Vrrp Router Priority And Preemption - Cisco Nexus 7000 Series Configuration Manual

Chapter 20 Configuring VRRP
Information About VRRP
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
Figure 20-2 shows a LAN topology in which VRRP is configured so that Routers A and B share the
traffic to and from clients 1 through 4. Routers A and B act as backups to each other if either router fails.
Figure 20-2 Load Sharing and Redundancy VRRP Topology
Router A Router B
Master for virtual router 1 Backup for virtual router 1
Backup for virtual router 2 Master for virtual router 2
10.0.0.1 10.0.0.2
Client 1 Client 2 Client 3 Client 4
Default gateway = Default gateway = Default gateway = Default gateway =
10.0.0.1 10.0.0.1 10.0.0.2 10.0.0.2
In this topology contains two virtual IP addresses for two VRRP groups that overlap. For VRRP group
1, Router A is the owner of IP address 10.0.0.1 and is the master. Router B is the backup to router A.
Clients 1 and 2 are configured with the default gateway IP address of 10.0.0.1.
For VRRP group 2, Router B is the owner of IP address 10.0.0.2 and is the master. Router A is the backup
to router B. Clients 3 and 4 are configured with the default gateway IP address of 10.0.0.2.

VRRP Router Priority and Preemption

An important aspect of the VRRP redundancy scheme is the VRRP router priority because the priority
determines the role that each VRRP router plays and what happens if the master router fails.
If a VRRP router owns the virtual IP address and the IP address of the physical interface, this router
functions as the master. The priority of the master is 255.
Priority also determines if a VRRP router functions as a backup router and the order of ascendancy to
becoming a master if the master fails.
For example, if router A, the master in a LAN topology, fails, VRRP must determine if backups B or C
should take over. If you configure router B with priority 101 and router C with the default priority of
100, VRRP selects router B to become the master because it has the higher priority. If you configure
routers B and C with the default priority of 100, VRRP selects the backup with the higher IP address to
become the master.
VRRP uses preemption to determine what happens after a VRRP backup router becomes the master.
With preemption enabled by default, VRRP will switch to a backup if that backup comes online with a
priority higher than the new master. For example, if Router A is the master and fails, VRRP selects
Router B (next in order of priority). If Router C comes online with a higher priority than Router B, VRRP
selects Router C as the new master, even though Router B has not failed.
If you disable preemption, VRRP will only switch if the original master recovers or the new master fails.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
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