Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Catalyst 3750 Layer 3 Switching question

I am wondering if someone can help me with a strange layer 3 switching situation on my Catalyst 3750. It is my core switch and is configured with 2 Vlans - 1 server vlan and 1 workstation vlan. The workstations dhcp from a dhcp server in the server vlan. Occasionally a PC will boot up, get an IP address from the DHCP server, but not be able to communicate with the server VLAN. I look at ipconfig on the workstation and everything looks fine - subnet mask/default gw are both correct. From the 3750 I can ping the workstation successfully, but from a server pings are unsuccessful. I performed a network capture at the workstation side and found that when the workstation in question does an unsuccessful ping to a server, the ping reply from the server is actually seen in the capture, even though the PC doesn't get a successful ping reply in Windows. Upon looking at the ping reply a bit closer, it is being sent to the wrong MAC address. I finally stumbled on to the fact that occasionally workstations have IP conflicts when they boot up. When I look at the capture, sure enough, the 3750 is sending the ping replies to the MAC address of the workstation that had a conflict. If I do a clear arp on the 3750, pings start to go through again and the PC can communicate with the server VLAN. Now obviously IP conflicts are not good and that needs to get cleared up, but I'm very curious as to how the 3750 can successfully ping the PC with the right MAC address, but when the 3750 needs to route a ping request from a server to the workstation, it sends the wrong mac address. It's like the arp table is different for traffic originated from the switch and traffic routed through the switch.

Any ideas? Is there a "show" command other than show arp that I can look at to see what is going on b/c when I do a show arp when things are broke, the 3750 has the right mac address for the workstation.

Thanks in advance,



Re: Catalyst 3750 Layer 3 Switching question

Just curious,

what kind of switching is used in this 3750?

cef? fast-switching? process-switching?

Let me know,


New Member

Re: Catalyst 3750 Layer 3 Switching question

I wish I could tell you but I can't seem to find out. I can tell you that it is using the default switching method. My guess is that is process-switching. Is there a way for me to tell?




Re: Catalyst 3750 Layer 3 Switching question


Try the following command:

sh ip interface vlan


New Member

Re: Catalyst 3750 Layer 3 Switching question

Thanks for the command, that was definitely it. Here is a snippet of one of my vlan interfaces:

IP fast switching is enabled

IP fast switching on the same interface is disabled

IP Flow switching is disabled

IP CEF switching is enabled

IP CEF Fast switching turbo vector

IP multicast fast switching is enabled

IP multicast distributed fast switching is disabled

IP route-cache flags are Fast, CEF

CreatePlease to create content