Change MTU size on HP PageWide - SMTP Failures - Inconsistent behavior

[ruh8n2]

This is a follow up to another article that is locked

https://h30434.www3.hp.com/t5/Printer-Setup-Software-Drivers/Change-MTU-size-on-HP-PageWide/m-p/6723...

My Printers are pagewide pros, but id bet this issue exists on a majority of HPs that have EWS and Scan-to-Email functionality, and specific to the conditions where the traffic has fragmentation (over GRE/IPsec tunnels)

Figured I would provide an update on my situation relating to MTU, and SMTP inconsistent behaviors between the admin web page and the scan-to-email feature.  If I had to guess, the code specifically sets Do Not Fragment when using scan-to-email.  Maybe they had a reason for it, but I cannot think of one. 

So in my case, I have a couple remote sites, they're configured to use the smtp relay in our main office.  We setup the device all remotely via the hp web services page.  SMTP traffic goes over an IPsec tunnel.  When we set it up, the test page goes thru without issue. When users go to use the printer to scan to email, they get a "Network Connection error. Check Network and try again"

A packet capture shows fragmentation. 

The solution would be to adjust the mtu in the tunnel / tunnel interfaces on the firewall and retry.

Bonus info

You can find the mtu needed to set by doing ping tests from the side where the printer is, to your smtp relay. 

ping <ip> -f -l <size length>. 

You should start at 1492 ( 8 - 1500) and work backwards till the ping no longer says "Packet needs to be fragmented but df set" and ping responds.  

In my case it was 1394 

From there you add 28 bits to that number to get the optimal MTU.

1394+28 = 1422 Optimal MTU

Thats what you're supposed to set as the MTU on the tunnel interfaces and that should resolve the problem.

You could also do the MSS calculation based off your tunnel settings.