Dell PowerConnect B-RX - Page 503

BigIron RX Series Configuration Guide 427
53-1002253-01
Overview of Virtual Switch Redundancy Protocol (VSRP)
15
Each Backup waits for a specific period of time, the Dead Interval, to receive a new Hello message
from the Master. If the Backup does not receive a Hello message from the Master by the time the
Dead Interval expires, the Backup sends a Hello message of its own, which includes the Backup's
VSRP priority, to advertise the Backup's intent to become the Master. If there are multiple Backups
for the VRID, each Backup sends a Hello message.
When a Backup sends a Hello message announcing its intent to become the Master, the Backup
also starts a hold-down timer. During the hold-down time, the Backup listens for a Hello message
with a higher priority than its own:
• If the Backup receives a Hello message with a higher priority than its own, the Backup resets
its Dead Interval and returns to normal Backup status.
• If the Backup does not receive a Hello message with a higher priority than its own by the time
the hold-down timer expires, the Backup becomes the new Master and starts forwarding Layer
2 traffic on all ports.
VSRP priority calculation
Each VSRP device has a VSRP priority for each VRID and its VLAN. The VRID is used during Master
election for the VRID. By default, a device’s VSRP priority is the value configured on the device
(which is 100 by default). However, to ensure that a Backup with a high number of up ports for a
given VRID is elected, the device reduces the priority if a port in the VRID’s VLAN goes down. For
example, if two Backups each have a configured priority of 100, and have three ports in VRID 1 in
VLAN 10, each Backup begins with an equal priority, 100. This is shown in Figure 76
FIGURE 76 VSRP priority
However, if one of the VRID’s ports goes down on one of the Backups, that Backup’s priority is
reduced. If the Master’s priority is reduced enough to make the priority lower than a Backup’s
priority, the VRID fails over to the Backup. Figure 77 shows an example.
VSRP
Master
VSRP
Backup
optional link
VSRP
Aware
VSRP
Aware
VSRP
Aware
F
F
F
B
B
B
Configured priority = 100
Actual priority = 100 * (3/3) = 100
Configured priority = 100
Actual priority = 100 * (3/3) = 100