VRRP failover-delay Overview
Failover is a backup operational mode in which the functions of a network device are assumed by a secondary device when the primary device becomes unavailable because of a failure or a scheduled down time. Failover is typically an integral part of mission-critical systems that must be constantly available on the network.
A fast failover requires a short delay. Thus, failover-delay configures the failover delay time, in milliseconds, for VRRP and VRRP for IPv6 operations. Junos OS supports a range of 50 through 2000 milliseconds for delay in failover time.
The VRRP process (vrrpd) running on the Routing Engine communicates a VRRP mastership change to the Packet Forwarding Engine for every VRRP session. Each VRRP group can trigger such communication to update the Packet Forwarding Engine with its own state or the state inherited form an active VRRP group. To avoid overloading the Packet Forwarding Engine with such messages, you can configure a failover-delay to specify the delay between subsequent Routing Engine to Packet Forwarding Engine communications.
The Routing Engine communicates a VRRP mastership change to the Packet Forwarding Engine to facilitate necessary state change on the Packet Forwarding Engine, such as reprogramming of Packet Forwarding Engine hardware filters, VRRP sessions and so on. The following sections elaborate the Routing Engine to Packet Forwarding Engine communication in two scenarios:
When failover-delay is not configured
Without failover-delay configured, the sequence of events for VRRP sessions operated from the Routing Engine is as follows:
- When the first VRRP group detected by the Routing Engine changes state, and the new state is master, the Routing Engine generates appropriate VRRP announcement messages. The Packet Forwarding Engine is informed about the state change, so that hardware filters for that group are reprogrammed without delay. The new master then sends gratuitous ARP message to the VRRP groups.
The delay in failover timer starts. By default, failover-delay timer is:
- 500 miliseconds—when the configured VRRP announcement interval is less than 1 second.
- 2 seconds—when the configured VRRP announcement interval is 1 second or more, and the total number of VRRP groups on the router is 255.
- 10 seconds—when the configured VRRP announcement interval is 1 second or more, and the number of VRRP groups on the router is more than 255.
- The Routing Engine performs one-by-one state change for subsequent VRRP groups. Every time there is a state change, and the new state for a particular VRRP group is master, the Routing Engine generates appropriate VRRP announcement messages. However, communication toward the Packet Forwarding Engine is suppressed until the failover-delay timer expires.
- After failover-delay timer expires, the Routing Engine sends message to the Packet Forwarding Engine about all VRRP groups that managed to change the state. As a consequence, hardware filters for those groups are reprogrammed, and for those groups whose new state is master, gratuitous ARP messages are sent.
This process repeats until state transition for all VRRP groups is complete.
Thus, without configuring failover-delay, the full state transition (including states on the Routing Engine and the Packet Forwarding Engine) for the first VRRP group is performed immediately, while state transition on the Packet Forwarding Engine for remaining VRRP groups is delayed by at least 0.5-10 seconds, depending on the configured VRRP announcement timers and the number of VRRP groups. During this intermediate state, receiving traffic for VRRP groups for state changes that were not yet completed on the Packet Forwarding Engine might be dropped at the Packet Forwarding Engine level due to deferred reconfiguration of hardware filters.
When failover-delay is configured
When failover-delay is configured, the sequence of events for VRRP sessions operated from the Routing Engine is modified as follows:
- The Routing Engine detects that some VRRP groups require a state change.
- The failover-delay starts for the period configured. The allowed failover-delay timer range is 5 through 2000 miliseconds.
- The Routing Engine performs one-by-one state change for the VRRP groups. Every time there is a state change, and the new state for a particular VRRP group is master, the Routing Engine generates appropriate VRRP announcement messages. However, communication toward the Packet Forwarding Engine is suppressed until the failover-delay timer expires.
- After failover-delay timer expires, the Routing Enigne sends message to the Packet Forwarding Engine about all VRRP groups that managed to change the state. As a consequence, hardware filters for those groups are reprogrammed, and for those groups whose new state is master, gratuitous ARP messages are sent.
This process repeats until state transition for all VRRP groups is complete.
Thus, when failover-delay is configured even the Packet Forwarding Engine state for the first VRRP group is deferred. However, the network operator has the advantage of configuring a failover-delay value that best suits the need of the network deployment to ensure minimal outage during VRRP state change.
 | Note: failover-delay influences only VRRP sessions operated by the VRRP process (vrrpd) running on the Routing Engine. For VRRP sessions distributed to the Packet Forwarding Engine, failover-delay configuration has no effect. |
