H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 16)

8
Principles of VRRP
• Routers in a VRRP group determine their roles by priority. The router with the highest priority is the
master, and the others are the backups. The master periodically sends VRRP advertisements to notify
the backups that it is working properly, and each of the backups starts a timer to wait for
advertisements from the master.
• In preemptive mode, when a backup receives a VRRP advertisement, it compares the priority in the
packet with its own priority. If the priority of the backup is higher, the backup becomes the master.
Otherwise, it remains as a backup. With the preemptive mode, a VRRP group always has a router
with the highest priority as the master for packet forwarding.
• In non-preemptive mode, a router in the VRRP group remains as a master or backup as long as the
master does not fail. A backup does not become the master even if it is configured with a higher
priority. The non-preemptive mode helps avoid frequent switchover between the master and
backups.
• If the timer of a backup expires but the backup still does not receive any VRRP advertisement, it
considers that the master fails. In this case, the backup considers itself as the master and sends VRRP
advertisements to start a new master election.
NOTE:
•
When multiple routers in a VRRP group declare that that they are the master because of inconsistent
configuration or 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.
• When a backup router receives an advertisement, it compares its priority with the advertised priority. If
its priority is higher than the advertised priority, it takes over the master.
VRRP tracking
NOTE:
To enable the VRRP tracking function, configure the routers in the VRRP group to operate in preemptive
mode first, so that only the router with the highest priority can always operate as the master for packet
forwarding.
Tracking a specified interface
The interface tracking function expands the backup functionality of VRRP. It provides backup not only
when the interface to which a VRRP group is assigned fails but also when other interfaces (such as uplink
interfaces) on the router become unavailable.
If the uplink interface of a router in a VRRP group fails, usually the VRRP group cannot be aware of the
uplink interface failure. If the router is the master of the VRRP group, hosts on the LAN are not able to
access external networks because of the uplink failure. This problem can be solved by tracking a
specified uplink interface. If the tracked uplink interface is down or removed, the priority of the master is
automatically decreased by a specified value and a higher priority router in the VRRP group becomes the
master.
Tracking a track entry
By monitoring a track entry, you can do the following:
• Monitor an uplink and change the priority of the router according to the state of the uplink. If the
uplink fails, hosts in the LAN cannot access external networks through the router. In this case, the