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Consider we are using 1.1.1.1 as our VIP which is the IP address of the load balancer. I'm aware that I should disable ARP replies on loop back interfaces of back end servers. However another issue confuses me with this topology:

  1. Router sends ARP request to find MAC address of 1.1.1.1
  2. Load balancer replies back with its own mac address. (Lets say AA:AA:AA:AA:AA:AA)
  3. Router caches this information (mapping of 1.1.1.1 to AA:AA:AA:AA:AA:AA)
  4. Router forwards packet to load balancer
  5. Load balancer changes destination MAC address from AA:AA:AA:AA:AA:AA to MAC address of back end server which is BB:BB:BB:BB:BB:BB.
  6. Back end server replies back to the client directly which means it sends reply packet with source IP of 1.1.1.1 (VIP) and source MAC address of BB:BB:BB:BB:BB:BB (This is the MAC address of back end server)
  7. Packet arrives at router. Router will notice that 1.1.1.1 should have MAC address of AA:AA:AA:AA:AA:AA (MAC address of load balancer) but now current packet says 1.1.1.1 has a MAC address of BB:BB:BB:BB:BB:BB which is conflicting with router ARP cache)

Now the question is what is the behavior of the router in this scenario?

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  • The packet doesn't contain the MAC address. The router will attach the destination MAC address based on what is in the ARP table. If the packet arrives at the router for IP 1.1.1.1, the router will place the ethernet header with destination MAC AA:AA:AA:AA:AA:AA from its ARP table.
    – John K.
    Jan 24, 2017 at 13:20
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    The question is, when the response comes back from 1.1.1.1 with a different MAC, will the router update its ARP table? It think the answer is it depends on the hardware. A router may act differently than a L3 switch. I'll have to lab it up.
    – Ron Trunk
    Jan 24, 2017 at 13:37
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 15, 2017 at 19:12

1 Answer 1

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ARP table entries are only updated by ARP packets, not by IP packets.

So the reply packet has no effect on the router's ARP table.

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  • Is this behavior kind of standard behavior? Or it is just how most routers function...
    – Majid Azimi
    Jan 24, 2017 at 13:42

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