User's Manual

65-3
65.1.2 VRRPv3 Working Mechanism
The working mechanism of VRRPv3 is the same with that of VRRPv2, which is mainly implemented via the
interaction of VRRP advertisement messages. It will be briefly described as follows:
Each VRRP router has a unique ID: VRIP, ranging from 1 to 255. This router has a unique virtual MAC
address outwardly, and the format of which is 00-00-5E-00-02-{VRID} (the format of virtual MAC address in
VRRPv2 is 00-00-5E-00-01-{VRID}). Master router is in charge of using this MAC address to respond to ND
neighbor request (it is ARP request in VRRPv2). Thus, no matter what switch is made, the terminal devices
will get the same IP and MAC address all the time, reducing the affection that the switch causes on terminal
devices.
There is only one kind of VRRP control message: VRRP advertisement. It uses IP multicast data packets to
encapsulate, and the format of multicast addresses is FF02:0:0:0:0:0:XXXX:XXXX. In order to keep a
consistence with the multicast address in VRRPv2 (224.0.0.18), the multicast addresses used by VRRPv3
advertisement messages can be FF02:0:0:0:0:0:0:12, and the advertisement is limited within the same LAN.
Thus, different VRID are guaranteed to be used repeatedly in different networks. In order to reduce the
overheads of network bandwidth, only master routers can send VRRP advertisement messages regularly.
Backup routers will start a new round of VRRP selection if it hasn’t received a VRRP advertisement in 3
advertisement intervals in a row or if it receives an advertisement with a priority of 0.
In a VRRP router group, the master router is selected according to priority. The range of priority in VRRP
protocol is 0-255. If the IP address of a VRRP router is the same to that of the virtual router interface, then the
virtual router will be called the IP address owner in the VRRP group; the IP address owner automatically has
the highest priority: 255. The priority of 0 is usually used when the IP address owner gives up the role of
master. The range of priority can be configured is 1-254. The configuration rule of priority can be set
according to the speed and cost of the link, the performance and reliability of the router and other
management policies. In the selection of the master router, the virtual router with high priority will win. So, if
there is an IP owner in the VRRP group, it will always be the master router. For the candidate routers having
the same priority, selection will be done according to the magnitude of IP addresses (the bigger IP address
takes precedence). VRRP also provides a preemptive priority policy. If such policy is configured, the backup
router with higher priority will preempt the role of new master router over the current master router with lower
priority.
In order to avoid the fault of returning a physical MAC address when Pinging virtual IP, it is regulated that
virtual IP can not be the real IP of the interface. Thus, all the interfaces participating of the backup group
selection will be backup by default.