3Com 7757 Configuration Manual page 556

556 C 52: VRRP C
HAPTER ONFIGURATION
You can configure a Switch 7750 to operate in preemptive mode. You can also set
the delay period. A backup switch waits for a period of time (the delay period)
before becoming a master switch. Setting a delay period aims at:
In an unstable network, backup switches in a backup group possibly cannot
receive packets from the master in time due to network congestions even if the
master operates properly. This causes the master of the backup group being
determined frequently. With the configuration of delay period, the backup switch
will wait for a while if it does not receive packets from the master switch in time. A
new master is determined only after the backup switches do not receive packets
from the master switch after the specified delay time.
Configuring authentication type and authentication key for a switch in a
backup group
VRRP provides following authentication types:
simple: Simple character authentication
■
md5: MD5 authentication
■
In a network under possible security threat, the authentication type can be set to
simple. Then the switch adds the authentication key into the VRRP packets before
transmitting them. The receiver will compare the authentication key of the packet
with the locally configured one. If they are the same, the packet will be taken as a
true and legal one. Otherwise it will be regarded as an illegal packet and be
discarded. In this case, a simple authentication key should not exceed eight
characters.
In a vulnerable network, the authentication type can be set to md5. The switch
then uses the authentication type provided by the Authentication Header, and
MD5 algorithm to authenticate the VRRP packets. In this case, you need to set an
authentication key in plain text comprising up to eight characters or an
authentication key of a 24-character encrypted string.
Packets that fail to pass the authentication are discarded. The switch then sends
trap packets to the network management system.
Configuring VRRP timer
The master switch advertises its normal operation state to the switches within the
VRRP backup group by sending VRRP packets once in each specified interval
(determined by the adver-interval argument). If the backup switches do not receive
VRRP packets from the master after a specific period (determined by the
master-down-interval argument), they consider the master is down and initiates
the process to determine the master switch.
You can adjust the frequency in which a master sends VRRP packets by setting the
corresponding VRRP timers (that is, the adver-interval argument). The
master-down-interval argument is usually three times of the adver-interval
argument. Excessive network traffic or differences between the timers of different
switches will result in master-down-interval timing out and state changing
abnormally. Such problems can be solved through prolonging the adver-interval
and setting delay time. If you configure the preemption delay for a backup switch,
the switch preempts the master after the period specified by the preemption delay