Multicast and Routing Guide K/KA/KB.15.15

NOTE: All routers in a given VR must belong to the same network (or subnet, in the case of a
multinetted VLAN.)
Virtual IP address (VIP)
The VIP associated with a VR must be a real IP address already configured in the associated VLAN
interface on the owner router in the VR. If the VIP is an IPv6 address, a link-local address must be
configured before adding a global IPv6 address. Also, the owner and all other (backup) routers
belonging to the VR have this IP address configured in their VRID contexts as the VIP. In Figure 53
(page 275), 10.10.100.1 is a real IP address configured on VLAN 100 in Router 1 and is the VIP
associated with VR 1.
If the configured owner in a VR becomes unavailable, it is no longer the master for the VR and a
backup router in the VR is elected to assume the role of master, as described under “Backup router”
(page 278).
A subnetted VLAN allows multiple VIPs. However, if there are 32 or fewer IP addresses in a VLAN
interface, and you want VRRP support on multiple subnets, the recommended approach is to
configure a separate VR instance for each IP address in the VLAN. In cases where VRRP support
is needed for more than 32 IP addresses in the same VLAN.
Master router
The current master router in a VR operates as the "real" or physical gateway router for the network
or subnet for which a VIP is configured.
Control of master selection
Selection of the master is controlled by the VRRP priority value configured in the VRID context of
each router in the VR. The router configured as the owner in the VR is automatically assigned the
highest VRRP priority (255) and, as long as it remains available, operates as the master router for
the VR. The other routers belonging to the VR as backups are assigned the default priority value
(100) and can be reconfigured to any priority value between 1 and 254, inclusive. If the current
master becomes unavailable, the protocol uses the priority values configured on the other, available
routers in the VR to select another router in the VR to take over the master function.
Function of the VRRP advertisement
The current master router sends periodic advertisements to inform the other routers in the VR of its
operational status. If the backup VRs fail to receive a master advertisement within the timeout
interval, the current master is assumed to be unavailable and a new master is elected from the
existing backups. The timeout interval for a VR is three times the advertisement interval configured
on the VRs in the network or subnet. In the default VRRP configuration, the advertisement interval
is one second and the resulting timeout interval is three seconds.
NOTE: All VRRP routers belonging to the same VR must be configured with the same advertisement
interval. As required in RFC 3768, if a locally configured advertisement interval does not match
the interval received in an inbound VRRP packet, the VR drops that packet.
Most IPv6 host configurations learn the default gateway IPv6 address using router advertisements.
The VR that becomes the master sends router advertisements for its virtual IP address.
Owner router
An owner router for a VR is the default master router for the VR and operates as the owner for all
subnets included in the VR. The VRRP priority on an owner router is always 255 (the highest.)
NOTE: On a multinetted VLAN where multiple subnets are configured in the same VR, the router
must be either the owner for all subnets in the VR or a backup for all subnets in the VR.
General operation 277