Simple arp question

CiscoCertsCiscoCerts Member Posts: 112
in CCNA & CCENT
when arp is used to find the mac address of a host its sent as a broadcast.

Does then only the matching ip holder send a reply? Or does everybody send a reply updating the router with all of the mac addresses on the LAN?
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  • CiscoCertsCiscoCerts Member Posts: 112
    One more question..
    PC A pings PC B, What are the arp table entries?

    PC A 192.168.0.5

    R1 fa0/0 192.168.0.1
    R1 s0/0 10.0.0.1

    R2 s0/0 10.0.0.2
    R2 fa0/0 172.16.0.1

    PC B 172.16.0.5

    Arp Tables
    PC A
    R1 Mac

    R1
    PC A Mac?? (the book doesn't list it but i think it's so)

    R2
    PC B Mac

    PC B
    R2 Mac (not mentioned, but again I think this is right)



    I appreciate your responses thanks!
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  • gojericho0gojericho0 Member Posts: 1,059 ■■■□□□□□□□
    Only the host matching the ip address of the ARP request. The host that sent the arp request will the cache the MAC to IP mapping for a variable time period. The routers mac address table will update an entry only if the source MAC address is not in the address table
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  • gojericho0gojericho0 Member Posts: 1,059 ■■■□□□□□□□
    R1
    PC A Mac?? (the book doesn't list it but i think it's so)

    This is correct during the reply. It is not entered when PC A first send the request because R1 does not need this information. R1 will need to encapsulate the data with whatever L2 protocol the serial link is using to get to R2
    PC B
    R2 Mac (not mentioned, but again I think this is right)

    Yup that is right because PC B will need to send the reply through R2
    [/quote]
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  • CiscoCertsCiscoCerts Member Posts: 112
    gojericho0 wrote:
    Only the host matching the ip address of the ARP request. The host that sent the arp request will the cache the MAC to IP mapping for a variable time period. The routers mac address table will update an entry only if the source MAC address is not in the address table
    Ah ha, thank you
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  • CiscoCertsCiscoCerts Member Posts: 112
    gojericho0 wrote:
    R1
    PC A Mac?? (the book doesn't list it but i think it's so)

    This is correct during the reply. It is not entered when PC A first send the request because R1 does not need this information. R1 will need to encapsulate the data with whatever L2 protocol the serial link is using to get to R2
    PC B
    R2 Mac (not mentioned, but again I think this is right)

    Yup that is right because PC B will need to send the reply through R2
    [/quote]

    Thank you very much sir :)
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  • tech-airmantech-airman Member Posts: 953
    CiscoCerts wrote:
    when arp is used to find the mac address of a host its sent as a broadcast.

    Does then only the matching ip holder send a reply? Or does everybody send a reply updating the router with all of the mac addresses on the LAN?

    CiscoCerts,

    It depends. Are we talking about an ARP within a single collision domain?
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  • CiscoCertsCiscoCerts Member Posts: 112
    tech-airman wrote:
    CiscoCerts wrote:
    when arp is used to find the mac address of a host its sent as a broadcast.

    Does then only the matching ip holder send a reply? Or does everybody send a reply updating the router with all of the mac addresses on the LAN?

    CiscoCerts,

    It depends. Are we talking about an ARP within a single collision domain?

    Single collision domain / single broadcast domain. Yes. So what if it wasn't, and we're talking about a hub how does that change things?
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  • tech-airmantech-airman Member Posts: 953
    CiscoCerts wrote:
    tech-airman wrote:
    CiscoCerts wrote:
    when arp is used to find the mac address of a host its sent as a broadcast.

    Does then only the matching ip holder send a reply? Or does everybody send a reply updating the router with all of the mac addresses on the LAN?

    CiscoCerts,

    It depends. Are we talking about an ARP within a single collision domain?

    Single collision domain / single broadcast domain. Yes. So what if it wasn't, and we're talking about a hub how does that change things?

    CiscoCerts,

    If the two hosts are separated by a hub, then it's basically as if the two hosts were connected with a crossover cable since a hub is a Layer 1 device. If the two hosts are separated by a switch, then the network layer broadcast would be "heard" by all other hosts logically connected to that switch, assuming the destination MAC address isn't in the CAM table yet. Now, if the two hosts are separated by a router, then host 1 will NEVER receive the actual MAC address of host 2 but instead will receive the MAC address of the router interface that host 1 is connected to. That's because by definition, routers block network layer broadcasts.
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  • CiscoCertsCiscoCerts Member Posts: 112
    I think you misunderstood. Let me try to clarify

    These two hosts were and are still seperated by routers

    Small diagram based on the first question:

    host A----hub//switch----router A
    router B
    host B



    My question was asking what's the difference in this scenario in conjunction with arp, because you said that it was different. for the sake this topic lets add in two more hosts:

    Host D / host A----hub//switch----router A
    router B
    hub/switch
    host B /host C


    At any point are host D and host C's arp tables containing host A or B's Mac addresses?
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