The IETF defined the Virtual Router Redundancy Protocol (VRRP) in RFC 2338 (later superseded by RFC 3768), an open standard First Hop Redundancy Protocol (FHRP) that offers network redundancy for Internet Protocol Version 4 and Internet Protocol version 6 networks’ default gateways. It functions essentially in the same way as Cisco’s proprietary HSRP.
What is Virtual Router Redundancy Protocol (VRRP)?
The VRRP-enabled routers on an IPv4 Local area network are dynamically given responsibilities for one or more virtual routers. In a multi-access segment, like Ethernet, it permits several physical routers to share a virtual IP address that serves as their default gateway. On end devices, this virtual IP address is set up as the gateway by default. The default router is referred to as the “first hop” since it is the first router a packet passes through on its way to distant networks.
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How VRRP Works
VRRP operates in an active/standby (or active/passive) model.
- Virtual Router: All client devices are shown a virtual router by VRRP, complete with a virtual MAC address and virtual IP address.
- Role Election: One physical router is chosen to serve as the master virtual router in a VRRP group, with the other routers serving as backup virtual routers. Traffic for the virtual IP and MAC addresses is only ever actively forwarded by the master virtual router.
- The master virtual router is chosen from among the routers having the greatest priority value.
- The router with the highest IP address in terms of numbers is chosen as the master if priority are equal.
- Advertisements: To negotiate responsibilities and keep track of each other’s progress, VRRP routers send each other advertisements, which are akin to hello messages. IP protocol number 112 is used to send these ads to the multicast destination address 224.0.0.18. These ads are automatically distributed once per second.
- Transparent Failover: A backup virtual router recognizes the failure if the primary virtual router malfunctions (for example, by ceasing to broadcast ads). The virtual IP and MAC addresses are then taken over by the backup router, which then acts as the master. Because end devices’ ARP tables and default gateway configuration usually don’t change, this move is transparent to them.
Benefits of VRRP
- High Availability and Redundancy: If a router fails, end devices will still be able to access the network since VRRP removes the default gateway’s single point of failure. Network resilience is increased as a result.
- Transparent Failover: By avoiding service interruption and removing the requirement for client reconfiguration, the failover procedure is transparent to end-user devices.
- Simplified Client Configuration: Network administration can be made easier by setting up end devices to use a single, static virtual IP address as their default gateway.
Drawbacks of VRRP
- Resource Inefficiency: Like HSRP, VRRP typically uses an active/standby approach for a specific subnet, which means that the CPU and bandwidth of backup routers remain essentially idle until a fault happens. Underutilization of the hardware that is available may result from this.
- Limited True Load Balancing: True per-packet load balancing is not offered by VRRP. Although it is possible to accomplish some sort of “load sharing” by setting up several VRRP groups in various Virtual Local Area Networks and switching between which router is the master for each VLAN, this is not as reliable as load-balancing-specific protocols like GLBP.
Types/Versions of VRRP
- VRRP Version 2: There is no VRRP version 1 standard; instead, Cisco IOS software supports VRRP Version 2, which is the default and most recent version. VRRPv2 specifies the IPv4 protocol.
- VRRP Version 3 (VRRPv3): Both IPv4 and IPv6 networks are supported by VRRP Version 3 (VRRPv3). VRRPv3 was still in draft form and not yet standardized when some sources were written.
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Configuration and Verification
VRRP Virtual MAC Address
The VRRP virtual MAC address is in the format 00-00-5e-00-01xx, where “xx” represents the two-digit hexadecimal VRRP group number. Unlike HSRP, VRRP does not offer the option to use the interface’s burned-in address (BIA) or a statically configured MAC address for VRRP groups.
VRRP Master Router Election
Priority: A VRRP group’s routers are given a priority value. The master virtual router is the router with the highest priority (for example, 105 over 100).
IP Address Tie-breaker: The router with the highest numerical IP address on the interface is chosen as the master if two routers share the same highest priority.
Preemption
By default, preemption is enabled for VRRP. This means that if a router with a higher priority comes online or recovers from a failure, it will automatically take over the master role. This contrasts with HSRP, where preemption is typically disabled by default and needs explicit configuration. Preemption can be disabled using the no vrrp [group-number] preempt command.
VRRP Load Balancing
Similar to HSRP, which usually entails setting up distinct VRRP groups for various VLANs and allocating the master role to distinct routers for each group, VRRP enables load balancing. This accomplishes a type of “load sharing” between several routers.
VRRP Interface Tracking
VRRP supports interface tracking, similar to HSRP, to monitor the operational status of other interfaces or IP routes.
- A tracked object (such as monitoring the reachability of an IP route or the line protocol of a Loopback interface) can be created by an administrator.
- VRRP can be set up to lower the router’s priority in the event that the tracked item on the master router fails. This might result in a fresh master election and enable a backup router to take over. By doing this, traffic is kept from reaching a router that is unable to forward it to its intended location.
- Depending on the platform and software, up to 500 monitored objects can be set up.
Configuration Commands (Basic Examples)
1. Configure IP address on the physical interface: Router(config-if)# ip address [address] [mask]
2. Create a VRRP group and assign the virtual IP address: Router(config-if)# vrrp [group-number] ip [virtual-ip-address]
3. Set priority (optional; default is 100): Router(config-if)# vrrp [group-number] priority [value]
4. Add a description (optional): Router(config-if)# vrrp [group-number] description [name]
5. Configure interface tracking (optional): Router(config)# track [object-number] interface [interface-type interface-number] line-protocol Router(config-if)# vrrp [group-number] track [object-number]
Verification Commands
• show vrrp
• show vrrp all
• show vrrp brief
• show vrrp interface [interface-type interface-number]
Debugging Commands
• debug vrrp [all|auth|errors|events|packets] provides real-time information on VRRP operation and specific events.
Applications of VRRP
A fundamental network service, VRRP guarantees the default gateway’s high availability. Therefore, VRRP’s ability to maintain a continuous link is naturally advantageous for all network applications and client interactions that depend on IP connectivity outside of their local subnet. The transparent failover offered by VRRP is advantageous for applications such as file transfers, email, online surfing, and real-time communication services (VoIP, video conferencing).
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