How The Vrrp Works; Different States Of A Vrrp Router - Enterasys Security Router X-PeditionTM User Manual

• Virtual Router - An abstract object managed by VRRP that acts as a default router for hosts on
a shared LAN. It consists of a VR Identifier and a set of associated IP address(es) across a
common LAN. A VRRP router may back up one or more VRs.
• IP Address Owner - The VRRP router that has the VR's IP address(es) as real interface
address(es). This is the router that, when up, will respond to packets addressed to one of these
IP addresses for ICMP pings, TCP connections, etc.
• VRRP Primary IP Address - An IP address selected from the set of real interface addresses. One
possible selection algorithm is to always select the first address. VRRP advertisements are
always sent using the primary IP address as the source of the IP packet.
• Virtual Router Master - The VRRP router that assumes the responsibility of forwarding packets
sent to the IP address(es) associated with the VR, and answers ARP requests for these IP
address. Note that if the IP address owner is available, then it will always become the master.

How the VRRP Works

Multiple IP routers on a single broadcast LAN comprise a single virtual router, which has a
unique virtual IP address and virtual MAC address. Hosts on the LAN configure the VR as their
default router (default gateway). Devices that provide support for a VR form a VRRP group. The
device acting as the VR is designated the master of the group.
At any one time, only one of the routers acts as the VR, forwarding packets from hosts on the
LAN. If that router goes down, the VRRP provides a method by which one of the other routers in
the group can take over the virtual IP address and MAC address in a timely manner.
When the VRRP is started, the IP router sends and receives VRRP advertisements until a master is
chosen. If the IP router does not become the master, it continues to listen to advertisements from
the master of the group.
If the IP router becomes the master of the group, it begins sending VRRP advertisements and adds
VRRP group information to the interface set. Once added, any Ethernet frame for the virtual MAC
address is received by the IP router. Any ARP requests for the virtual IP address are responded to
using the virtual MAC address.
If the IP router ceases to be the group master, it removes the VRRP group information from the
system and continues to listen for VRRP advertisements from the new master.

Different States of a VRRP Router

Underlying how VRRP operates are three states the VRRP router experiences: initialize, backup,
and master. Initialize, the first state, involves these steps:
• A VRRP router checks the virtual IP address to learn if it is the master.
• If it owns that address, it realizes it is the master and its priority is 255.
• If the priority equals 255, the VRRP router advertises itself as the master, broadcasts an ARP
message to all IP addresses associated with the VR's IP address, starts the advertisement timer
and transitions to the master state.
• If priority is less than 255, the VRRP router transitions to backup state.
In the backup state, a VRRP router monitors the VR master to confirm it is alive, does not respond
to ARP requests or accept packets for the IP address(es) associated with the VR, and discards
packets destined for the VR's MAC address. If an advertisement is received that the priority
equals 0, then the VRRP router performs the following:
• Advertises that it is the master VR,
IP Routing Protocols
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