Gratuitous ARP requests on X-Series chassis

Products

• XOS

Issue/Introduction

Discussion about the use of gratuitous ARP requests on X-Series chassis and their configuration

A gratuitous ARP request is an ARP request packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. 

Gratuitous ARP requests are sent by Crossbeam to update other machines' ARP tables and to inform switches of the MAC address on a given switch port. The master VAP in the VAP group is responsible for sending gratuitous ARPs (as it is for other ARP communication).

Cause

The Crossbeam X-Series chassis implements gratuitous ARPs to support two different redundancy features:

1) VRRP

An IP address configured in the VRRP context will move from one chassis to another upon VRRP failover. In order to facilitate this change the master VAP sends a gratuitous ARP on each circuit that has a VRRP IP address attached to it.

In addition to gratuitous ARPs triggered by a failover, Crossbeam also broadcasts gratuitous ARPs every 30 seconds on each VRRP-enabled circuit. Periodic ARP announcements are required in some Layer 2 environments. The default interval can be adjusted in /crossbeam/etc/cbsvapcfgd.cf on VAPs:

#Periodic gratuitous arps interval
# default is 30
#periodic_grat_arp_interval=30

To make the change, you need to rsh to each VAP, edit the file /crossbeam/etc/cbsvapcfgd.cf and restart the cbsvapcfgd daemon.

2) Redundant Interfaces

This feature allows you to specify the backup for a physical interface. When the backup interface assumes the role of the failed port, it sends a gratuitous ARP request to inform attached Layer 2 or Layer 3 devices about the interface change. Based on the parameter mac-usage (master or active) either Layer 2 or Layer 3 devices need to be updated about the interface change.