3Com MSR 50 Series, Configuration Manual

Page: 1977 / 2443

Introduction to VRRP 1977
while the others are the backups. The master sends VRRP advertisement
packets periodically to notify the backups that it is working properly, and each
of the backups starts a timer to wait for advertisement packets from the
master.
■
In preemption mode, when a backup receives a VRRP advertisement packet, it
compares the priority in the packet with that of its own. If its priority is higher,
it becomes the master; otherwise, it remains a backup.
■
In non-preemption mode, the router in the standby group remains as a master
or backup as long as the master does not fail. The backup will no become the
master even if the former is configured with a higher priority.
■
If the timer of a backup expires but the backup still does not receive any VRRP
advertisement packet, it considers that the master fails. In this case, the backup
router considers itself as the master router and sends VRRP advertisements to
start the election process to elect a new master router for forwarding packets.
VRRP Tracking 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 standby group is assigned
fails but also when other interfaces on the router become unavailable. This is
achieved by tracking interfaces. When a monitored interface goes down, the
priority of the router owning the interface is automatically decreased by a
specified value
, allowing a higher priority router in the standby group to become
the master.
Tracking a Track object
You can monitor the upper link by monitoring a Track object. 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 changes to negative,
and the priority of the router decreases by a specified value, allowing a higher
priority router in the VRRP standby group to become the master to maintain the
proper communication between the hosts in the LAN and the external network.
n
For details of Track object tracking, refer to “Track Configuration” on page 2207 .
VRRP Application
(Taking IPv4-Based VRRP
for Example)
Master/backup
In master/backup mode, only one router, the master, provides services. When the
master fails, a new master is elected from the original backups. This mode requires
only one standby group, in which each router holds different priorities and the one
with the highest priority becomes the master, as shown in
Figure 587 .