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the network above shown has PC1(10.10.10.1) AND PC2(10.10.10.2) IN VLAN 10 10.10.10.0/24 AND PC3(20.20.20.3) and PC4(20.20.20.4) in VLAN 20 20.20.20.0/24 Network diagram

I have configured all the configuration of inter-VLAN routing. Sub-interfaces on router for VLAN10 and VLAN 20, switch-port to router as trunk and switch-port to to PC's as access but the only thing I have not set is the Default gateway of PC1 and PC 2 as router sub-interface 10.10.10.254 and for PC3 and PC4 as sub-interface 20.20.20.254 and I except that that PC1 should ping PC3 AND PC4 using Proxy ARP which is not working . WHY ? Since router knows both network(VLAN's)10.10.10.0/24 and 20.20.20.0/24 so it must give it's MAC address to PC1 to ping PC3 and PC4

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4 Answers 4

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Let me start by saying proxy ARP is at best a sloppy solution. They only time I found it useful as a feature is when I was dealing with devices on the network that could not utilize classless netmasks or couldn't set a default route.

Yes, it can "cover" many client configuration or bad design problems, but it doesn't fix those problems. It also doesn't "cover" all of them and it can make troubleshooting issues more difficult.

Getting back to your question, the most likely reason this isn't working is that your client's aren't ARPing. My guess is that you have given them what is often considered a "standard" network mask of /24. In your example, there is likely no solution to get proxy ARP to work as clients should not accept a network mask less than /8 so your client will never think the destination is on the local network and send out an ARP request.

Why? A client uses it's IP address and network mask to determine if a destination is on the local network or not. If it is on the local network, the client checks it's ARP table for any entries for the destination and if one doesn't exist, will send out an ARP request to get this information. This is where the router with proxy ARP enabled can respond, but if there is no ARP request then the router cannot provide a proxy ARP response.

If the destination is not on the local network, then the client will check it's routing table to see where to forward the traffic. This is typically your default gateway.

Now, with the IP addresses you used, when the client checked the destination against it's IP/mask, it would find the destination is not on the local network. Going to the routing table, it won't have a specific entry for the destination network (clients won't by default) and no default route/gateway. It will then fail with a "no route to host" type of message.

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  • Reposted the majority of the answer I posted to this similar question. While the questions are similar, the reason this isn't working is really the same.
    – YLearn
    Commented Sep 4, 2015 at 14:35
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after a lot of search i can figure out that , you can only make use of proxy arp if PC1 in your example can think that PC 3,4 are directly connected to it to start send ARP request to obtain there MAC address .
for example in your topology PC1 which is (10.10.10.1/24) can't think that PC 3,4 are directly connected to it because there ips are (20.20.20.3,4/24) but if you want pc 1 use proxy arp to send packets to pc 3,4 you must make PC1 ip 10.10.x.x/16 and make PC 3,4 something like 10.10.20.X/24 and make router sub interfaces in same IP range of each segment in this case pc1 can think pc3,4 are directly connected to it and start to send proxy ARP . enter image description here

or avoid all this headache may you use Default GW

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ARP should work if you want to ping an PC in the same VLAN, but, if you want to communicate with PC in other networks, you should set a default route to a gateway of that network, in this case, the subinterface that you have configured. Only in that way, you will be able to do what you want.

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  • The main advantage of proxy ARP is that it can be added to a single router on a network and does not disturb the routing tables of the other routers on the network. Proxy ARP must be used on the network where IP hosts are not configured with a default gateway or do not have any routing intelligence. [refer to ](cisco.com/c/en/us/support/docs/ip/…)
    – Gadeliow
    Commented Sep 4, 2015 at 14:03
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Until you set a route in your PCs they don't know what to do with the packets. Type "route print" in a command prompt of your windows PC. You will see a route to your local network but no route to anywhere else. Once you set the default route then the machine knows where to send packets.

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  • PLEASE Look at this link this is written learningnetwork.cisco.com/thread/60350 --->>>>>Proxy arp was a function used back whenever hosts didn't have a default gateway setting. As such, the router would actually assist in the forwarding process of frames. Instead of "ARPing" for the default gateway and going from there, the router would forward the ARP request across to the other network it attached.
    – Shivam Chauhan
    Commented Sep 4, 2015 at 12:47
  • So why the packets are not routed using proxy arp. Does proxy does not work now?
    – Shivam Chauhan
    Commented Sep 4, 2015 at 12:51
  • @Shivam Chauhan, proxy arp is a security risk. You shouldn't run it unless you absolutely need it.
    – Ron Maupin ♦
    Commented Sep 4, 2015 at 13:22
  • So we can conclude that proxy ARP and default gateway does the same thing but there way are different and also security measures as you mentioned
    – Shivam Chauhan
    Commented Sep 4, 2015 at 13:50
  • Proxy-arp still requires a route. It'll be a 0/0 route to an interface, effectively saying "The entire internet is on this link." The only difference today is that route doesn't exist by default -- long ago, machines only had one interface, so logically, there was only one place to send anything.
    – Ricky
    Commented Sep 4, 2015 at 20:46

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