Multicast and Routing Guide K/KA/KB.15.15

Backup router
There must be at least one backup router. A given VR instance on a backup router must be
configured with the same VIP as the owner for that VR (and both routers must belong to the same
network or subnet.) Router 2 in Figure 53 (page 275) illustrates this point.
VR priority operation
In a backup router's VR configuration, the virtual router priority defaults to 100. (The priority for
the configured owner is automatically set to the highest value: 255.) In a VR where there are two
or more backup routers, the priority settings can be reconfigured to define the order in which
backups are reassigned as master in the event of a failover from the owner.
Preempt mode
Where multiple backup routers exist in a VR, if the current master fails and the highest-priority
backup is not available, VRRP selects the next-highest priority backup to operate as master. If the
highest-priority backup later becomes available, it preempts the lower-priority backup and takes
over the master function. If you do not want a backup router to have this preemptive ability on a
particular VR, you can disable this operation with the no preempt-mode command. (Preempt
mode applies only to VRRP routers configured as backups.)
Virtual router MAC address
When a VR instance is configured, the protocol automatically assigns a MAC address based on
the standard MAC prefix for VRRP packets, plus the VRID number (as described in RFC 3768.)
The first five octets form the standard MAC prefix for VRRP, and the last octet is the configured
VRID. that is:
00-00-5E-00-01- VRid
For example, the virtual router MAC address for the VR in Figure 53 (page 275) is
00-00-5E-00-01-01.
VRRP and ARP for IPv4
The master for a given VR responds to ARP requests for the VIPs with the VR's assigned MAC
address. The virtual MAC address is also used as the source MAC address for the periodic
advertisements sent by the current master.
The VRRP router responds to ARP requests for non-VIPs (IP addresses on a VLAN interface that are
not configured as VIPs for any VR on that VLAN) with the system MAC address.
VRRP and neighbor discovery for IPv6
Neighbor Discovery (ND) is the IPv6 equivalent of the IPv4 ARP for layer 2 address resolution, and
uses IPv6 ICMP messages to do the following:
• Determine the link-layer address of neighbors on the same VLAN interface.
• Verify that a neighbor is reachable.
• Track neighbor (local) routers.
Neighbor Discovery enables functions such as the following:
• Router and neighbor solicitations and discovery
• Detecting address changes for devices on a VLAN
• Identifying a replacement for a router or router path that has become unavailable
• Duplicate address detection (DAD)
• Router Advertisement processing
278 Virtual Router Redundancy Protocol (VRRP)