Nortel Web OS 10.0, Application Manual

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Web OS 10.0 Application Guide
Chapter 11: High Availability
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VRRP Operation
The host shown in Figure 11-1 is configured with the virtual interface router’s IP address as its
default gateway. The master forwards packets destined to remote subnets and responds to ARP
requests. 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 virtual
interface router's IP address. The backup does not forward any traffic on behalf of the virtual
interface router nor does it respond to ARP requests.
If the owner is not available, the backup becomes the master and takes over responsibility for
packet forwarding and responding to ARP requests. However, because this switch is not the
owner, it does not have a real interface configured with the virtual interface router's IP address.
Selecting the Master VRRP Router
Each VRRP router that is not an owner is configured with a priority between 1–254. According
to the VRRP standard, an owner has a priority of 255. A bidding process determines which
VRRP router is or becomes the master—the VRRP router with the highest priority. Owners
have 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 receive
an advertisement for three advertisement intervals, it initiates a bidding process to determine
which 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, it
can 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. The
current 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 be
down, or all communications links between the master and the backup can be down. If the
master has failed, it is clearly desirable for the backup (or one of the backups, if there is more
than one) to become the master.
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If the master is healthy but communication between the master and the backup has
failed, 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.