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Improving the Convergence Time for VRRP

You can enable faster convergence time for the configured Virtual Router Redundancy Protocol (VRRP), thereby reducing the traffic restoration time to less than 1 second. To improve the convergence time for the VRRP, perform the following tasks:

• Configure the distributed periodic packet management process—When the VRRP process is busy and does not send VRRP advertisements, the backup VRRP routers might assume that the master router is down and take over as the master router, causing unnecessary flaps. To address this problem and to reduce the load on the VRRP process, Junos OS uses the distributed periodic packet management (PPM) process to send VRRP advertisements on behalf of the VRRP process.

  To configure the distributed PPM process, include the delegate-processing statement at the [edit protocols vrrp] hierarchy level.

• Disable the skew timer—The skew timer in VRRP is used to ensure that two backup routers do not switch to the master state at the same time in case of a failover situation. When there is only one master router and one backup router in the network deployment, you can disable the skew timer, thereby reducing the time required to transition to the master state.

  To disable the skew timer, include the skew-timer-disable statement at the [edit protocols vrrp] hierarchy level.

• Configure the number of fast advertisements that can be missed by a backup router before it starts transitioning to the master state—The backup router waits until a certain number of advertisement packets are lost after which it transitions to the master state. This waiting time can be fatal in scenarios such as router failure or link failure. To avoid such a situation and to enable faster convergence time, in Junos OS Release 12.2 and later, you can configure a fast advertisement interval value that specifies the number of fast advertisements that can be missed by a backup router before it starts transitioning to the master state.

  To configure the fast advertisement interval, include the global-advertisements-threshold statement at the [edit protocols vrrp] hierarchy level.

• Configure inheritance of VRRP groups—Junos OS enables you to configure VRRP groups on the various subnets of a virtual LAN (VLAN) to inherit the state and configuration of one of the groups, which is known as the active VRRP group. When the vrrp-inherit-from statement is included in the configuration, only the active VRRP group, from which the other VRRP groups inherit the state, sends out frequent VRRP advertisements and processes incoming VRRP advertisements. Use inherit groups for scaled configurations. For example, if you have 1000 VRRP groups with an advertisement interval of 100 ms, then use inherit groups.

  To configure inheritance for a VRRP group, include the vrrp-inherit-from statement at the [edit interfaces interface-name unit logical-unit-number family inet address address vrrp-group group-id] hierarchy level.

Inheritance of VRRP groups is supported with all types of interfaces. Other measures to reduce convergence time, such as VRRP distribution, disabling skew timer, and reducing advertisement threshold, are not applicable when VRRP is configured over integrated routing and bridging (IRB) interfaces, aggregated Ethernet interfaces, and multichassis link aggregation group (MC-LAG) interfaces. Compared to other routers, the convergence time and the traffic restoration time are less for MX Series routers with MPCs. Reduction in convergence time is applicable for all types of configurations at the physical interface but the convergence time might not be less than 1 second for all the configurations. The convergence time depends on the number of groups that are transitioning from the backup to the master state and the interval at which these groups are transitioning.