MTU question

[bma]

Hi

In the NT workstation or windows 98, if do

ping -f -l 1260 x.x.x.x (x.x.x.x is any inside machines or internet site ip address), I get reply.

but I do ping -f -l 1280 x.x.x.x, I get packets needs to be fragment but DF bit set.

In the NT servers, if do samething,

ping -f -l 1460 x.x.x.x, get response.

ping -f -l 1480 x.x.x.x, get packets needs to be fragment but DF bit set.

Is it right, or something limit in the windows workstation or windows 98? What is for the largest packets size should setup in the lan?

Thanks

ben

[rwcrowe]

I assume all the boxes are connected via ethernet. Are the NT servers, workstations and 98 boxes on the same subnet ? If not then you need to check the MTU settings on any routers in between them. If there are any packet filtering devices between them, like firewalls, make sure that Path MTU is not disabled. I dont remember the ICMP type or code right off the top of my head.

The NT server behavior seems fine. Ethernet MTU is 1500 bytes. Subtract 40 for the ethernet header, etc, and you get 1460.

http://www.cisco.com/warp/public/105/38.shtml

[bma]

Yes, all boxes are connected via ehternet

I get same result in the same subnet or different subnet. All subnet (or vlans) links with Cisco 6509 as route, no firewall between subnet. Do you think we need enable Path MTU in the Cisco 6509?

Thanks

ben

[Harold Ritter]

Ben,

Enabling PMTUD on the 6500 won't help since it only enables path mtu discovery for packets originated on the 6500 itself (IE: telnet or BGP session initiated from the 6500) not IP packets going through it.

As for your original issue, I suspect the MTU size of your NIC card on the workstations might have been altered as it is often the case when you install a VPN software.

Hope this helps,

[bma]

Thanks,the vpn software made the issue.

ben