Default Gateway Configuration

[shajisiva]

[Kasun Bandara]

Hi,

are you using any simulation or actual routers? simulations can behave different way than actual OS. in PKT tracer your scenario not happening and working perfect way. end routers need #ip default-gateway command to route to other networks

[shajisiva]

I am using 3 physical routers. (1841 Series )

[Georg Pauwen · Accepted solution]

Hello,

in the second scenario, the top/middle router probably has proxy ARP enabled. With that enabled, hosts do not need to be on the same subnet in order to communicate.

What is IP proxy ARP?
Proxy ARP is a technique by which a proxy server on a given network answers the Address Resolution Protocol (ARP) queries for an IP address that is not on that network. The proxy is aware of the location of the traffic's destination and offers its own MAC address as the (ostensibly final) destination.

[shajisiva]

Hello 

Thank you for the reply 

You are right - When I disabled ARP Proxy , the routers wouldn't able to reach without default gateway. 

For further clarification - 

Question 1 :

Which server could have been acted as a proxy server in this environment ? I guess it is Router 1 because when I gave no ip proxy arp on R-A alone ; I still able to ping R-B without default gateway .

If it is Router A , why PC's cant able to communicate without Proxy ARP ?

Question 2 :

In second scenarion while sending the first ICMP packet from R-A ; the first thing it will do is perform the AND operation to identify whether the target IP belongs to different network . Since it is a different network R-A will look for default gateway . As we haven't configured default gateway the packet must be dropped there it self. Why the packet didnt drop and went for ARP resolution ? (ie, why the packet is passed to L2 even if it didn't find a default gateway address ? )