Why can I ping an APIPA when I'm not in that subnet?

[kylebrogers]

I have something odd going on.  My PC is in our data VLAN/subnet (let's say 10.10.10.0/24) and there is a server in our servers VLAN/Subnet (let's say 192.168.1.0/24).  A secondary NIC on the server (also on a port in the servers VLAN) does not have a static IP and there is no DHCP on that subnet so it gets your typical APIPA address (169.254.x.x).  At this point everything is normal.

But if I try and ping that APIPA address from my desktop, I get a response. The ARP table on the router shows an entry for that IP and the MAC of the unconfigured server NIC.  I've verified that my PC does not have a 2nd NIC and is not configured with a secondary IP in the APIPA range.  That wouldn't really matter anyway since we're on different VLANs.  

There is no proxy ARP configuration on the router/switch.  This also shouldn't matter since Windows should be going straight to its gateway because the APIPA address isn't in its own subnet.

Anybody have any idea how this is happening?  

 

[manish arora]

umm interesting, APIPA is link local and shouldn't be communicating besides its local subnet, are you sure that your Ping is going to the server ? Setup a wireshark or tcpdump and see if icmp sourced from your PC is reaching the server on that APIPA configured nic for sure.

Seeing the ARP in the router or switch should be of any concern because a machine generally uses arp to check for duplicate ip addresses even in case of APIPA address configuration.

Manish