Proxy arp question.

CATYO · ‎05-12-2016

Hello.

I have a question about proxy arp.

topology is like below.

PC1---------------|

SW--------------(proxy arp is enabled)R1

PC2---------------|

PC1 : 1.1.1.1/24

PC2 : 1.1.1.2/24

R1 : 1.1.1.254 (GW)

at the very beginning of communication, SW has no mac address table.

So if PC1 requests who is 1.1.1.2?

then SW will take that message and broadcast it.

finally, PC2 receives that request and reply it to PC1 like I am 1.1.1.2 and my mac address is XOXOXOXO

but very next time, R1 will also receive broadcast and its subnet is in R1's router table.

So R1 will send reply to PC1 directly, I am 1.1.1.2 and my mac address is CISCOCISCO

in this case, although R1 sends reply it to PC1, SW also take this packet and install the information its mac-address table.

the issue is after all of this done, PC1 send a packet to PC2 and it will drop.

Because packet will travel PC1 -> SW -> R1 and drop.

I'd like to know my explain is correct and why R1 drop the packet? R1 may think this is same subnet so i don't care??

how to resolve this issue? disable proxy arp is best option for this?

Thank you

Best regards,

Joseph W. Doherty · ‎05-12-2016

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Posting

If R1 is 1.1.1.254/24, it shouldn't proxy between hosts on the same network.

Carlos Villagran · ‎05-12-2016

Hi!

Please refer to RFC 1027 in the following link:

https://tools.ietf.org/html/rfc1027

    If the network interfaces on which the request was received and
    through which the route to the target passes are the same, the
    gateway must not reply.  In this case, either the target host is on
    the same physical network as the gateway (and thus the host should
    reply for itself), or this gateway is not on the most direct path to
    the desired network, i.e., there is another gateway on the same
    physical network that is on a more direct path and the other gateway
    should respond.

Hope it helps, best regards!

JC