Web OS 10.0 Application GuideChapter 11: High Availability n 251212777-A, February 2002VRRP OperationThe host shown in Figure 11-1 is configured with the virtual interface router’s IP address as itsdefault gateway. The master forwards packets destined to remote subnets and responds to ARPrequests. In this example, the master is also the virtual interface router’s IP address owner—therefore it also responds to ICMP ping requests and IP datagrams destined for the virtualinterface router's IP address. The backup does not forward any traffic on behalf of the virtualinterface router nor does it respond to ARP requests.If the owner is not available, the backup becomes the master and takes over responsibility forpacket forwarding and responding to ARP requests. However, because this switch is not theowner, it does not have a real interface configured with the virtual interface router's IP address.Selecting the Master VRRP RouterEach VRRP router that is not an owner is configured with a priority between 1–254. Accordingto the VRRP standard, an owner has a priority of 255. A bidding process determines whichVRRP router is or becomes the master—the VRRP router with the highest priority. Ownershave a higher priority than the range permitted for non-owners. If there is an IP address owner,it is always the master for the virtual interface router, as long as it is available.The master periodically sends advertisements to an IP multicast address. As long as the back-ups receive these advertisements, they remain in the backup state. If a backup does not receivean advertisement for three advertisement intervals, it initiates a bidding process to determinewhich VRRP router has the highest priority and takes over as master.If, at any time, a backup determines that it has higher priority than the current master does, itcan preempt the master and become the master itself, unless configured not to do so. In pre-emption, the backup assumes the role of master and begins to send its own advertisements. Thecurrent master sees that the backup has higher priority and will stop functioning as the master.A backup router can stop receiving advertisements for one of two reasons—the master can bedown, or all communications links between the master and the backup can be down. If themaster has failed, it is clearly desirable for the backup (or one of the backups, if there is morethan one) to become the master.N OTE – If the master is healthy but communication between the master and the backup hasfailed, there will then be two masters within the virtual router. To prevent this from happening,configure redundant links to be used between the switches that form a virtual router.
