Mikrotik L2TP and Protocol Field Compression (PFC)

RouterOS General
1

Hello,

I am using a Cisco and Mikrotik router in a LAC/LNS configuration.

Despite the L2TP tunnel initialising, traffic over the tunnel fails. With l2tp logging enabled on the Mikrotik, I saw the line “received unsupported protocol 0x2145”

Ive gone into more detail on my blog but it seems like the Mikrotik doesn’t support Protocol Field Compression and is attempting to read the field as 2 bytes (0x2145) instead of 1 (0x21) and subsequently rejecting the frame.

Is anyone familiar with this issue? Is it possible to enable PFC, or ensure it’s not negotiated? There is a thread on the linux-kernal mailing list suggesting some changes were made in 2018. Have these been applied to newer 6.X versions of RouterOS?

I can confirm its working in RouterOS 7 :smiley: but fixing it in 6 would be ideal.

As a work around I am disabling PFC negotation on the Cisco router:

interface Group-Async0
  ppp pfc local forbid

Thanks

2

Is anyone else running an L2TP tunnel with a Cisco router as the client and a Mikrotik as the server?

Thanks

3

For anyone wondering I’ve had a ticket open with support but no response in almost 2 months. (SUP-69140)

4

Can’t speak to the lack of Mikrotik’s response… It may be the needed support for PFC isn’t in the older kernel used by V6. So if I was a betting man, you may never see protocol field compression with ppp in V6, especially if it works in V7 - but who knows.