ARP question

bencmcbbencmcb Posts: 43Users Awaiting Email Confirmation ■■□□□□□□□□
When an ARP request broadcast is sent, do all the hosts on the LAN record the sender's IP and MAC address into their ARP tables even if the ARP broadcast request was not meant for them or do those hosts just discard the message without changing anything?

Comments

  • ImYourOnlyDJImYourOnlyDJ Posts: 180Member
    It is my understanding that a PC will ignore all ARP broadcasts unless they have the matching destination IP Address.
  • Jon_CiscoJon_Cisco Posts: 1,772Member ■■■■■■■■□□
    I do not believe the other machines will record the data. I don't recall it being expressed that way or any question about it during my study.
  • emazemaz Posts: 34Member ■■□□□□□□□□
    I agree with ImYourOnlyDJ, if the destination address is not that of the machine that receives it, it will drop it. The switch on the other hand will place the mac-address of the source in its mac table if it does not already have it.
  • markulousmarkulous Posts: 2,389Member ■■■■■■■■□□
    I believe it only caches ARP entries that it has responded to (or visa versa).
  • ImYourOnlyDJImYourOnlyDJ Posts: 180Member
    emaz wrote: »
    I agree with ImYourOnlyDJ, if the destination address is not that of the machine that receives it, it will drop it. The switch on the other hand will place the mac-address of the source in its mac table if it does not already have it.

    Thanks for agreeing with me :p. Just wanted to make to make sure the OP did not confuse the MAC Address Table with the ARP table which was his original question. A switch will grab MAC addresses for the MAC Address Table, but these will not be added to the ARP table unless the switch has responded to the request (eg management IP address).
  • fredrikjjfredrikjj Posts: 879Member
    bencmcb wrote: »
    When an ARP request broadcast is sent, do all the hosts on the LAN record the sender's IP and MAC address into their ARP tables even if the ARP broadcast request was not meant for them or do those hosts just discard the message without changing anything?

    Refreshes existing entries, but doesn't add new ones.

    I have some notes from Internetworking by Comer, and they say:

    When an ARP request is broadcast other machines use the source's binding to update their own cache.
    If for example A sends the request, machines with A in their cache will update the entry, but other
    machines will ignore it.

    Section 6.9 in the latest edition if you want to read the source.
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