The Gratuitous ARP is sent as a broadcast message to a node to announce or update its IP to MAC mapping to the entire network.
Check the below scenarios:-
Redundant IP Addresses
- In this topology, only IP address is redundant. Two devices are sharing a single IP address, although each device has their own MAC addresses.
ARP Mapping - 20.0.0.1 -------0063.ffff.aaaa
ARP Mapping - 20.0.0.1 -------0063.ffff.bbbb
- Two Routers (Router1 and Router2) are sharing the IP address 20.0.0.1, and hosts are using the shared IP address as their default gateway.
- When one of the routers went down, the other router will send Gratuitous ARP.
- Purpose of the Gratuitous ARP packet is to tell everyone on the network, the IP 20.0.0.1 is now being served by the other router’s MAC address.
- After receiving the Gratuitous ARP, all the hosts will update their ARP tables with the new mapping, so they will continue to send traffic to their default gateway IP address via new live Router.
- The process will be continue, if the opposite router will be down.
Redundant IP and MAC Addresses
- In this topology, both the IP address and the MAC address are redundant. Two devices are sharing both an IP address and a MAC address.
Switch Mac Address Table:
Port 2 -------0063.ffff.1111
Switch Mac Address Table:
Port 1 -------0063.ffff.1111
- Two Routers(Router1 and Router2), but this time they are sharing the IP address 20.0.0.1 and the MAC address 0063.ffff.1111.
- The hosts will be use the shared IP address as their default gateway, but in this example their ARP mapping will never be need to change, the shared IP address 20.0.0.1 will always map to the shared MAC address 0063.ffff.1111.
- The hosts’ ARP mapping never need to be update. Actually shared MAC address will be associated with the correct port, switches learn MAC address mappings from the Source MAC address of any received frame.
- If a router fails, then another router sends a Gratuitous ARP, after that switch will updates its MAC address table with the new location of the device which contain the shared MAC address.
- Frames sent by the hosts addressed to 0063.ffff.1111 will always egress the correct switch port.
Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.
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