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IP Configuration: VRRP
Configurable Elements of VRRP
21
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 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.
VRRP Advertisements
The virtual router master sends VRRP advertisements to routers which are in the same group (configured
with the same virtual router identification).
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