Vrrp Timers - 3Com S7906E Configuration Manual

Working mode
A router in a VRRP group works in one of the following two modes:
Non-preemptive mode
When a router in the VRRP group becomes the master, it stays as the master as long as it operates
normally, even if a backup is assigned a higher priority later.
Preemptive mode
When a backup finds its priority higher than that of the master, the backup sends VRRP advertisements
to start a new master election in the VRRP group and becomes the master. Accordingly, the original
master becomes a backup.
Authentication mode
To avoid being attacked by unauthorized users, VRRP authenticates the received packets by adding
authentication keys into the packets. VRRP provides two authentication modes:
simple: Simple text authentication
A router sending a packet fills an authentication key into the packet, and the router receiving the packet
compares its local authentication key with that of the received packet. If the two authentication keys are
the same, the received VRRP packet is considered real and valid; otherwise, the received packet is
considered invalid.
md5: MD5 authentication
The router computes the digest of a packet to be sent using the authentication key and MD5 algorithm
and saves the result in the authentication header. The router receiving the packet performs the same
operation using the authentication key and MD5 algorithm, and compares the result with the content in
the authentication header. If the results are the same, the router receiving the packet considers the
packet an authentic and valid VRRP packet; otherwise, the router considers the packet invalid.
On a secure network, you do not need to set the authentication mode.

VRRP Timers

VRRP timers include VRRP advertisement interval timer and VRRP preemption delay timer.
VRRP advertisement interval timer
The master in a VRRP group sends VRRP advertisements periodically to inform the other routers in the
VRRP group that it operates properly.
You can adjust the interval for sending VRRP advertisements by setting the VRRP advertisement
interval timer. If a backup receives no advertisements in a period three times the interval, the backup
regards itself as the master and sends VRRP advertisements to start a new master election.
VRRP preemption delay timer
To avoid members in a VRRP group from changing their states frequently and make backups have
enough time to collect information (such as routing information), each backup waits for a period of time
(the preemption delay time) after it receives an advertisement with the priority lower than the local
priority, then sends VRRP advertisements to start a new master election in the VRRP group and finally
becomes the master.
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