Vrrp Tracking; Principles Of Vrrp - 3Com S7906E Configuration Manual

Auth Type: Authentication type. 0 means no authentication, 1 means simple text authentication,
and 2 means MD5 authentication. VRRPv3 does not support MD5 authentication.
Adver Int: Interval for sending advertisement packets. For VRRPv2, the interval is in seconds and
defaults to 1; for VRRPv3, the interval is in centiseconds and defaults to 100.
Checksum: 16-bit checksum for validating the data in VRRP packets.
IP Address/IPv6 Address: Virtual IPv4 or IPv6 address entry of the VRRP group. The Count IP
Addrs or Count IPv6 Addrs field defines the number of the virtual IP v4 or IPv6 addresses.
Authentication Data: Authentication key. Currently, this field is used only for simple authentication
and is 0 for any other authentication modes.

Principles of VRRP

With VRRP enabled, the routers decide their respective roles in the VRRP group by priority and IP
address. The router with the highest priority becomes the master, and the others are the backups;
if the priority is the same, the router with a higher 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 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 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 object
By monitoring a Track object, you can:
Monitor an uplink and modify the priority of the router according to the state of the uplink. If there is
a fault on the uplink, 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.
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