IP Addressing Configuration Guide, Cisco IOS XE 17.x
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The GARP retry feature provides customer support and minimizes control plane traffic impact, but only when utilized on BD-VIF
interfaces.
GARP for NAT mapping does not support the use of the same IP address in multiple VRFs within the same BD-VIF.
GARP retry messages are sent only from the active box in a Box-to-Box High Availability (HA) configuration.
Information About Initiating GARP for NAT Mapping
Overview
Initiating Gratuitous Address Resolution Protocol (GARP) for Network Address Translation (NAT) Mapping is a feature that uses
the Address Resolution Protocol (ARP) and GARP to map MAC addresses to IP addresses within a local network. This feature proactively
updates and notifies devices about address changes in the network, ensuring accurate mapping between MAC and IP addresses.
By utilizing GARP, devices in the Application Centric Infrastructure (ACI) Fabric can efficiently discover and associate MAC
addresses with IP addresses. This feature ensures that devices are promptly informed or notified about any changes in address
assignments, allowing for seamless connectivity and efficient network operations.
Overall, GARP-based NAT Mapping in ACI Fabric simplifies the process of MAC-to-IP address resolution, enhances network efficiency,
and ensures smooth communication between devices within the network.
Gratuitous ARP (GARP)
GARP is a part of the ARP that is primarily used to update host devices in a network about changes to IP to MAC address mappings.
A network device sends a GARP request when the device's IP address changes, during failovers, or when the IP address initially
becomes active on the network. This request ensures that other devices in the network update their ARP tables with the new
mapping. In the context of NAT, GARP can be used to initiate or update NAT mappings across the network devices.
Initiating GARP for NAT Mapping in ACI Fabric
For a router connected to an ACI fabric in a cloud deployment, it takes a while to discover networks created due to changes
in NAT mapping. With this functionality, whenever there is a change in NAT mapping, the router triggers a GARP message.
This message, in turn, enables the ACI fabric to discover the MAC address that corresponds to the IP address, thereby enabling
seamless connectivity and efficient network operation.
Figure 1. Initiating GARP for NAT Mapping in ACI Fabric Process Flow
This feature is part of a broader configuration that brings together GARP and NAT mechanisms.
The process is initiated with the activation of the feature using the GARP global ip arp nat-garp-retry feature enable command. This command offers control over several optional parameters, including the number of NAT GARP retry messages, the
interval between these messages, and the maximum number of GARP command executions.
Following this activation, the system requests GARP messages using the garp-interface option with the ip nat inside source static command. This happens on the BD-VIF interface during NAT mapping configuration.
How to Configure the Initiation of GARP for NAT Mapping
Configuring the Initiation of GARP for NAT Mapping
Perform the following steps to configure the initiation of GARP for NAT Mapping.
SUMMARY STEPS
ip arp nat-garp-retry feature enable
ip arp nat-garp-retry retries , ip arp nat-garp-retry interval,ip arp nat-garp-retry entries
Device(config)# ip arp nat-garp-retry feature enable
Configure the GARP retry feature and the ARP retry database. Set the interval for ARP gratuitous retry.
Note
It is essential to enable GARP based NAT mapping at the system level.
Step 2
ip arp nat-garp-retry retries , ip arp nat-garp-retry interval,ip arp nat-garp-retry entries
Example:
Device(config)# ip arp nat-garp-retry retries
Device(config)# ip arp nat-garp-retry interval
Device(config)# ip arp nat-garp-retry entries
These are optional arguments or keywords that provide further control over the ip arp nat-garp-retry command:
retries: Specifies the number of NAT GARP Retry messages. The default value is 2, and the permissible range is from 1 to 5. However,
it is not recommended to set the value above 3.
intervals: Configures intervals between NAT GARP Retry messages. The default is 5 seconds, with a permissible range of 1 to 30 seconds
entries: Defines the number of NAT mappings to be supported. The maximum number of BD-VIF interfaces for GARP initiation is capped
at 3000 to optimize control plane load.