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Understanding VRRP

Overview of VRRP

Configuring end hosts on your network with static default routes minimizes configuration effort and complexity and reduces processing overhead on the end hosts. When hosts are configured with static routes, the failure of the default gateway normally results in a catastrophic event, isolating all hosts that are unable to detect available alternate paths to their gateway. Using Virtual Router Redundancy Protocol (VRRP) enables you to dynamically provide alternative gateways for end hosts if the primary gateway fails.

VRRP (defined in RFC 3768) provides dynamic failover of IP addresses from one router to another in the event of failure. You can implement VRRP to provide a highly available default path to a gateway without needing to configure dynamic routing or router discovery protocols on end hosts.

Switches configured with VRRP share a virtual IP address, which is the address you configure as the default route on the hosts. At any time, one of the switches is the VRRP master, meaning that it owns the virtual IP address and is the active default gateway. The other devices are backups. The switches dynamically assign master and backup roles based on priorities that you configure (1 through 255). If the master fails, the backup switch with the highest priority becomes the master within a few seconds. This is done without any interaction with the hosts.

In VRRP operation, the master sends advertisements to the backup switches at regular intervals. The default interval is 1 second. If the backup switches do not receive an advertisement for a set period, the backup with the highest priority takes over as master within a few seconds and begins forwarding packets. This is done without any interaction with the hosts.

Note: Priority 255 cannot be set for routed VLAN interfaces (RVIs).

You can configure two QFabric systems to participate in a VRRP configuration as if they were two standalone switches. One benefit of this configuration is if you use VMware’s vMotion, virtual machines can transition between hosts connected to the QFabric systems without updating their default gateway information. For example, a virtual machine running on a host connected to a QFabric system in data center A can transition to a host connected to a QFabric system in data center B without needing to resolve a new gateway IP address and MAC address.

Sample VRRP Topology

Figure 1 illustrates a basic VRRP topology. In this example, switches A and B are running VRRP and share the virtual IP address 10.1.1.1. The default gateway for each of the clients is 10.1.1.1.

Figure 1: Basic VRRP Topology

1. When any of the servers wants to send traffic out of the LAN, it sends the traffic to the default gateway address of 10.1.1.1. This is a virtual IP address (VIP) owned by VRRP group 100. Because switch A is the master of the group, the VIP is associated with the “real” address 10.1.1.251 on switch A, and traffic from the servers is actually sent to this address. (Switch A is the master because it has been configured with a higher priority value.)
2. If there is a failure on switch A that prevents it from forwarding traffic to or from the servers—for example, if the interface connected to the LAN fails—switch B becomes the master and assumes ownership of the VIP. The servers continue to send traffic to the VIP, but because the VIP is now associated with the “real” address 10.1.1.252 on switch B (because of change of master), the traffic is sent to switch B instead of switch A.
3. If the problem that caused the failure on switch A is corrected, switch A becomes the master again and reasserts ownership of the VIP. In this case, the servers resume sending traffic to switch A.

Notice that no configuration changes are required on the servers for them to switch between sending traffic to switch A and switch B. When the VIP moves between 10.1.1.251 and 10.1.1.252, the change is detected by normal TCP-IP behavior and no configuration or intervention is required on the servers.