running VRRP Failed

Publication Date:  2013-10-30 Views:  47 Downloads:  0
Issue Description

On a network shown in the attachment, to improve reliability, the traffic was sent from the switch upstream to two NE80Es (version: V600R001C00SPC800) and VRRP was enabled to provide gateway redundancy protection.

In the upstream direction: The gateway on the user side used the VRRP virtual address. For the Layer 2 packets, the destination MAC address was the VRRP virtual MAC address and the source MAC address was the MAC address of the user's interface.

In the downstream direction: For the Layer 2 packets sent from the Internet, the destination MAC address was the host MAC address of the user and the source MAC address was the router's interface MAC address.

The intermediate Layer 2 switch forwards data based on the MAC address. Specifically, it learns the user's MAC address for forwarding downstream packets and the VRRP virtual MAC address for forwarding upstream packets.

The host ARP aging time was 20 minutes by default. The equipment had to send an ARP request packet in every 20 minutes to request the VRRP virtual address. The primary VRRP router then replied with an ARP response packet with the VRRP virtual MAC address as the source MAC address. Therefore, the switch received the Layer 2 packet with the VRRP virtual address as the source MAC address in every 20 minutes, but the default time for MAC addresses was 5 minutes. The 15- minute time difference may cause the switch to forward the Layer 2 packets as unknown unicast packets. The issue had to be pay attention to and resolved. 
Handling Process
None
Root Cause
The ARP aging time was not equal to the default MAC aging time on an intermediate Layer 2 device. 
Solution

By default, a router sent a gratuitous ARP packet to each VRRP group to update the switch's MAC address table. Huawei could change the interval at which the ARP packet was sent for updating MAC address tables of switches. Also, customer can choose to disable the function.

[PE]vrrp gratuitous-arp timeout ?

INTEGER<30-1200> Timeout value(s)

disable Cancel sending gratuitous arp
Suggestions
Disabling the transmission of gratuitous ARP packets is not recommended as the intermediate Layer 2 device may forward a large number of unknown unicast packets.

END