H3C Technologies H3C S6800 Series Switches - Manual (Page 27)

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Master election
Routers in a VRRP group determine their roles by priority. When a router joins a VRRP group, it has a
backup role. The router role changes according to the following situations:
• If the backup does not receive any VRRP advertisement when the timer (3 × advertisement interval
+ Skew_Time) expires, it becomes the master.
• If the backup receives a VRRP advertisement with a greater or the same priority within the timer (3
× advertisement interval + Skew_Time), it remains a backup.
• If the backup receives a VRRP advertisement with a smaller priority within the timer (3 ×
advertisement interval + Skew_Time), it remains a backup when operating in non-preemptive mode,
or becomes the master when operating in preemptive mode.
The elected master starts a VRRP advertisement interval to periodically send VRRP advertisements to
notify the backups that it is operating correctly. Each of the backups starts a timer to wait for
advertisements from the master.
After a backup receives a VRRP advertisement, it compares only the priority in the packet with its own
priority.
When multiple routers in a VRRP group declare that they are the master because of network problems,
the one with the highest priority becomes the master. If two routers have the same priority, the one with
the highest IP address becomes the master.
VRRP application
Master/backup
In master/backup mode, only the master forwards packets, as shown in Figure 9 . When the master fails,
a new master is elected from among the backups. This mode requires only one VRRP group, and each
router in the group has a different priority. The one with the highest priority becomes the master.
Figure 9 VRRP in master/backup mode
Assume that Router A is acting as the master to forward packets to external networks, and Router B and
Router C are backups in listening state. When Router A fails, Router B and Router C elect a new master
to forward packets for hosts on the subnet.