Gratuitous arp

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Gratuitous ARP is an ARP reply sent without a preceding ARP request, typically to announce or refresh an address-to-MAC mapping.

Gratuitous ARP (GARP) is an ARP reply sent without a preceding request. During an FHRP failover, the new active router broadcasts GARP so switches relearn the virtual MAC address on the correct port. section 19.1.3 section 19.4

FHRP failover

If R1 fails, R2 becomes active and sends GARP messages sourced from the virtual MAC address and destined for ffff.ffff.ffff. Hosts keep using the same VIP and ARP entry, while switches update their MAC address tables. section 19.1.3

Figure 19.4 An FHRP failover. (1) A hardware failure occurs on R1. (2) R2 takes over as the active router. (3) R2 sends GARP messages, sourced from the virtual MAC and destined for the broadcast MAC. (4) The switches learn the virtual MAC on the appropriate ports—those leading toward the new active router. (5) End hosts’ default gateways and ARP table entries for the VIP remain the same.

Follow the failover sequence from hardware failure through GARP transmission, switch relearning, and unchanged host gateway settings. section 19.1.3

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Explains GARP’s unsolicited-reply form, broadcast destination, virtual-MAC source, and resulting switch-table update.

Examples

Uses an R1-to-R2 FHRP takeover in which broadcast GARP causes switches to relearn the virtual MAC on the new router’s port.

What you can do after

Understand how GARP preserves transparent host connectivity while switches redirect frames after an FHRP failover.

How it connects

Connects ARP with FHRP failover, virtual IP addresses, virtual MAC addresses, switches, and MAC address tables.

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Related concepts

• arp replykind of · An ARP reply is a protocol message that provides or asserts the MAC address associated with an IPv4 address.
• failoverused by · Failover is the transfer of service or workload from a failed or unavailable component to a functioning alternate or standby component.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.