Quanta Cloud Technology QuantaMesh QNOS5, Configuration Manual

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7.3. VRRP
The Virtual Router Redundancy (VRRP) protocol is designed to handle default router (L3 switch) failures by
providing a scheme to dynamically elect a backup router. VRRP can help minimize black hole periods due to
the failure of the default gateway router during which all traffic directed towards it is lost until the failure is
detected.
7.3.1. VRRP Operation in the Network
VRRP eliminates the single point of failure associated with static default routes by enabling a backup router
to take over from a master router without affecting the end stations using the route. The end stations will
use a virtual IP address that will be recognized by the backup router if the master router fails. Participating
routers use an election protocol to determine which router is the master router at any given time. A given
port may appear as more than one virtual router to the network, also, more than one port on a switch may
be configured as a virtual router. Either a physical port or a routed VLAN may participate.
With VRRP, a virtual router is associated with one or more IP addresses that serve as default gateways. In the
event that the VRRP router controlling these IP addresses (formally known as the master) fails, the group of
IP addresses and the default forwarding role is taken over by a Backup VRRP Router.
7.3.1.1. VRRP Router Priority
The VRRP router priority is a value from 1–255 that determines which router is the master. The greater the
number, the higher the priority. If the virtual IP address is the IP address of a VLAN routing interface on one
of the routers in the VRRP group, the router with IP address that is the same as the virtual IP address is the
interface owner and automatically has a priority of 255. By default, this router is the VRRP master in the
group.
If no router in the group owns the VRRP virtual IP address, the router with the highest configured priority is
the VRRP master. If multiple routers have the same priority, the router with the highest IP address becomes
the VRRP master.
If the VRRP master fails, other members of the VRRP group will elect a master based on the configured router
priority values. For example, router A is the interface owner and master, and it has a priority of 255. Router B
is configured with a priority of 200, and Router C is configured with a priority of 190. If Router A fails,
Router B assumes the role of VRRP master because it has a higher priority.
7.3.1.2. VRRP Preemption
If preempt mode is enabled and a router with a higher priority joins the VRRP group, it takes over the VRRP
master role if the current VRRP master is not the owner of the virtual IP address. The preemption delay
controls how long to wait to determine whether a higher priority Backup router preempts a lower priority
Master. In certain cases, for example, during periods of network congestion, a backup router might fail to
receive advertisements from the master. This could cause members in the VRRP group to change their
states frequently, i.e. flap. The problem can be resolved by setting the VRRP preemption delay timer to a
non-zero value.