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
• Neighbor reachability
• Autoconfiguration of unicast addresses
• Resolution of destination addresses
• Changes to link-layer addresses.
An instance of Neighbor Discovery is triggered on a device when a new or changed IPv6 address is detected.
VRRPv3 provides a faster failover to a backup router by not using standard ND procedures. A failover to a backup
router can occur in approximately three seconds without any interaction with hosts and with a minimum of
VRRPv3 traffic.
Duplicate address detection (DAD)
Duplicate Address Detection verifies that a configured unicast IPv6 address is unique before it is assigned to a
VLAN interface. When the owner router fails, the backup VRRP router assumes the master role. When the owner
router becomes operational, DAD will fail as there is a backup VRRP router in the master role that responds to the
DAD request. To avoid this, virtual routers that are in owner mode (priority = 255) will not send DAD requests for
the VLAN interface on which the owner VR is configured.
General operating rules
• IP routing (IPv4) or IPv6 unicast-routing (IPv6) must be enabled on the router before enabling VRRP.
• IP must be enabled on a VLAN before creating a VR instance on the VLAN.
• VIP:
Chapter 16 Virtual Router Redundancy Protocol (VRRP)
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