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  1. Senior Member
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    #1

    Default ARP question regarding routers

    Here's a simple network diagram: Host1 - SwitchA - RouterA - RouterB - RouterC - SwitchB - Host2

    So when Host1 wants to communicate with Host2, Host1 sees that Host 2 isn't on its network so it sends an ARP to find the mac address of the default gateway which is RouterA. I get that. ARP is at layer 2. The routers are all consulting their routing tables which don't care about mac addresses. The next ARPing will take place when the packet arrives at RouterC and it looks at the Destination Address in the packet and then will ARP to find the mac address so the frame can be rebuilt with Host2's mac so the frame can be sent to Host2 by SwitchB.

    Am I thinking about this correctly or do the Routers need to ARP the mac address of the next hop interface to rebuild the frame? My google skills are very weak right now and I can't seem to find an answer. A video I've seen suggested ARP between routers but that doesn't make sense to me.

    Thanks much for listening to my ramblings. It has been a long week.
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  3. Senior Member dontstop's Avatar
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    #2
    The routers are all consulting their routing tables which don't care about mac addresses
    Regardless of what protocols are used in between A, B, C we still need to rebuild the layer 2 headers which are stripped off at ingress (input) of RouterA and rewritten at egress (output) of RouterA. Be it Ethernet/HDLC/PPP/Frame Relay we will rebuild those headers whatever way makes sense. If RouterB is on a P2P link then we can just use the next hop egress interface. If it's on a shared segment Ethernet then we would as normal need to ARP for the next hop IP address of the RouterB.

    HTH
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  4. Senior Member
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    #3
    Thanks for that dontstop. Yes the frame is rebuilt regardless but it won't ARP unless it is Ethernet between the two routers? My main confusion is when interfaces are ARPing.
    Last edited by AvgITGeek; 09-15-2018 at 11:07 PM.
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  5. Senior Member
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    #4
    do the Routers need to ARP the mac address of the next hop interface to rebuild the frame?

    yes

    like the host that looks up the mac in it's mac table and if it doesn't have it, it will do an arp for the gateway's mac
    a router maintains information bases with the mac address of the next hop, if it has it, it will use it. if it doesn't have it, it will arp for it.
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  6. Junior Member Registered Member
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    #5
    ARP is used to retrieve the MAC address of the destination host. At layer 3 it knows the destination IP but when it has to add the L2 header it does not know the mac address of the destination host. So to get the mac address it sends an ARP packet. After getting the response it will update its ARP tables for future lookups.
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