IP Configuration: VRRP
Configurable Elements of VRRP
351 Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
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All the VRRP routers supporting the same virtual router must have the same configuration. If
the configurations are different, the configuration of the master is used. A backup VRRP
router syslogs a message when its configuration is different from the configuration of the
master.
Source IP Address In a VRRP Router
Each VRRP router supporting a virtual router uses their own IP address as the source IP
address in their outgoing VRRP messages for the virtual router. VRRP routers of the same
virtual router communicate to each other in VRRP messages. If a VRRP router is the owner of
the IP address of the virtual router, then IP address is one of the virtual router IP addresses. If a
VRRP router is not the owner of the IP address of the virtual router, then the IP address is the
IP address of the VRRP router interface to the same IP subnet of the virtual router.
If the source IP address was manually configured, the configuration is removed and the default
source IP address is taken (lowest VRRP router's IP address defined on the interface). If the
source IP address was a default one, a new default source IP address is taken.
VRRP Router Priority and Preemption
An important aspect of the VRRP redundancy scheme is the ability to assign each VRRP
router a VRRP priority. The VRRP priority must express how efficiently a VRRP router would
perform as a backup to a virtual router defined in the VRRP router. If there are multiple
backup VRRP routers for the virtual router, the priority determines which backup VRRP
router is assigned as master if the current master fails.
If a virtual router is the owner of the IP address, its VRRP priority is automatically assigned
with priority of 255 by the system, and the VRRP router (on which this virtual router is
assigned) automatically functions as a virtual router master if it is up.
In the “Basic VRRP Topology” figure, if Router A, the virtual router master, fails, a selection
process takes place to determine if virtual router backups B or C must take over. If Routers B
and C are configured with the priorities of 101 and 100, respectively, Router B is elected to
become virtual router master because it has the higher priority. If both have the same priority,
the one with the higher IP address value is selected to become the virtual router master.
By default, a preemptive feature is enabled, which functions as follows:
• Enabled—When a VRRP router is configured with higher priority than the current
master is up, it replaces the current master.
• Disabled—Even if a VRRP router with a higher priority than the current master is up,
it does not replace the current master. Only the original master (when it becomes
available) replaces the backup.
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