What is proxy arp explain with scenario?

Take a close at the hosts you can see that PC1 has a /24 subnet mask & PC2 has a /8 subnet mask. When PC1 tries to reach the server at 10.2.2.100 the following will happen.

PC1> ping 10.2.2.200

·         PC1 perform AND operation to the destination (10.2.2.100) and decides that the server is in another subnet.

·         PC1 decides to send the packets towards its default gateway (10.1.1.254).

·         PC1 checks its ARP table to see if there is an entry for 10.1.1.254, if does not find & sends an ARP request.

·         The router responds to the ARP request which is sent by PC1, sending its MAC address of its interface fast-Ethernet 0/0.

This is how ARP works normally, when PC2 tries to send an IP packet towards the server something else will happen:

 PC2> ping 10.2.2.100

·         PC2 performs AND operation to the IP address of the server(10.2.2.100) and decides that the server is in the same subnet.

·         PC2 will send an ARP request if there is no entry for 10.2.2.100. PC2 does not send ARP request for the gateway.

The server however is not on the same subnet (10.1.1.0 /24) & router do not forward broadcast traffic. Hence, ARP request never make it to the server.

So, this is where proxy ARP comes to the rescue !

·         The router sees the ARP request from PC2 on the F 0/0 interface & sees that this is an ARP request for some device in the 10.2.2.0 /24 subnet on F1/1 interface.

·         Because directly connected route is present in the Routing table, then router realizes that it knows how to reach the 10.2.2.0 /24 subnet(Server 10.2.2.100) & decides to respond to the ARP request in order to help PC-2.

·         The router sends an ARP reply to PC2 with MAC address of the Fast-Ethernet 0/0 interface.

In this way, communication is successful between PC-2  & Server.

Hope this post will be helpful to you. Please ask questions for further clarification.