ARP

[dj214]

Hello All:

Posted this a week or so ago but still no resolution. Have segmented our LAN 10.0.1.x and 10.0.2.x. Cannot FTP to server in other subnet. Times out. If I ping it first with a single packet, it can ftp no problem. I am assuming the problem is ARP and the ICMP request and reply satisfy the IP to MAC resolution.

Arp cache on router is OK and entries correct. What I did see through packet capture that when I intiate the FTP connection....the client box makes several ARP requests to a non-existent IP. It then looks to the router but the IP on the router is listed as an external IP that resolved to another domain. Really strange. The interfaces FA0/0 is configured as 10.0.2.1 and FA0/1 as 10.0.1.1. Access lists configured etc.

What am I missing?? would it help if I posted the running-config to look at??

Thanks

[tbaranski]

What is the IP that the client ARPs for when an FTP session is initiated? Does this IP exist on the network? And can you clarify what you mean by the "It then looks to the router but..." sentence? If it's possible to post the packet captures, that may help.

[dj214]

It first sends several ftp requests to connect to port 21 on remote machine. After not receiving response, acknowledgement, it arps to router (fa0/0 which is configured on this subnet as 10.0.2.1) and the only other machine on this subnet.

Does this clear up your questions?

As mentioned earlier, if I ping the remote machine 1 time, viola, connecction possbile.

[tbaranski]

What MAC address is the client using when it sends the FTP requests? Is it 10.0.2.1's MAC address or something else? What happens if you clear the ARP cache right before iniating the FTP session? Are you positive that the subnet masks of both the PCs as well as the router's interfaces are correct?

This doesn't at all make sense, so all I can really do is ask questions at this point and hope that one of the answers makes the problem apparent.