For IPv6 hosts on a LAN to learn about one or more default routers, IPv6-enabled routers send router advertisements using the IPv6 ND protocol. The routers multicast these router advertisements every few minutes.
The ND protocol uses a mechanism called neighbor unreachability detection to detect the failure of a neighbor node (router or host) or the failure of the forwarding path to a neighbor. Nodes can monitor the status of a forwarding path by sending unicast ND neighbor solicitation messages to the neighbor node. To reduce traffic, nodes only send neighbor solicitations to neighbors to which they actively send traffic and only after the node receives no positive indication that the neighbors are up for a period of time. A host takes a minimum of 5 seconds to learn that a router is unreachable before it switches to another default router, but this minimum value increases ND traffic. This delay can cause service disruption.
VRRP for IPv6 provides a faster switchover to an alternate default router than is possible using the ND protocol. With VRRP for IPv6, a backup router can take over for a failed default router in approximately three seconds (using default parameters). The switchover is accomplished without interaction with the hosts and with a minimum amount of VRRP traffic.
The IPv6 VRRP implementation is similar to the existing IPv4 VRRP operation, including support for holddown timer, critical IP, fast advertisements, and backup master. With backup master enabled, the backup switch routes all traffic according to its routing table. The backup master switch does not perform Layer 2 switching for the traffic to the VRRP master.
You must specify a link-local address to associate with the virtual router. Optionally, you can also assign global unicast IPv6 addresses to associate with the virtual router. Network prefixes for the virtual router are derived from the global IPv6 addresses assigned to the virtual router.
One active master switch exists for each IPv6 network prefix. All other VRRP interfaces in a network are in backup mode.
VRRP uses a virtual IP address shared between two or more routers connecting the common network prefix to the enterprise network. With the virtual IP address as the default gateway on end hosts, VRRP provides dynamic default gateway redundancy in the event of failover.
The VRRP router with higher priority is called the master router. In case of equal priority the router with higher link-local address becomes the master router. The master router forwards packets sent to the virtual router IP addresses.
The following figure shows the minimum VRRP topology.
Traffic flows between users A and users B.
Router A uses VRRP global addresses as next hops for users B, and Router B for users A.
The VRRP master forwards the traffic and sends VRRP advertisements in the VLAN to announce to the backups that it is the master. If the master is no longer available, the backup takes over and becomes master. The only change occurs to the state of VRRP.
The VRRP router then transitions to the controlling state.
Note
The VRRP virtual IP address can be the same as the local IP address or Primary IP address of the interface on which VRRP is enabled. In this case, the router is the VRRP address owner on that interface and the priority is set to the maximum value of 255.In the controlling state, the VRRP router functions as the forwarding router for the IP addresses associated with the virtual router. The router responds to ND neighbor solicitation and ND router solicitation messages for these IP addresses, forwards packets with a destination MAC address equal to the virtual router MAC address, and accepts packets addressed to IP addresses associated with the virtual router.
If you initialize the VRRP router and the priority is not 255, the router transitions to the backup state to ensure that all Layer 2 switches in the downstream path relearn the new origin of the VRRP MAC addresses.
In the backup state, a VRRP router monitors the availability and state of the master router. The backup does not respond to ND neighbor solicitation and ND router solicitation messages for virtual router IP addresses and discards packets with a MAC address equal to the virtual router MAC address. The backup does not accept packets addressed to IP addresses associated with the virtual router. If a shutdown occurs, it transitions back to the initialize state. If the master router goes down, the backup router sends the VRRP advertisement and unsolicited ND neighbor advertisements and ND router advertisements described in the preceding paragraphs and transitions to the controlling state.
When you initialize a VRRP router, the master router continues to send advertisement messages at the advertisement interval period.
The other VRRP routers transition to the backup state in the following situations:
if the priority in the received advertisement is greater than the local priority
if the priority in the received advertisement is the same as the local priority and the primary IP address of the sender is greater than the local primary IP address
The backup routers use the advertisements from the master router as a keepalive to monitor the status of the master router. If the backup router does not receive an advertisement during the master downtime interval, calculated as 3 * advertisement interval, then the master router is declared down.
If a shutdown occurs, the master router sends a VRRP advertisement with a priority of 0 and transitions to the initialize state.
The priority value 0 indicates that the master router has stopped participating in VRRP. This value triggers the backup router to transition to the master state without waiting for the current master to time out.
Edge devices learn the VRRP MAC address towards the master. After the backup becomes master and sends the advertisements, this causes the VRRP MAC address to move on the edge devices and point towards the new master.
The critical IPv6 address is an interface that has primary impact on VRRP. If you enable critical IPv6 and the status of the critical IP changes, the master and backup relationship also changes.
If you configure and enable critical IPv6 address, the master transitions to backup if the critical IPv6 is down, and the backup becomes the master. After the critical IPv6 address of the original master resumes, if the hold-down timer is configured to 0, it becomes the master immediately. Otherwise, the original master transitions to the master state after the hold-down timer time out.
You can specify the local router IP interface uplink from the VRRP router to the network as the critical IP address. This ensures that, if the local uplink interface fails, VRRP initiates a master router failover to one of the backup routers.
The critical address can be one of the global unicast IPv6 addresses assigned to any local IPv6 interfaces.
The holddown timer is a proprietary enhancement to VRRP.
After a master transitions to backup by critical IP changing, one of the backup routers will be elected as the master router. After the critical IPv6 of the original master is restored, the original master remains in the backup state for a period of time that you configure by using the holddown-timer parameter. The router becomes the master immediately if you use the command ipv6 vrrp <1–255> action preempt.
The holddown timer allows the master router enough time to detect and update the dynamic routes. The timer delays the preemption of the master over the backup, when the master becomes available. If the hold-timer is configured to 0, it becomes the master router immediately. Otherwise, it transitions to the master state only after the holddown timer times out.
The holddown timer does not apply during failovers caused by VRRP router priority change. The holddown timer applies only to failovers caused by a critical IP failure.
Configure all of your routers to use identical values for the holddown timer.
Important
Do not use VRRP backup master and critical IP at the same time. Use one or the other. The critical IP address must be a local address.
The standard implementation of VRRP supports one active master switch for each IPv6 subnet. All other VRRP interfaces in a network are in backup mode.
A deficiency occurs when VRRP-enabled switches use SMLT. If VRRP switches are aggregated into two SMLT switches, the end host traffic is load-shared on all uplinks to the aggregation switches (based on the Multilink Trunk [MLT] traffic distribution algorithm).
However, VRRP usually has only one active routing interface enabled. All other VRRP routers are in backup mode. Therefore, all traffic that reaches the backup VRRP router is forwarded over Virtual Inter-Switch Trunk (vIST) toward the master VRRP router. In this case, vIST potentially does not have enough bandwidth to carry all the aggregated traffic.
To resolve this issue, assign the backup router as the backup master router. The backup master router can actively load-share the routing traffic with a master router.
Because the two VRRP peer nodes exchange MAC address tables, the VRRP backup master can forward traffic directly, on behalf of the master router. The switch in the backup master state routes all traffic received on the backup master IP interface according to its routing table. The backup master switch does not perform Layer 2 switching for the traffic to the VRRP master.
If you enable SMLT on the backup master router, the incoming host traffic is forwarded over the SMLT links as usual.
Important
Do not use VRRP backup master and critical IP at the same time. Use one or the other.
You can configure the advertisement time interval (in seconds) between sending advertisement messages. This interval permits fast network convergence with standardized VRRP failover. However, losing connections to servers for more than a second can result in missing critical failures. Customer network uptime in many cases requires faster network convergence, which means network problems must be detected within hundreds of milliseconds.
To meet these requirements, the fast advertisement interval are provided.
The fast advertisement interval is similar to the advertisement interval parameter except for the unit of measure and the range. The fast advertisement interval is expressed in milliseconds and the range is from 200 to 1,000 milliseconds. This unit of measure must be in multiples of 200 milliseconds.
To configure fast advertisement, you must specify a fast advertisement interval and explicitly enable the fast advertisement option. After you enable fast advertisement, the fast advertisement interval is used instead of the advertisement interval.
If you enable fast advertisement, VRRP can only communicate with other products that have the same configuration.
When you configure VRRP for IPv6 on an interface you can configure the accept-mode parameter, which controls whether the VRRP master or backup master accepts packets destined for the IPv6 address associated with the virtual router.
By default, accept-mode is disabled. The accept-mode parameter does not affect the Neighbor Discovery packets. The master router forwards packets with a destination link-layer MAC address that matches the virtual MAC address, and accepts packets forwarded over the virtual interswitch trunk (vIST) toward the master router, if accept-mode is enabled. If you disable accept-mode, you cannot ping the virtual IPv6 address. If you enable accept-mode, the master router accepts packets addressed to the IPv6 address that is associated with the virtual router.
When you configure VRRP for IPv6 on an interface, you can configure the accept-mode parameter. By default, accept-mode is disabled. If you disable accept-mode, the master router does not drop neighbor solicitations or neighbor advertisements. The master router forwards packets with a destination link-layer MAC address that matches the virtual MAC address. If you disable accept-mode, you cannot ping the virtual IPv6 address.