Take the 2-minute tour ×
Network Engineering Stack Exchange is a question and answer site for network engineers. It's 100% free, no registration required.

Does gratuitous ARP work like a normal ARP request?

Why is gratuitous ARP used for HSRP?

share|improve this question
3  
Did you read up on this somewhere? If you didn't, you should. If you did and something isn't clear, please be more specific in your question. –  Gerben May 2 at 7:07
    
@Gerben i'm clear about how arp works, but this concept is my confusing part especially in HSRP how this ARP works.. –  Trojan May 2 at 7:46

1 Answer 1

up vote 9 down vote accepted

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"advance notification", it updates the ARP cache of other systems before they ask for it (no ARP request) or to update outdated information.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:

  • Both source and destination IP in the packet are the IP of the host issuing the gratuitous ARP
  • The destination MAC address is the broadcast MAC address (ff:ff:ff:ff:ff:ff)
    • This means the packet will be flooded to all ports on a switch
  • No reply is expected

Gratuitous ARP is used for some reasons:

  • Update ARP tables after a MAC address for an IP changes (failover, new NIC, etc.)
  • Update MAC address tables on L2 devices (switches) that a MAC address is now on a different port
  • Send gratuitous ARP when interface goes up to notify other hosts about new MAC/IP bindings in advance so that they don't have to use ARP requests to find out
  • When a reply to a gratuitous ARP request is received you know that you have an IP address conflict in your network

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.

share|improve this answer
    
Thanks a lot man... –  Trojan May 3 at 18:26
    
How to view the Gratuitous ARP packets ? –  Trojan May 3 at 18:31
    
Use a network sniffer like Wireshark –  Sebastian May 4 at 12:02
    
is there any wireshark captures for gratuitous ARP.. –  Trojan May 4 at 13:13
1  
You can see an example packet on the Wireshark Homepage –  Sebastian May 4 at 18:27

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.