Unexpected PING responce

bapatsubodh · ‎11-17-2009

hi,

Most of the times we are struck in the problem that we are not able to ping the device but in this weird case I am getting ping responce which is not expected. In our network of L2 switches and only one L3 switch which acts as default gateway for all VLAN. We have one device (not PC or any network device) where we have configured only the ip addres but there is no way to configure subnet mask or default gateway. I was expecting that it will not be able to communicate with devices from any other VLAN as there is no DG to this device. But to my surprise I was aple to ping to this device from other VLANS also. This remains a mystery to me. Some L2 or L3 swicth is forwarding the frames and the some how it is getting a ping responce.

Please share the experience.

Any link on cisco.com is highly appreciable.

Thanks in advance.

Subodh

Peter Paluch · ‎11-17-2009

Subodh,

You are probably experiencing the ProxyARP in action. The device will ask for MAC address of every IP it talks to using ARP. The L3 switch does not own the IP address in question, however, according to its routing table, it known the route to it, therefore it responds using its own MAC address. As a result, the device sends its IP packet to the L3 switch, exactly as if the default gateway was correctly configured.

There are various disadvantages to the ProxyARP, notably a big ARP traffic and a large ARP cache on the device that relies on the ProxyARP function, and usually, it is better turned off. However, in your case, it is probably the only way to ensure that your device is able to talk to other networks.

Read here for further information:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml

Best regards,

Peter