Does gratuitous ARP work like a normal ARP request?
Why is gratuitous ARP used for HSRP?
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Gratuitous ARP is a sort of "advance notification", it updates the ARP cache of other systems before they ask for it (no ARP request) or to update outdated information.
When talking about gratuitous ARP, the packets are actually special ARP request packets, not ARP reply packets as one would perhaps expect. Some reasons for this are explained in RFC 5227.
The gratuitous ARP packet has the following characteristics:
Gratuitous ARP is used for some reasons:
As for the second part of your question, HSRP, VRRP etc. use gratuitous ARP to update the MAC address tables on L2 devices (switches). Also there is the option to use the burned-in MAC address for HSRP instead of the "virtual"one. In that case the gratuitous ARP would also update the ARP tables on L3 devices/hosts.
A Gratuitous ARP is an ARP Response that was not prompted by an ARP Request. The Gratuitous ARP is sent as a broadcast frame, as a way for a node to announce or update its IP to MAC mapping to the entire network.
There are many use cases for Gratuitous ARP, all typically having to do with some sort of need to update the ARP Mapping or Switchport MAC address tables.
HSRP (and many other redundancy protocols) use Gratuitous ARP frequently. This animation will illustrate how:
Notice RouterA and RouterB are sharing the IP Address
10.0.0.1 and the MAC address
0053.ffff.1111. The Host's are configure with a default gateway of
10.0.0.1, and they each have an ARP mapping indicating the
10.0.0.1 IP address maps to the
0053.ffff.1111 MAC address. Since both the MAC address and IP address are shared between the two routers, this ARP Mapping will never have to be updated.
However, when RouterA is the active HSRP router, the MAC address is being served by the device connected to switchport 1. If RouterA fails, then RouterB sends a Gratuitous ARP to update the switch's MAC address table that the MAC address
0053.ffff.1111 is now being served by the device connected to switchport 2.
The process occurs again if RouterB fails, and RouterA needs to take back over serving the IP/MAC
While not strictly something you asked about, given the comments in this Q&A I thought it wise to mention something about duplicate address detection:
There is an often confused point regarding duplicate address detection and Gratuitous ARP. When a computer is trying to see if another host on the network is using a particular IP address it does not send a Gratuitous ARP.
The reason for this is by definition, a Gratuitous ARP is meant to update the ARP mappings of other devices on the network. If an IP is not unique, we wouldn't want the "test for uniqueness" to cause an incorrect ARP entry to be updated.
In reality, a specific set of ARP Messages are used in duplicate address detection: an ARP Probe and ARP Announcement.
The crucial difference between these two packets and an actual Gratuitous ARP, is the ARP Probe does not have a complete IP/MAC mapping, therefore it is impossible for an ARP Probe to accidentally erroneously update an ARP Cache.
The general process is the ARP Probe is sent a few times, which would prompt the owner of the IP in question to send a Gratuitous ARP if the IP is already in use. If no response is received, then the sender sends an ARP Announcement to officially claim the IP address on the network.
The ARP Announcement is even more similar to a Gratuitous ARP, but it (also) is not technically the same because the ARP Announcement has an opcode of
1, indicating a request (where as a Gratuitous ARP will have an opcode of
If you want some video there is a Great explanation here.