VRRP Master Election
When a VRRP configured network starts, VRRP uses an election algorithm to dynamically assign master responsibility to one of the VRRP routers on the network.
VRRP priority—Possible values are 0 through 255. Value 255 is reserved for the VRRP router IP address owner, and value 0 is reserved for the master router, to indicate it is releasing master responsibility. Values 1–254 can be configured on backup routers to influence which backup router becomes the master when the master VRRP router is no longer available. The higher number has higher priority. The default value for backup routers is 100.
Higher IP address—If multiple backup routers have the same configured priority, the router with the highest IP address becomes the master.
If the master router becomes unavailable, the election process begins and the backup router that wins the election assumes the role of master.
NoteIn VRRP IPv6, master election happens based on the interface link local address when the priorities are the same. The highest link local address switch is selected as the VRRP master.
A new master is elected when one of the following things happen:
- VRRP is disabled on the master router.
- Loss of communication occurs between master and backup router(s).
If VRRP is disabled on the master interface, the master router sends an advertisement with the priority set to 0 to all backup routers. This signals the backup routers that they do not need to wait for the master down interval to expire, and the master election process can begin immediately.
The master down interval is set using the following formula: 3 x advertisement interval + skew time.
The advertisement interval is a user-configurable option, and the skew time is (256-priority/256).
NoteThe formula for VRRPv2; ((256 - priority) / 256). The VRRPv3 Skew time calculation will be different. Please refer to RFC 5798 (http://tools.ietf.org/html/rfc5798), Skew time. (((256 - priority) x Master_Adver_Interval) / 256)
NoteAn extremely busy CPU can create a short dual master situation. To avoid this, increase the advertisement interval.