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Configuring Passive ARP Learning for Backup VRRP Routers

By default, the backup VRRP router drops ARP requests for the VRRP-IP to VRRP-MAC address translation. This means that the backup router does not learn the ARP (IP-to-MAC address) mappings for the hosts sending the requests. When it detects a failure of the primary router and transitions to become the new primary router, the backup router must re-learn all the entries that were present in the ARP cache of the primary router. In environments with many directly attached hosts, such as metro Ethernet environments, the number of ARP entries to learn can be high. This can cause a significant transition delay, during which the traffic transmitted to some of the hosts might be dropped.

Passive ARP learning enables the ARP cache in the backup router to hold approximately the same contents as the ARP cache in the primary router, thus preventing the problem of learning ARP entries in a burst. To enable passive ARP learning, include the passive-learning statement at the [edit system arp] hierarchy level:

We recommend setting passive learning on both the backup and primary VRRP routers. Doing so prevents the need to manually intervene when the primary router becomes the backup router. While a router is operating as the primary router, the passive learning configuration has no operational impact. The configuration takes effect only when the router is operating as a backup router.

For information about configuring gratuitous ARP and the ARP aging timer, see the Junos OS Administration Library for Routing Devices.