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As you may know, in IPv6 networks, the Neighbor Discovery protocol is used instead of ARP. When a router becomes the Master, an unsolicited ND Neighbor Advertisement with the Router Flag is sent for each IPv6 address associated with the virtual router.

VRRP state machine

VRRP state transition flow

As you can see from the diagram, each VRRP node can be in one of three states:

• Init state
• Backup state
• Master state

Init state

The purpose of this state is to wait for a Startup event. When this event is received, the following actions are taken:

• if priority is 255,
• * for IPv4 send advertisement packet and broadcast ARP requests
• * for IPv6 send an unsolicited ND Neighbor Advertisement for each IPv6 address associated with the virtual router and set target address to link-local address associated with VR.
• * transit to MASTER state;
• else transit to BACKUP state.

Backup state

When in the backup state,

• in IPv4 networks, a node is not responding to ARP requests and is not forwarding traffic for the IP associated with the VR.
• in IPv6 networks, a node is not responding to ND Neighbor Solicitation messages and is not sending ND Router Advertisement messages for VR-associated IPv6 addresses.

Routers' main task is to receive advertisement packets and check if the master node is available.

The backup router will transit itself to the master state in two cases:

• If priority in advertisement packet is 0;
• When Preemption_Mode is set to yes and Priority in the ADVERTISEMENT is lower than the local Priority

After the transition to Master state node is:

• in IPv4 broadcasts gratuitous ARP request;
• in IPv6 sends an unsolicited ND Neighbor Advertisement for every associated IPv6 address.

In other cases, advertisement packets will be discarded. When the shutdown event is received, transit to Init state.

Master state

When the MASTER state is set, the node functions as a forwarding router for IPv4/IPv6 addresses associated with the VR.

In IPv4 networks, the Master node responds to ARP requests for the IPv4 address associated with the VR. In IPv6 networks Master node:

• responds to ND Neighbor Solicitation message for the associated IPv6 address;
• sends ND Router Advertisements for the associated IPv6 addresses.

If advertisement packet is received by master node:

• If priority is 0, send advertisement immediately;
• If priority in advertisement packet is greater than nodes priority then transit to the backup state
• If priority in advertisement packet is equal to nodes priority and primary IP Address of the sender is greater than the local primary IP Address, then transit to the backup state
• Ignore advertisement in other cases

When the shutdown event is received, send the advertisement packet with priority=0 and transit to Init state.