PPPoE and MTU > 1488

MrYan · Fri Sep 23, 2016 2:16 pm

Upgraded to 6.37 this morning and noticed that even with max-mtu=1500 on the pppoe-client interface that the MTU changes to 1480 after between 3-5 seconds (seen using monitor command on interface).

Thought it was a problem with 6.37 so downgraded back to 6.36.3 but that exhibited the same problem. This has worked in the past so I'm not sure what has changed to break it.

One thing I did notice is that if I set max-mtu to 1488 or lower it doesn't change MTU after 3-5 seconds, but if set to 1489 it changes.
A packet trace shows that if the max-mtu is set to > 1488 then a large LCP echo request is sent 3 times. The BRAS I'm connected to seems to return a malformed packet and I guess this is what caused the Mikrotik to fallback to a 1480 byte MTU. If max-mtu is set to 1488 bytes or less this large LCP echo request is not sent by the Mikrotik device.

I see on the forums that others have had problems in the past with 1488 byte MTUs but this seems to have been fixed in subsequent versions of ROS.

A Google search turns up instances where people say that the MTU should be setup to 1488 if you are doing PPPoE over DSL to fit completely within a round number of ATM cells. Not sure if this is why the Mikrotik PPPoE client does this or not.

Anyone have any other insight?

rodolfo · Sat Sep 24, 2016 1:19 pm

The Mtu seems auto-adjusted by the discovery mechanism.
If interfaces or tunnels You use does not allow 1500, You must set MRRU.
Mikrotik say to leave unchanged mtu and mru to defaults
See also http://forum.mikrotik.com/viewtopic.php?t=96276

pe1chl · Sat Sep 24, 2016 2:01 pm

This large LCP request is RFC4638 compliant. It is sent to validate that your link is indeed capable of
transferring the large (larger than ethernet standard) packet. Make sure your BRAS supports RFC4638
and that your ethernet MTU is large enough along the path between router and BRAS.
(so on your ethernet interface on the MikroTik, on your DSL modem, etc)

MrYan · Sat Sep 24, 2016 2:56 pm

The interface MTU on the Mikrotik and DSL modem (Vigor 130) is set to 1508 bytes so the path to the BRAS should be capable of sending > 1492 byte packets.

I see in the PPPoE discovery that the Mikrotik advertises PPP-Max-Payload for RFC 4638 negotiation (05dc == 1500):

Frame 1: 38 bytes on wire (304 bits), 38 bytes captured (304 bits)
Ethernet II, Src: Routerbo_8d:70:a6 (d4:ca:6d:8d:70:a6), Dst: Broadcast (ff:ff:ff:ff:ff:ff)
PPP-over-Ethernet Discovery
    0001 .... = Version: 1
    .... 0001 = Type: 1
    Code: Active Discovery Initiation (PADI) (0x09)
    Session ID: 0x0000
    Payload Length: 18
    PPPoE Tags
        Host-Uniq: 016a0001
        PPP-Max-Payload: 05dc

And the BRAS (BT Alcatel) negotiates a 1500 byte MRU (the Mikrotik ACKs this):

Frame 5: 60 bytes on wire (480 bits), 60 bytes captured (480 bits)
Ethernet II, Src: Alcatel-_c0:36:b4 (0c:a4:02:c0:36:b4), Dst: Routerbo_8d:70:a6 (d4:ca:6d:8d:70:a6)
PPP-over-Ethernet Session
Point-to-Point Protocol
PPP Link Control Protocol
    Code: Configuration Request (1)
    Identifier: 124 (0x7c)
    Length: 19
    Options: (15 bytes), Maximum Receive Unit, Authentication Protocol, Magic Number
        Maximum Receive Unit: 1500
        Authentication Protocol: Challenge Handshake Authentication Protocol (0xc223)
        Magic Number: 0x09051c66

But the Mikrotik doesn't do the same (not sure if it needs to) - it only negotiates the magic number:

Frame 6: 32 bytes on wire (256 bits), 32 bytes captured (256 bits)
Ethernet II, Src: Routerbo_8d:70:a6 (d4:ca:6d:8d:70:a6), Dst: Alcatel-_c0:36:b4 (0c:a4:02:c0:36:b4)
PPP-over-Ethernet Session
Point-to-Point Protocol
PPP Link Control Protocol
    Code: Configuration Request (1)
    Identifier: 3 (0x03)
    Length: 10
    Options: (6 bytes), Magic Number
        Magic Number: 0x0fbf18cd

I see in section 5.2 of RFC 4638:

If the MRU is negotiated to a value larger than 1492 octets, the
sending side SHOULD have the option of sending one or more MRU-sized
Echo-Request packets once the session is opened.

Not sure why Mikrotik decided to use 1488 instead of 1492 here.

Matt.

pe1chl · Sat Sep 24, 2016 3:06 pm

I have a similar setup, MikroTik router, Draytek Vigor 130 VDSL2 modem, but my provider uses a Juniper BRAS.
It works OK here:

/interface pppoe-client print detail
Flags: X - disabled, R - running
0 R ;;; PPPoE-link XS4ALL-internet
name="pppoe-xs4all-inet" max-mtu=1500 max-mru=1500 mrru=1500
interface=eth1-vdsl user="x" password="x" profile=default
keepalive-timeout=60 service-name="" ac-name="" add-default-route=yes
default-route-distance=1 dial-on-demand=no use-peer-dns=no
allow=pap,mschap2

Apparently there is still some "bottleneck" for you that clamps the ethernet frame size.
BTW, I have MTU 1598 on the ethernet port of the MikroTik and I do the VLAN tag insertion
on the Draytek modem. (provider requires a VLAN tag on the PPPoE frames)
When you do the VLAN tag insertion on the MikroTik you need more headroom on the ethernet MTU.

MrYan · Mon Sep 26, 2016 9:53 am

The VLAN insertion is done on the Vigor (not on the Mikrotik). I've tried with larger MTU (1520 which is the maximum the Vigor supports) with no change to behaviour.

I'm sure you are correct - the path between me and the exchange is probably short 4 bytes (most likely a VLAN tag) but getting BT to do anything about it is unlikely.

Still no closer to finding out why the Mikrotik choses 1488 bytes for the large LCP echo request though.

Matt.

pe1chl · Mon Sep 26, 2016 11:19 am

Are you sure that BT support the 1500 byte MTU? You can try with the Vigor making the PPPoE connection
and see if that works at 1500 byte MTU. When you do not want IPv6 that may even be the entire solution.
(set the Vigor to have the MikroTik as DMZ host with "Active True IP")

MrYan · Mon Sep 26, 2016 2:39 pm

Yeah, BT do support 1500 byte MTU. It's worked in the past - not sure when it stopped - I only noticed when I upgraded to 6.37 but was the same on downgrade. Most likely, BT have changed something in their network.

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