Configuring VRRP

The Virtual Router Redundancy Protocol (VRRP) is an election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on a LAN, allowing several routers on a multiaccess link to utilize the same virtual IP address. A VRRP router is configured to run the VRRP protocol in conjunction with one or more other routers attached to a LAN. In a VRRP configuration, one router is elected as the virtual router master, with the other routers acting as backups in case the virtual router master fails.

This module explains the concepts related to VRRP and describes how to configure VRRP in a network.

Restrictions for VRRP

  • VRRP is designed for use over multiaccess, multicast, or broadcast capable Ethernet LANs. VRRP is not intended as a replacement for existing dynamic protocols.

  • VRRP is supported on Ethernet, Fast Ethernet, Bridge Group Virtual Interface (BVI), and Gigabit Ethernet interfaces, and on Multiprotocol Label Switching (MPLS) Virtual Private Networks (VPNs), VRF-aware MPLS VPNs, and VLANs.

  • Because of the forwarding delay that is associated with the initialization of a BVI interface, you must configure the VRRP advertise timer to a value equal to or greater than the forwarding delay on the BVI interface. This setting prevents a VRRP router on a recently initialized BVI interface from unconditionally taking over the master role. Use the bridge forward-time command to set the forwarding delay on the BVI interface. Use the vrrp timers advertise command to set the VRRP advertisement timer.

Restrictions for VRRP on Cisco Routers

  • Stateful Switchover (SSO) with VRRP is not supported.

Restrictions for VRRPv2 on Cisco RSP3 Module

  • Sub second timer (msec) is not supported for VRRPv2.
  • Bidirectional Forwarding Detection (BFD) is not supported for VRRPv2.
  • SSO and ISSU are not supported for VRRPv2.
  • Maximum of 8 Active FHRP groups are supported on RSP3 Module.
  • Maximum of 4 FHRP groups are supported per L3 interface(BDI-logical or Gig-physical) on RSP3 Module.

Information About VRRP

VRRP Operation

There are several ways a LAN client can determine which router should be the first hop to a particular remote destination. The client can use a dynamic process or static configuration. Examples of dynamic router discovery are as follows:

  • Proxy ARP—The client uses Address Resolution Protocol (ARP) to get the destination it wants to reach, and a router will respond to the ARP request with its own MAC address.

  • Routing protocol—The client listens to dynamic routing protocol updates (for example, from Routing Information Protocol [RIP]) and forms its own routing table.

  • ICMP Router Discovery Protocol (IRDP) client—The client runs an Internet Control Message Protocol (ICMP) router discovery client.

The drawback to dynamic discovery protocols is that they incur some configuration and processing overhead on the LAN client. Also, in the event of a router failure, the process of switching to another router can be slow.

An alternative to dynamic discovery protocols is to statically configure a default router on the client. This approach simplifies client configuration and processing, but creates a single point of failure. If the default gateway fails, the LAN client is limited to communicating only on the local IP network segment and is cut off from the rest of the network.

VRRP can solve the static configuration problem. VRRP enables a group of routers to form a single virtual router. The LAN clients can then be configured with the virtual router as their default gateway. The virtual router, representing a group of routers, is also known as a VRRP group.

VRRP is supported on Ethernet, Fast Ethernet, BVI, and Gigabit Ethernet interfaces, and on MPLS VPNs, VRF-aware MPLS VPNs, and VLANs.

The figure below shows a LAN topology in which VRRP is configured. In this example, Routers A, B, and C are VRRP routers (routers running VRRP) that comprise a virtual router. The IP address of the virtual router is the same as that configured for the Ethernet interface of Router A (10.0.0.1).

Figure 1. Basic VRRP Topology

Because the virtual router uses the IP address of the physical Ethernet interface of Router A, Router A assumes the role of the virtual router master and is also known as the IP address owner. As the virtual router master, Router A controls the IP address of the virtual router and is responsible for forwarding packets sent to this IP address. Clients 1 through 3 are configured with the default gateway IP address of 10.0.0.1.

Routers B and C function as virtual router backups. If the virtual router master fails, the router configured with the higher priority will become the virtual router master and provide uninterrupted service for the LAN hosts. When Router A recovers, it becomes the virtual router master again. For more detail on the roles that VRRP routers play and what happens if the virtual router master fails, see the VRRP Router Priority and Preemption section.

The figure below shows a LAN topology in which VRRP is configured so that Routers A and B share the traffic to and from clients 1 through 4 and that Routers A and B act as virtual router backups to each other if either router fails.

Figure 2. Load Sharing and Redundancy VRRP Topology

In this topology, two virtual routers are configured. (For more information, see the Multiple Virtual Router Support section.) For virtual router 1, Router A is the owner of IP address 10.0.0.1 and virtual router master, and Router B is the virtual router backup to Router A. Clients 1 and 2 are configured with the default gateway IP address of 10.0.0.1.

For virtual router 2, Router B is the owner of IP address 10.0.0.2 and virtual router master, and Router A is the virtual router backup to Router B. Clients 3 and 4 are configured with the default gateway IP address of 10.0.0.2.

VRRP Benefits

Redundancy

VRRP enables you to configure multiple routers as the default gateway router, which reduces the possibility of a single point of failure in a network.

Load Sharing

You can configure VRRP in such a way that traffic to and from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers.

Multiple Virtual Routers

VRRP supports up to 255 virtual routers (VRRP groups) on a router and up to four virtual routers on every interface, subject to the platform supporting multiple MAC addresses. Multiple virtual router support enables you to implement redundancy and load sharing in your LAN topology.

Multiple IP Addresses

The virtual router can manage multiple IP addresses, including secondary IP addresses. Therefore, if you have multiple subnets configured on an Ethernet interface, you can configure VRRP on each subnet.

Preemption

The redundancy scheme of VRRP enables you to preempt a virtual router backup that has taken over for a failing virtual router master with a higher priority virtual router backup that has become available.

Authentication

VRRP message digest 5 (MD5) algorithm authentication protects against VRRP-spoofing software and uses the industry-standard MD5 algorithm for improved reliability and security.

Advertisement Protocol

VRRP uses a dedicated Internet Assigned Numbers Authority (IANA) standard multicast address (224.0.0.18) for VRRP advertisements. This addressing scheme minimizes the number of routers that must service the multicasts and allows test equipment to accurately identify VRRP packets on a segment. The IANA assigned VRRP the IP protocol number 112.

VRRP Object Tracking

VRRP object tracking provides a way to ensure the best VRRP router is the virtual router master for the group by altering VRRP priorities to the status of tracked objects such as the interface or IP route states.

Multiple Virtual Router Support

You can configure up to 255 virtual routers on a router and up to four virtual routers on every interface. The actual number of virtual routers that a router can support depends on the following factors:

  • Router processing capability

  • Router memory capability

  • Router interface support of multiple MAC addresses

In a topology where multiple virtual routers are configured on a router interface, the interface can act as a master for one virtual router and as a backup for one or more virtual routers.

VRRP Router Priority and Preemption

An important aspect of the VRRP redundancy scheme is VRRP router priority. Priority determines the role that each VRRP router plays and what happens if the virtual router master fails.

If a VRRP router owns the IP address of the virtual router and the IP address of the physical interface, this router will function as a virtual router master.

Priority also determines if a VRRP router functions as a virtual router backup and the order of ascendancy to becoming a virtual router master if the virtual router master fails. You can configure the priority of each virtual router backup with a value of 1 through 254 using the vrrp priority command.

For example, if Router A, the virtual router master in a LAN topology, fails, an election process takes place to determine if virtual router backups B or C should take over. If Routers B and C are configured with the priorities of 101 and 100, respectively, Router B is elected to become virtual router master because it has the higher priority. If Routers B and C are both configured with the priority of 100, the virtual router backup with the higher IP address is elected to become the virtual router master.

By default, a preemptive scheme is enabled whereby a higher priority virtual router backup that becomes available takes over for the virtual router backup that was elected to become virtual router master. You can disable this preemptive scheme using the no vrrp preempt command. If preemption is disabled, the virtual router backup that is elected to become virtual router master remains the master until the original virtual router master recovers and becomes master again.

VRRP Advertisements

The virtual router master sends VRRP advertisements to other VRRP routers in the same group. The advertisements communicate the priority and state of the virtual router master. The VRRP advertisements are encapsulated in IP packets and sent to the IP Version 4 multicast address assigned to the VRRP group. The advertisements are sent every second by default; the interval is configurable.

Although the VRRP protocol as per RFC 3768 does not support millisecond timers, Cisco routers allow you to configure millisecond timers. You need to manually configure the millisecond timer values on both the primary and the backup routers. The master advertisement value displayed in the show vrrp command output on the backup routers is always 1 second because the packets on the backup routers do not accept millisecond values.

You must use millisecond timers where absolutely necessary and with careful consideration and testing. Millisecond values work only under favorable circumstances, and you must be aware that the use of the millisecond timer values restricts VRRP operation to Cisco devices only.


Note

Millisecond timer value is not supported for VRRPv2 on RSP3 Module.

In Service Software Upgrade--VRRP


Note
This section is not supported for VRRPv2 on ASR 900 RSP3 Module.

VRRP supports In Service Software Upgrade (ISSU). In Service Software Upgrade (ISSU) allows a high-availability (HA) system to run in stateful switchover (SSO) mode even when different versions of software are running on the active and standby Route Processors (RPs) or line cards.

ISSU provides the ability to upgrade or downgrade from one supported release to another while continuing to forward packets and maintain sessions, thereby reducing planned outage time. The ability to upgrade or downgrade is achieved by running different software versions on the active RP and standby RP for a short period of time to maintain state information between RPs. This feature allows the system to switch over to a secondary RP running upgraded (or downgraded) software and continue forwarding packets without session loss and with minimal or no packet loss. This feature is enabled by default.

For detailed information about ISSU, see the In Service Software Upgrade Process document in the High Availability Configuration Guide.

How to Configure VRRP

Customizing VRRP

Customizing the behavior of VRRP is optional. Be aware that as soon as you enable a VRRP group, that group is operating. It is possible that if you first enable a VRRP group before customizing VRRP, the router could take over control of the group and become the virtual router master before you have finished customizing the feature. Therefore, if you plan to customize VRRP, it is a good idea to do so before enabling VRRP.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    interface type number

    4.    ip address ip-address mask

    5.    vrrp group description text

    6.    vrrp group priority level

    7.    vrrp group preempt [delay minimum seconds]

    8.    vrrp group timers advertise [sec] interval

    9.    vrrp group timers learn

    10.    exit


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example: 
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example: 
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 interface type number


    Example: 
    Router(config)# interface GigabitEthernet 0/0/0
     

    Enters interface configuration mode.

     
    Step 4 ip address ip-address mask


    Example: 
    Router(config-if)# ip address 172.16.6.5 255.255.255.0
     

    Configures an IP address for an interface.

     
    Step 5 vrrp group description text


    Example: 
    Router(config-if)# vrrp 10 description working-group
     

    Assigns a text description to the VRRP group.

     
    Step 6 vrrp group priority level


    Example: 
    Router(config-if)# vrrp 10 priority 110
     

    Sets the priority level of the router within a VRRP group.

    • The default priority is 100.

     
    Step 7 vrrp group preempt [delay minimum seconds]


    Example: 
    Router(config-if)# vrrp 10 preempt delay minimum 380
     

    Configures the router to take over as virtual router master for a VRRP group if it has a higher priority than the current virtual router master.

    • The default delay period is 0 seconds.

    • The router that is IP address owner will preempt, regardless of the setting of this command.

     
    Step 8 vrrp group timers advertise [sec] interval


    Example: 
    Router(config-if)# vrrp 10 timers advertise 110
     

    Configures the interval between successive advertisements by the virtual router master in a VRRP group.

    • The unit of the interval is in seconds unless the sec keyword is specified. The default interval value is 1 second.

    Note   

    All routers in a VRRP group must use the same timer values. If the same timer values are not set, the routers in the VRRP group will not communicate with each other and any misconfigured router will change its state to master.

     
    Step 9 vrrp group timers learn


    Example: 
    Router(config-if)# vrrp 10 timers learn
     

    Configures the router, when it is acting as virtual router backup for a VRRP group, to learn the advertisement interval used by the virtual router master.

     
    Step 10 exit


    Example: 
    Router(config-if)# exit
     

    Exits interface configuration mode.

     

    Enabling VRRP

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    interface type number

      4.    ip address ip-address mask

      5.    vrrp group ip ip-address [secondary]

      6.    end

      7.    show vrrp [brief] | group]

      8.    show vrrp interface type number [brief]


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Router> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 configure terminal


      Example:
      Router# configure terminal
       

      Enters global configuration mode.

       
      Step 3 interface type number


      Example:
      Router(config)# interface GigabitEthernet 0/0/0
       

      Enters interface configuration mode.

       
      Step 4 ip address ip-address mask


      Example:
      Router(config-if)# ip address 172.16.6.5 255.255.255.0
       

      Configures an IP address for an interface.

       
      Step 5 vrrp group ip ip-address [secondary]


      Example:
      Router(config-if)# vrrp 10 ip 172.16.6.1
       

      Enables VRRP on an interface.

      • After you identify a primary IP address, you can use the vrrp ip command again with the secondary keyword to indicate additional IP addresses supported by this group.

      Note   

      All routers in the VRRP group must be configured with the same primary address and a matching list of secondary addresses for the virtual router. If different primary or secondary addresses are configured, the routers in the VRRP group will not communicate with each other and any misconfigured router will change its state to master.

       
      Step 6 end


      Example:
      Router(config-if)# end
       

      Returns to privileged EXEC mode.

       
      Step 7 show vrrp [brief] | group]


      Example:
      Router# show vrrp 10
       

      (Optional) Displays a brief or detailed status of one or all VRRP groups on the router.

       
      Step 8 show vrrp interface type number [brief]


      Example:
      Router# show vrrp interface GigabitEthernet 0/0/0
       

      (Optional) Displays the VRRP groups and their status on a specified interface.

       

      Disabling a VRRP Group on an Interface

      Disabling a VRRP group on an interface allows the protocol to be disabled, but the configuration to be retained. This ability was added with the introduction of the VRRP MIB, RFC 2787, Definitions of Managed Objects for the Virtual Router Redundancy Protocol .

      You can use a Simple Network Management Protocol (SNMP) management tool to enable or disable VRRP on an interface. Because of the SNMP management capability, the vrrp shutdown command was introduced to represent a method via the command line interface (CLI) for VRRP to show the state that had been configured using SNMP.

      When the show running-config command is entered, you can see immediately if the VRRP group has been configured and set to enabled or disabled. This is the same functionality that is enabled within the MIB.

      The no form of the command enables the same operation that is performed within the MIB. If the vrrp shutdown command is specified using the SNMP interface, then entering the no vrrp shutdown command reenables the VRRP group.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface type number

        4.    ip address ip-address mask

        5.    vrrp group shutdown


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example:
        Router> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.

         
        Step 2 configure terminal


        Example:
        Router# configure terminal
         

        Enters global configuration mode.

         
        Step 3 interface type number


        Example:
        Router(config)# interface GigabitEthernet0/0/0
         

        Enters interface configuration mode.

         
        Step 4 ip address ip-address mask


        Example:
        Router(config-if)# ip address 172.16.6.5 255.255.255.0
         

        Configures an IP address for an interface.

         
        Step 5 vrrp group shutdown


        Example:
        Router(config-if)# vrrp 10 shutdown
         

        Disables the VRRP group on an interface.

        • The command is now visible on the router.

        Note   

        You can have one VRRP group disabled, while retaining its configuration, and a different VRRP group enabled.

         

        Configuring VRRP Text Authentication

        Before You Begin

        Interoperability with vendors that may have implemented the RFC 2338 method is not enabled.

        Text authentication cannot be combined with MD5 authentication for a VRRP group at any one time. When MD5 authentication is configured, the text authentication field in VRRP hello messages is set to all zeros on transmit and ignored on receipt, provided the receiving router also has MD5 authentication enabled.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    terminal interface type number

          4.    ip address ip-address mask [secondary]

          5.    vrrp group authentication text text-string

          6.    vrrp group ip ip-address

          7.    Repeat Steps 1 through 6 on each router that will communicate.

          8.    end


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example:
          Router> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.

           
          Step 2 configure terminal


          Example:
          Router# configure terminal
           

          Enters global configuration mode.

           
          Step 3 terminal interface type number


          Example:
          Router(config)# interface Ethernet 0/1
           

          Configures an interface type and enters interface configuration mode.

           
          Step 4 ip address ip-address mask [secondary]


          Example:
          Router(config-if)# ip address 10.0.0.1 255.255.255.0
           

          Specifies a primary or secondary IP address for an interface.

           
          Step 5 vrrp group authentication text text-string


          Example:
          Router(config-if)# vrrp 1 authentication text textstring1
           

          Authenticates VRRP packets received from other routers in the group.

          • If you configure authentication, all routers within the VRRP group must use the same authentication string.

          • The default string is cisco.

          Note   

          All routers within the VRRP group must be configured with the same authentication string. If the same authentication string is not configured, the routers in the VRRP group will not communicate with each other and any misconfigured router will change its state to master.

           
          Step 6 vrrp group ip ip-address


          Example:
          Router(config-if)# vrrp 1 ip 10.0.1.20
           

          Enables VRRP on an interface and identifies the IP address of the virtual router.

           
          Step 7 Repeat Steps 1 through 6 on each router that will communicate.  

          —

           
          Step 8 end


          Example:
          Router(config-if)# end
           

          Returns to privileged EXEC mode.

           

          Enabling the Router to Send SNMP VRRP Notifications

          The VRRP MIB supports SNMP Get operations, which allow network devices to get reports about VRRP groups in a network from the network management station.

          Enabling VRRP MIB trap support is performed through the CLI, and the MIB is used for getting the reports. A trap notifies the network management station when a router becomes a Master or backup router. When an entry is configured from the CLI, the RowStatus for that group in the MIB immediately goes to the active state.

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    snmp-server enable traps vrrp

            4.    snmp-server host host community-string vrrp


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 enable


            Example:
            Router> enable
             

            Enables privileged EXEC mode.

            • Enter your password if prompted.

             
            Step 2 configure terminal


            Example:
            Router# configure terminal
             

            Enters global configuration mode.

             
            Step 3 snmp-server enable traps vrrp


            Example:
            Router(config)#
 snmp-server enable traps vrrp
             

            Enables the router to send SNMP VRRP notifications (traps and informs).

             
            Step 4 snmp-server host host community-string vrrp


            Example:
            Router(config)#
 snmp-server host myhost.comp.com public vrrp
             

            Specifies the recipient of an SNMP notification operation.

             

            Configuration Examples for VRRP

            Example: Configuring VRRP

            In the following example, Router A and Router B each belong to three VRRP groups.

            In the configuration, each group has the following properties:

            • Group 1:
              • Virtual IP address is 10.1.0.10.
              • Router A will become the master for this group with priority 120.
              • Advertising interval is 3 seconds.
              • Preemption is enabled.
            • Group 5:
              • Router B will become the master for this group with priority 200.
              • Advertising interval is 30 seconds.
              • Preemption is enabled.
            • Group 100:
              • Router A will become the master for this group first because it has a higher IP address (10.1.0.2).
              • Advertising interval is the default 1 second.
              • Preemption is disabled.

            Router A

            Router(config)# interface GigabitEthernet 1/0/0 
Router(config-if)# ip address 10.1.0.2 255.0.0.0 
Router(config-if)# vrrp 1 priority 120 
Router(config-if)# vrrp 1 authentication cisco 
Router(config-if)# vrrp 1 timers advertise 3 
Router(config-if)# vrrp 1 timers learn 
Router(config-if)# vrrp 1 ip 10.1.0.10 
Router(config-if)# vrrp 5 priority 100 
Router(config-if)# vrrp 5 timers advertise 30
Router(config-if)# vrrp 5 timers learn
Router(config-if)# vrrp 5 ip 10.1.0.50 
Router(config-if)# vrrp 100 timers learn
Router(config-if)# no vrrp 100 preempt 
Router(config-if)# vrrp 100 ip 10.1.0.100
Router(config-if)# no shutdown

            Router B

            Router(config)# interface GigabitEthernet 1/0/0 
Router(config-if)# ip address 10.1.0.1 255.0.0.0 
Router(config-if)# vrrp 1 priority 100 
Router(config-if)# vrrp 1 authentication cisco 
Router(config-if)# vrrp 1 timers advertise 3 
Router(config-if)# vrrp 1 timers learn 
Router(config-if)# vrrp 1 ip 10.1.0.10 
Router(config-if)# vrrp 5 priority 200 
Router(config-if)# vrrp 5 timers advertise 30
Router(config-if)# vrrp 5 timers learn
Router(config-if)# vrrp 5 ip 10.1.0.50 
Router(config-if)# vrrp 100 timers learn
Router(config-if)# no vrrp 100 preempt 
Router(config-if)# vrrp 100 ip 10.1.0.100
Router(config-if)# no shutdown

            Example: VRRP Text Authentication

            The following example shows how to configure VRRP text authentication using a text string: 

            Router(config)# interface GigabitEthernet 0/0/0
Router(config)# ip address 10.21.8.32 255.255.255.0
Router(config-if)# vrrp 10 authentication text stringxyz
Router(config-if)# vrrp 10 ip 10.21.8.10

            Example: Disabling a VRRP Group on an Interface

            The following example shows how to disable one VRRP group on GigabitEthernet interface 0/0/0 while retaining VRRP for group 2 on GigabitEthernet interface 1/0/0: 

            Router(config)# interface GigabitEthernet 0/0/0
Router(config-if)# ip address 10.24.1.1 255.255.255.0
Router(config-if)# vrrp 1 ip 10.24.1.254
Router(config-if)# vrrp 1 shutdown
Router(config-if)# exit
Router(config)# interface GigabitEthernet 1/0/0
Router(config-if)# ip address 10.168.42.1 255.255.255.0
Router(config-if)# vrrp 2 ip 10.168.42.254

            Example: VRRP MIB Trap

            Router(config)# snmp-server enable traps vrrp
Router(config)# snmp-server host 10.1.1.0 community abc vrrp