Routes with greater prefix and Proxy ARP ~= IP Mobility

Inside an Autonomous System, it is possible to move a machine inside a network, keeping its IP address even though it goes to a network segment that doesn’t serve the corresponding Network.

Something like this:

RouterA -------- Network Segment
         Host A (, GW:

It is possible to move Host A to another network segment, lets say to interface FastEthernet of RouterB. Of course this would require address and possibly other configuration changes to Host A. Changin the IP address of a server is not always a good idea ™.

Lets say that we move Host A to interface FastEthernet of Router B. Supposing that a routing protocol is setup and works in the Autonomous System, Host A may keep its IP address by configuring RouterB (cisco commands):

RouterB(config)# ip route
   FastEthernet 0/1

RouterB(config)# interface FastEthernet 0/1
RouterB(config-if)# ip proxy-arp      # This is the default

RouterB(config)# router eigrp 1
RouterB(config-router)# redistribute static

RouterA(config)# interface FastEthernet 0/1
RouterA(config-if)# ip proxy-arp

This will work because:

  • Whenever Host A tries to reach a host in its subnet (ARP request), Router B will respond with its mac address. This is what Proxy ARP does.
  • All routers within the A.S. will learn the route. Even Router A will prefer to use this one instead of the directly connected since it has a longest preffix (routing table lookups are longest matching preffix lookups)
  • Since Router A will also learn this route and since it has Proxy ARP enabled, it will respond to ARP requests for with its address