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Advanced Configuration and Management Guide
When you configure a VRID, the software automatically assigns its MAC address. When a VRID becomes active,
the Master router broadcasts a gratuitous ARP request containing the virtual router’s MAC address for each IP
address associated with the virtual router. In Figure 12.2, Router1 sends a gratuitous ARP with MAC address
00-00-5e-00-01-01 and IP address 192.53.5.1. Hosts use the virtual router’s MAC address in routed traffic they
send to their default IP gateway (in this example, 192.53.5.1).
Virtual Router IP Address
Unlike Standby Router Protocol (SRP), VRRP does not use virtual IP addresses. Thus, there is no virtual IP
address associated with a virtual router. Instead, you associate the virtual router with one or more real interface
IP addresses configured on the router that owns the real IP address(es). In Figure 12.2, the virtual router with
VRID1 is associated with real IP address 192.53.5.1, which is configured on interface e1/6 on Router1. VRIDs are
interface-level parameters, not system-level parameters, so the IP address you associate with the VRID must
already be a real IP address configured on the Owner’s interface.
NOTE: You also can associate a virtual router with a virtual interface. A virtual interface is a named set of
physical interfaces. See
“Configuring VLANs” on page 16-1 for more information.
When you configure the Backup router for the VRID, specify the same IP address as the one you specify on the
Owner. This is the IP address used by the host as its default gateway. The IP address cannot also exist on the
Backup router. The interface on which you configure the VRID on the Backup router must have an IP address in
the same sub-net.
NOTE: If you delete a real IP address used by a VRRP entry, the VRRP entry also is deleted automatically.
NOTE: When a Backup takes over forwarding responsibilities from a failed Master router, the Backup forwards
traffic addressed to the VRID MAC address, which the host believes is the MAC address of the router interface for
its default gateway. However, the Backup cannot reply to IP pings sent to the IP address(es) associated with the
VRID. Because the IP address(es) are owned by the Owner, if the Owner is unavailable, the IP addresses are
unavailable as packet destinations.
Master Negotiation
The routers within a VRID use the VRRP priority values associated with each router to determine which router
becomes the Master. When you configure the VRID on a router interface, you specify whether the router is the
Owner of the IP address(es) you plan to associate with the VRID or a Backup. If you indicate that the router is the
Owner of the IP address(es), the software automatically sets the router’s VRRP priority for the VRID to 255, the
highest VRRP priority. The router with the highest priority becomes the Master.
Backup routers can have a priority from 3 – 254, which you assign when you configure the VRID on the Backup
router’s interfaces. The default VRRP priority for Backup routers is 100.
Because the router that owns the IP addresses associated with the VRID always has the highest priority, when all
the routers in the virtual router are operating normally, the negotiation process results in the Owner of the VRID’s
IP address(es) becoming the Master router. Thus, the VRRP negotiation results in the normal case, in which the
hosts’ path to the default route is to the router that owns the interface for that route.
Hello Messages
VRRP routers use Hello messages for negotiation to determine the Master router. VRRP routers send Hello
messages to IP Multicast address 224.0.0.18. The frequency with which the Master sends Hello messages is the
Hello Interval. Only the Master sends Hello messages. However, a Backup uses the Hello interval you configure
for the Backup if it becomes the Master.
The Backup routers wait for a period of time called the Dead Interval for a Hello message from the Master. If a
Backup router does not receive a Hello message by the time the dead interval expires, the Backup router assumes
that the Master router is dead and negotiates with the other Backups to select a new Master router. The Backup
router with the highest priority becomes the new Master.
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