MX - Do MX care about DHCP option 26 ?

[RaphaelL]

Hi ,

I'm trying to test DHCP option 26. 

I have 1 DHCP device ( another MX ) configured with DHCP option 26 with a value of 1280.

My second MX ( the DHCP client ) sends a DHCP Discover ( D-O-R-A ) and requests in DHCP option 55 ( PRL ) the option 26. 

The DHCP server responds ( D-O-R-A ) with the option 26 , but the MX ends up ignoring that value. The MTU stays unchanged. I'm wondering if that has ever worked or simply bugged in MX18.107.x

[GIdenJoe]

Just out of curiosity, are you running DF-bit pingtests to determine the upstream MTU or is there a way to see it on the MX itself what MTU it is using?

[RaphaelL]

I have a packet capture running on the "WAN" side of the MX and I can see the MX sourcing packets > 1400 bytes. It "should" be using my option 26 which is set to 1280. 

[RaphaelL]

Edit : 

I tried a more common MTU value ( 1492 = PPPoE ) and still the MX won't honor it : 

Might have to try with a different firmware version ( running 18.107.4 and 18.107.5 )

[RaphaelL]

I was a bit wrong.  The MX will honor the value but it won't perform MSS clamping , which I'm trying to confirm with Support if that's the expected behavior.