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Why ARP need ARP cross LAN request?

When I am reading a document:

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You see the host A(10.1.0.1/8) send ARP Request for 20.2.0.1/8's MAC address. In general, the ARP request is for MAC address in the same LAN. why there need the cross LAN's ARP request? further on will need the ARP proxy. because I think the host A send to host B data, it can through the Router Forwarding.


What's the 10.1.0.1/8 tries to ARP 20.1.0.1's benefits?

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It would be an unusual situation for the host A to ARP for B, because B is on a different network (A is on 10.0.0.0/8 and B is on 20.0.0.0/8).

Nevertheless if A's operating system permits it, A might ARP for B. The router R could be configured to answer the "who has 20.2.0.1?" ARP query with its own ethernet address (that of G0/0/0, the one on the same network as A). In this situation, we say that R is a proxy for B.

Read RFC 1027 "Using ARP to Implement Transparent Subnet Gateways" for details.

It is much, much, more common for A to have a route to R for all traffic not on its own network 10.0.0.0/8. In which case A will ARP for 10.1.0.2, to get the ethernet address of R, and then send B's traffic to R.

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You see the host A(10.1.0.1/8) send ARP Request for 20.2.0.1/8's MAC address.

This is not normal.

Normally, host A considers 10.0.0.0/8 as its local subnet, so 20.2.0.1 is remote. This makes the host query its local routing table for an appropriate gateway entry. Assuming there's a default gateway 0.0.0.0/0 -> 10.1.0.2, it would then ARP the gateway's IP and use the discovered MAC address for encapsulating the IP packet. The router would receive the encapsulating frame, de-encapsulate the packet and forward it onwards.

If 10.1.0.1/8 tries to ARP 20.1.0.1 there's likely a weird routing entry in its local table or its network mask isn't set correctly. Without any special configuration on the router ("ARP proxy") the ARP request would simply be ignored.

Additionally, a /8 mask for an operational subnet is nonsense. /8 prefixes may make sense for corporate HQ "default" routes but an actual subnet with 16 million addresses is nuts.

  • but what's the 10.1.0.1/8 tries to ARP 20.1.0.1's benefits? – aircraft Sep 18 '18 at 10:58
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    There's no benefit. Something is wrong there. – Zac67 Sep 18 '18 at 11:55
  • so, through router ARP proxy to get other LAN's host's MAC address is meaningless. – aircraft Sep 18 '18 at 11:57
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    Not exactly. If there's no other way you can get host A to communicate with host B on another subnet you could set up proxy ARP (to resolve to the gateway's MAC and make the router transparent) but that's an ugly kludge. The normal way is to route. Host B's actual MAC address on another segment is meaningless, yes. – Zac67 Sep 18 '18 at 12:02
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    I agree entirely with all of Zac's comments; only adding that I have seen industrial equipment and older operating systems which, unfortunately, actually do ARP for everything, local or not. "Ugly kludge" is a polite way of putting it. – jonathanjo Sep 18 '18 at 12:19

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