Handbook

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NOTE: Every VIR must be assigned to an IP interface, and every IP interface must be assigned to a
VLAN. If no port in a VLAN has link up, the IP interface of that VLAN is down, and if the IP interface
of a VIR is down, that VIR goes into INIT state.
VRRP operation
Only the virtual router master responds to ARP requests. Therefore, the upstream routers only forward
packets destined to the master. The master also responds to ICMP ping requests. The backup does not
forward any traffic, nor does it respond to ARP requests.
If the master is not available, the backup becomes the master and takes over responsibility for packet
forwarding and responding to ARP requests.
Selecting the master VRRP router
Each VRRP router is configured with a priority between 1–254. A bidding process determines which VRRP
router is or becomes the master—the VRRP router with the highest priority.
The master periodically sends advertisements to an IP multicast address. As long as the backups receive
these advertisements, they remain in the backup state. If a backup does not receive an advertisement
for three advertisement intervals, it initiates a bidding process to determine which VRRP router has the
highest priority and takes over as master.
If, at any time, a backup determines that it has higher priority than the current master does, it can
preempt the master and become the master itself, unless configured not to do so. In preemption, the
backup assumes the role of master and begins to send its own advertisements. The current master sees
that the backup has higher priority and will stop functioning as the master.
A backup router can stop receiving advertisements for one of two reasons—the master can be down, or
all communications links between the master and the backup can be down. If the master has failed, it is
clearly desirable for the backup (or one of the backups, if there is more than one) to become the master.
NOTE: If the master is healthy but communication between the master and the backup has failed,
there will then be two masters within the virtual router. To prevent this from happening, configure
redundant links to be used between the switches that form a virtual router.
Failover methods
With service availability becoming a major concern on the Internet, service providers are increasingly
deploying Internet traffic control devices, such as application switches, in redundant configurations.
Traditionally, these configurations have been hot-standby configurations, where one switch is active and
the other is in a standby mode. A non-VRRP hot-standby configuration is shown in the figure below:
Figure 24 Non-VRRP hot-standby configuration
While hot-standby configurations increase site availability by removing single points-of-failure, service
providers increasingly view them as an inefficient use of network resources because one functional
application switch sits by idly until a failure calls it into action. Service providers now demand that
vendors' equipment support redundant configurations where all devices can process traffic when they
are healthy, increasing site throughput and decreasing user response times when no device has failed.