H3C Technologies H3C S12500-X Series Switches - Manual (Page 49)

41
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 tracking
To enable VRRP tracking, configure the routers in the VRRP group to operate in preemptive mode first, so
that only the router with the highest priority operates as the master for packet forwarding. For more
information about track entries, see High Availability Configuration Guide.
The VRRP tracking function uses network quality analyzer (NQA) or bidirectional forwarding detection
(BFD) to monitor the state of the master, and establishes the collaboration between the VRRP device state
and NQA or BFD through the track function. It implements the following:
• Monitors the upstream link and changes the priority of the router according to the state of the link.
If the upstream link fails, the hosts on the subnet cannot access external networks through the router
and the state of the track entry becomes Negative. The priority of the master decreases by a
specified value. Then, a router with a higher priority in the VRRP group becomes the master to
maintain the proper communication between the hosts on the subnet and external networks.
• Monitors the state of the master on the backups. When the master fails, a backup immediately takes
over as the master to ensure uninterrupted communication.
When the track entry changes from Negative to Positive or Notready, the router automatically restores its
priority. For more information about track entries, see " Configuring Track ."
VRRP application
Master/backup
In master/backup mode, only the master forwards packets, as shown in Figure 14 . 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.