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    Transmission of GARP Packets Overview

    Gratuitous Address Resolution Protocol (GARP) requests provide duplicate IP address detection. A GARP request is a broadcast request for a router’s own IP address. If a router sends an Address Resolution Protocol (ARP) request for its own IP address and no ARP replies are received, the router’s assigned IP address is not being used by other nodes. If a router sends an ARP request for its own IP address and an ARP reply is received, the router’s assigned IP address is already being used by another node.

    A GARP is an ARP broadcast in which the source and destination MAC addresses are the same. It is used primarily by a host to inform the network about its IP address. A spoofed gratuitous ARP message can cause network mapping information to be stored incorrectly, causing a network malfunction.

    GARP is a method of establishing an association between a logical IP address and a hardware address whenever an interface is created or the state of the interface shifts to the operationally up state. On the other hand, ARP dynamically binds the IP address (the logical address) to the correct MAC address. The device that transmits a GARP populates both the source and destination fields with its own information. The devices that receive the GARP requests might update the ARP caches with the new information contained in the GARP packets.

    By default, updating the ARP cache on GARP replies is disabled on the router. On Ethernet interfaces, you can enable transmission of GARP packets on a specific interface by using the ip gratuitous-arps command in Interface Configuration mode and specify the number of GARP packets to be sent, depending on the changes to IP interface settings. If an IP address is configured directly on the physical Ethernet interface and a VLAN major interface is not configured on the Ethernet interface for VLAN encapsulation, transmission of GARP packets does not take place.

    When you create an IP interface or the administrative status of the interface transitions to the up state, three GARP packets are transmitted for each IP address. Each GARP packet is sent at an interval of 10 seconds. By default, the router generates GARP requests. An IP interface can support up to a maximum of 16 secondary IP addresses. Therefore, with the maximum number of secondary IP addresses configured, a total of 48 GARP messages for each IP interface are sent. In a fully scaled environment, such a transmission of a large number of GARP messages creates a storm of GARP packets in the entire broadcast domain, which contains dynamic subscriber line access multiplexers (DSLAMs) and other BRAS devices within the same Metro Ethernet network. In such a network, reducing the number of GARP packets transmitted for interface changes reduces performance impact on the router and improves the processing efficiency of the router.

    GARP Packets Transmission Scenarios

    The following scenarios describe the manner in which GARP packets are generated, based on the default configuration settings for transmission of GARP packets and the network topology:

    • Three GARP packets are sent when you configure a new primary or secondary IP address on an IP interface.
    • Three GARP packets are transmitted when an IP interface state transitions from the down state to the up state.
    • Three GARP packets are sent for each IP address of the numbered interface when a new unnumbered interface associated with the numbered interface is created.
    • Three GARP packets are sent for all the unnumbered interfaces whenever any secondary IP address on the numbered interface that it is associated with is modified.
    • Three GARP packets are sent for all the unnumbered interfaces for all the IP addresses whenever the primary IP address of the numbered interface that it is associated with is modified.

    In all of the these scenarios, you can modify the number of GARP packets to be transmitted to be less than three by using the ip gratuitous-arps command.

    The following two scenarios describe the method of transmission of GARP packets, regardless of whether the sending of GARP packets is disabled. In such cases, even if you configure the no ip gratuitous-arps command to disable sending GARPs, these packets are sent to denote the changes in system and interface conditions.

    • One GARP packet is always sent for each virtual address of a VRRP interface. If you configure VRRP on a virtual router and associate the IP address with the VRRP instance ID (VRID) using the ip vrrp command in Interface Configuration mode, one GARP packet is always transmitted for each virtual address of the interface enabled for VRRP.
    • Three GARP packets are always sent when a failover occurs to the secondary link of the redundant port on GE-2 and GE-HDE line modules that are paired with GE-2 SFP I/O modules, 2xGE APS I/O SFP modules, and GE-2 APS I/O SFP modules, with physical link redundancy.

    Published: 2014-08-13