Alteon Application Switch Operating System Application Guide
High Availability
Document ID: RDWR-ALOS-V2900_AG1302
509
VRRP Priority
This section describes the following topics:
•
How VRRP Priority Decides Which Alteon is the Master, page 509
•
Transitioning from the INIT State Based on VRRP Priority, page 509
•
Determining How to Configure Priority, page 509
•
Determining VRRP Priority for Ports Outside the VLAN, page 510
How VRRP Priority Decides Which Alteon is the Master
According to the VRRP standard, a virtual interface IP address owner has a priority of 255. You
configure each renter with a priority of between 1 and 254. If the IP address owner is available, it
always become the virtual router master.
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, 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 stops 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 communication links between the master and the backup are down, but the master is
healthy, Alteon may select a second master within the virtual router. To prevent this, configure
redundant links between the VRRP devices within the virtual router.
Transitioning from the INIT State Based on VRRP Priority
If there is no port in the virtual router's VLAN with an active link, the interface for the VLAN fails,
thus placing the virtual router into the INIT state.
The INIT state identifies that the virtual router is waiting for a startup event. If it receives a startup
event, it will either transition to master if its priority is 255 (the IP address owner), or transition to
the backup state if it is not the IP address owner.
Determining How to Configure Priority
A virtual router's priority is an initial value that increases or decreases depending on the parameters
that are tracked. For example, if you configure the virtual router to track the link state of the
physical ports, the virtual router's priority decreases by two priority points if the link to one port
fails.
To ensure that a decrease in priority causes failover from the current master to the backup virtual
router, set the priority of the master Alteon one point higher than the backup. For example, priority
101 for the master, and 100 for the backup. If the master and backup Alteons are set to priorities
110 and 100 respectively, a single port failure only decreases the master's priority to 108. Since 108
is still higher than the backup's priority of 100, the master does not fail due to the loss of one port
link.
