Chapter 34: Virtual Router Redundancy Protocol
410 Section VII: Routing
VRRP on the Switch
VRRP is disabled by default. When a virtual router is created on the
switch, it is enabled by default, but the VRRP module must be enabled
before it is operational. The VRRP module or a specific virtual router can
be enabled or disabled afterwards by using the ENABLE VRRP and
DISABLE VRRP commands.
A virtual router must be created on at least two switches before it operates
correctly. To create a virtual router for an IP address over an Ethernet
interface, so that the switch participates in the virtual router, use the
CREATE VRRP command.
To destroy a virtual router on the LAN, it must be removed from all
participating switches. To remove a virtual router so that the switch no
longer participates in it, use the DESTROY VRRP command.
If the switch in the master role for the virtual router becomes unavailable,
the master role is taken by the switch with the highest priority amongst the
available switches. The priority is a value from 1 to 255, with a default of
100. The highest value of 255 is reserved for the switch that owns the
virtual router’s IP address. The new master takes over all the
responsibilities of the original master. Hosts on the LAN can continue
sending packets to the same virtual MAC address with which they
associate the configured first hop IP address, even though the switch that
owns the IP address is not currently available. When the preferred switch
that owns the IP address becomes available again, it resumes the role of
master.
By default, when a switch becomes available with a higher priority than the
master, it takes over as master. This is referred to as preempt mode and
can be set on or off. Even with preempt mode off, the switch that owns the
IP address always becomes the master when available. If two switches
are configured with the same priority, the one with the highest IP address
has higher priority. Preempt mode must be the same for all switches in the
virtual router. Set the priority and preempt mode when you create the
virtual router; modify it later by using the SET VRRP command.
The frequency with which the master sends advertisement packets must
be set to the same value for all switches in the virtual router. The default
advertisement interval of 1 second is recommended for most networks.
This is set with the ADINTERVAL parameter in the CREATE VRRP and
SET VRRP commands.
Each of the switches in the virtual router can be configured for plaintext
authentication or none. No authentication is suitable when there is minimal
security risk, and the configuration is so simple (for example, two switches
on a LAN) that there is little chance of configuration errors. Plaintext
password authentication protects against accidental misconfiguration and