H3C Technologies H3C SecPath F1000-E - Manual (Page 834)

5
Principles of VRRP
• Routers in a VRRP group decide their respective roles by priority and IP address. The router with the
highest priority becomes the master, and the others are the backups. If the routers have the same
priority, the one with the highest IP address becomes the master. The master sends VRRP
advertisements periodically 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 that of its own. If the priority of the backup is higher, the backup becomes the master;
otherwise, it remains a backup.
• In non-preemptive mode, the router in the VRRP group remains as a master or backup as long as the
master does not fail. The backup does not become the master even if the backup is configured with
a higher priority.
• 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.
VRRP Tracking
Tracking a Track object
By monitoring a Track object, you can:
• Monitor an uplink and modify the priority of a router according to the state of the uplink. If there is
a fault on the upper link, hosts in the LAN cannot access the external network through the router. In
this case, the state of the monitored Track object is negative and the priority of the router decreases
by a specified value. After that, a higher priority router in the VRRP group becomes the master to
maintain the proper communication between the hosts in the LAN and the external network.
• Monitor the master on a backup. If there is a fault on the master, the backup working in the
switchover mode switches to the master immediately to maintain normal communication.
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, normally the VRRP group cannot be aware of the
uplink failure. If the router is the master of the VRRP group, hosts on the LAN are not able to access the
external network because of the uplink failure. You can solve the problem through the function of tracking
a specified interface. In this case, it is the uplink interface. After you configure to monitor the uplink
interface, when the uplink interface is in the down or removed status, the priority of the master is
automatically decreased by a specified value and a higher priority router in the VRRP group becomes the
master.
VRRP Application (Taking IPv4-Based VRRP for Example)
Master/backup
In master/backup mode, only one router, the master, forwards packets. When the master fails, a new
master is elected from the original backups. This mode requires only one VRRP group, in which each
router holds a different priority and the one with the highest priority becomes the master, as shown in
Figure 4 .