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.

I have been informed by certain networking professionals that ARP replies can and sometimes actually are broadcast packets instead of unicast. If and when would you use a broadcast ARP reply?

share|improve this question
add comment

2 Answers 2

up vote 3 down vote accepted

According to the RFC826 (An Ethernet Address Resolution Protocol) the sender has to:

put the local hardware and protocol addresses in the sender fields

But as you pointed out why an host would use the broadcast address in an ARP reply? Maybe because this could seem clever: using the L2 broadcast address all the hosts in the LAN would immediately know a new (IP,MAC) pair saving time. Actually this method is not desired by the RFC (even if they refer to periodic broadcasting):

Periodic broadcasting is definitely not desired. Imagine 100 workstations on a single Ethernet, each broadcasting address resolution information once per 10 minutes (as one possible set of parameters). This is one packet every 6 seconds. This is almost reasonable, but what use is it? The workstations aren't generally going to be talking to each other (and therefore have 100 useless entries in a table);

It also depends on the host's OS to add in the ARP table a tuple learned by a broadcasted message.

EDIT: As stated in the comments the RFC 5227 updates the RFC 826 and it says

In some applications of IPv4 Address Conflict Detection (ACD), it may be advantageous to deliver ARP Replies using broadcast instead of unicast because this allows address conflicts to be detected sooner than might otherwise happen

share|improve this answer
3  
RFC 5227 updates 826, and it says "In some applications of IPv4 Address Conflict Detection (ACD), it may be advantageous to deliver ARP Replies using broadcast instead of unicast because this allows address conflicts to be detected sooner than might otherwise happen." –  Gerben Apr 1 at 9:16
add comment

One of the uses of broadcast ARP is the gratuitous ARP replies sent by some clustering or link aggregation (i.e. teaming) solutions. This is the process they will typically use when they want to move traffic from one interface to another.

This process will update all hosts on the local subnet (including the gateway) as well as causing the L2 switches to update their MAC address tables. The result is that all the traffic should switch over to the new interface smoothly and quickly.

share|improve this answer
add comment

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.