HSRP Gratuitous ARP

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Information About HSRP Gratuitous ARP

HSRP and ARP

HSRP works when the hosts are configured for proxy ARP. When the active HSRP device receives an ARP request for a host that is not on the local LAN, the device replies with the MAC address of the virtual router. If the active device becomes unavailable or its connection to the remote LAN goes down, the device that becomes the active device receives packets addressed to the virtual router and transfers them accordingly. If the Hot Standby state of the interface is not active, proxy ARP responses are suppressed.

HSRP Gratuitous ARP

The HSRP Gratuitous ARP feature configures HSRP to check that the entries in the ARP cache are correct and to send periodic gratuitous ARP packets from one or more HSRP active groups. By default, HSRP sends out three gratuitous ARP packets from an HSRP group when the group state changes to Active. HSRP sends the first gratuitous ARP packet when the group becomes active. The second two gratuitous ARP packets are sent 2 and 4 seconds later.

The HSRP Gratuitous ARP feature enhances the capability of HSRP so that the number and frequency of gratuitous ARP packets sent by an active HSRP group are configurable. Use the standby arp gratuitous command in interface configuration mode to configure a specific number of gratuitous ARP packets to be sent at a specified interval.

Use the standby send arp command in EXEC mode to configure HSRP to send a single gratuitous ARP packet for each active group. When the standby send arp command is configured, HSRP checks that the entries in the ARP cache are correct prior to sending a gratuitous ARP packet. If an ARP entry is incorrect, HSRP will try to readd it. Static or alias ARP entries cannot be overwritten by HSRP.

Configuring the standby send arp command ensures that a host ARP cache is updated prior to heavy CPU-usage processes or configurations are started.

When CPU usage is above 50 percent due to heavy ARP traffic combined with moderate software switched IP traffic, ARP refresh requests could fail, causing some application servers to lose their default gateway ARP entries and fail to communicate with the rest of the network. In some scenarios, operations such as enabling a large access list can cause ARP requests from hosts to be delayed, causing the host to have no default gateway for a short time. A periodic gratuitous ARP packet sent from the HSRP active device refreshes the host ARP cache before it expires.

How to Configure HSRP Gratuitous ARP

Configuring HSRP Gratuitous ARP

Perform this task to configure HSRP to check that the entries in the ARP cache are correct and to send periodic gratuitous ARP packets from one or more HSRP active groups. By default, HSRP sends out three gratuitous ARP packets from an HSRP group when the group state changes to Active. HSRP sends the first gratuitous ARP packet when the group becomes active. The second two gratuitous ARP packets are sent 2 and 4 seconds later.

SUMMARY STEPS

    1.    enable

    2.    standby send arp [interface-type interface-number [group-number]]

    3.    configure terminal

    4.    interface type number

    5.    standby arp gratuitous [count number] [interval seconds]

    6.    end

    7.    show standby arp gratuitous [type-number]


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 standby send arp [interface-type interface-number [group-number]]


    Example:
    Device# standby send arp Ethernet 1/1 1
     

    (Optional) Configures HSRP to send a single gratuitous ARP packet for each active HSRP group.

     
    Step 3 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 4 interface type number


    Example:
    Device(config)# interface Ethernet1/1
     

    Configures an interface type and enters interface configuration mode.

     
    Step 5 standby arp gratuitous [count number] [interval seconds]


    Example:
    Device(config-if)# standby arp gratuitous count 3 interval 4
     

    Configures the number of gratuitous ARP packets sent by an active HSRP group, and how often they are sent.

     
    Step 6 end


    Example:
    Device(config-if)# end
     

    (Optional) Returns to privileged EXEC mode.

     
    Step 7 show standby arp gratuitous [type-number]


    Example:
    Device# show standby arp gratuitous ethernet1/1
     

    (Optional) Displays the number and configured interval of gratuitous ARP packets sent by HSRP.

     

    Example

    The following is sample output from the show standby arp gratuitous command: 

    Device# show standby arp gratuitous ethernet 1/1

HSRP Gratuitous ARP
Interface          Interval  Count    
Ethernet1/1 4         3

    Configuration Examples for HSRP Gratuitous ARP

    Example: Configuring HSRP Gratuitous ARP

    The following example shows how to configure HSRP to check that the entries in the ARP cache are correct and to send three gratuitous ARP packets at 4-second intervals when an HSRP group on the interface changes to active state: 

    Device> enable
Device# standby send arp Ethernet 1/1 1
Device# configure terminal
Device(config)# interface Ethernet 1/1
Device(config-if)# standby arp gratuitous count 3 interval 4
Device(config-if)# end
Device# show standby arp gratuitous ethernet 1/1

HSRP Gratuitous ARP 
Interface   Interval   Count 
Ethernet1/1 4           3

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Commands List, All Releases

    HSRP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

    Cisco IOS First Hop redundancy Protocols Command Reference

    HSRP for IPv6

    “HSRP for IPv6” module

    Troubleshooting HSRP

    Hot Standby Router Protocol: Frequently Asked Questions

    Standards

    Standards

    Title

    No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

    --

    MIBs

    MIBs

    MIBs Link

    CISCO-HSRP-MIB CISCO-HSRP-EXT-MIB

    To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

    http:/​/​www.cisco.com/​go/​mibs

    RFCs

    RFCs

    Title

    RFC 792

    Internet Control Message Protocol

    RFC 1828

    IP Authentication Using Keyed MD5

    RFC 2281

    Cisco Hot Standby Router Protocol

    Technical Assistance

    Description

    Link

    The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for HSRP Gratuitous ARP

    The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

    Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

    Table 1 Feature Information for HSRP Gratuitous ARP

    Feature Name

    Releases

    Feature Information

    HSRP Gratuitous ARP

    12.2(33)SXI

    12.2(50)SY

    15.0(1)SY

    The HSRP Gratuitous ARP feature configures HSRP to check that the entries in the ARP cache are correct and to send periodic gratuitous ARP packets from one or more HSRP active groups.

    The following commands were introduced by this feature: show standby arp gratuitous, standby arp gratuitous, standby send arp.

    The following commands were modified by this feature: show standby, debug standby events.