5

Firstly what I understand, Default Gateway--> When we set the default gateway in PC the network layer check the destination IP address and if it is not on same broadcast domain as the source IP address it set destination MAC address to be of the router which further make routing decision.

Proxy ARP -->{{It is used when host do not have default gateway set}} When the destination IP address's MAC address is not in the table of PC it sent an ARP request to know its MAC address. THE switch broadcast the request and it also goes to router and the router knows the destination IP address as it is directly connected to it so it gives its MAC address as proxy.

Now my question is that MY network is like this-->> Host A---Switch1----Router----Switch2---Host B and Host A has IP address of 192.168.1.1 (no default gateway set) and same for host B with IP address 192.168.2.1(no default gateway set) so why can't Host A ping Host B using Proxy ARP.Router has interfaces as 192.168.1.254 and 192.168.2.254

IS proxy ARP not enabled by default but it is given in cisco.com it is enabled by default so why can not then

AND Also

which is given preference default gateway method or proxy ARP is default gateway is set and router knows path to destination

3

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. So the solution would be to switch the network masks on the clients to /16.

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, if you used a /24 network mask, 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.

  • Sir i found that when I enabled the proxy arp manually in interfaces of router by the command ip proxy-arp ping worked without default gateway set so is it given wrong in cisco.com proxy arp column that proxy arp is enabled by default or there are some changes. – Shivam Chauhan Sep 4 '15 at 19:03
  • @ShivamChauhan, YLearn is correct that proxy arp is a mess to deal with, and it is now a security risk, too. I believe it is no longer enabled by default because of these problems. You need to be careful of the documentation since Cisco does change this sort of thing all the time, and you need to make sure that the documentation you are looking at matches the IOS version that you are playing with, – Ron Maupin Sep 5 '15 at 4:28
  • Sir I checked the proxy-ARP with the command "" show ip interface fastethernet 0/0 "" and it showed Proxy ARP is enabled so why then the ping did not worked.. – Shivam Chauhan Sep 5 '15 at 17:49
  • @ShivamChauhan, are the IP addresses still the same as you listed in the question? What are the subnet masks? And is proxy ARP enabled on both router interfaces? Can you edit the router configuration into the question? – YLearn Sep 5 '15 at 18:06
  • The router interface towards HOST A is 192.168.1.254 and towards HOST B is 192.168.2.254 the subnet mask in both the networks is 255.255.255.0 and proxy-ARP is enabled on both router interfaces((I did not do it manually but it is enabled by default ((I checked it with command "" show ip interface fastethernet 0/0 "" towards HOST A and also towards fastethernet 0/1 towards HOST B and it showed ""Proxy arp is enabled"")))) – Shivam Chauhan Sep 5 '15 at 19:13
-2

Proxy ARP must be set on the router.

  • If proxy ARP is set then default gateway is not required . PLEASE look at the link at above comment – Shivam Chauhan Sep 4 '15 at 12:52
  • But you said that you has not set default gateway. Then the only way is to use Proxy ARP. My comment was about you shut SET ON on the router, Proxy ARP. – Orlando Gaetano Sep 4 '15 at 12:55
  • IT is written in Cisco.com website that proxy ARP is ON by default on all routers. – Shivam Chauhan Sep 4 '15 at 13:02
  • If you know the command to check Proxy ARP is on or not then please tell – Shivam Chauhan Sep 4 '15 at 13:02
  • On cisco devices, is enabled by default, and you can configured globally(i guess), or on interface level. You must check if, is not set, on this case, the interface must show "no ip proxy-arp". In this link you can check what i am telling you. cisco.com/c/en/us/support/docs/ip/… – Orlando Gaetano Sep 4 '15 at 13:05

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