Fibre MTU

siggi

Hey y’all,

I’m using a MikroTik Hex S router with OpenWrt firmware, which is just about infinitely tweakable. The hardware is MT7621 based, and can work with Ethernet frames up to 2 kbytes, as I understand.

I was curious whether I could bump the PPPoE MTU to 1500 from the recommended 1492, and it turns out that’s no problem. When the MTU is set to 1492, I see the router negotiating it, but at 1500 there’s no negotiation at all, so it seems 1500 is the default MTU for PPPoE.
This is very much a micro optimisation, probably doesn’t make a measurable difference, though there’s a slight decrease in framing overhead.

Siggi

ToNIX

Ebox supports baby jumbo frames without a problem.

I also use a Mikrotik, you can boost the MTU of the pppoe interface to 1520 in the interfaces list (or the MTU of the wan interface, I don’t remember which one I specifically adjusted. The other one will follow, or change both). They will auto adjust to the max possible and you’ll be using baby jumbo frames.

WalterDnes

siggi From https://client.ebox.ca/home (scroll down near bottom of page)

To establish the connection, make sure that the PPPoE configuration is done using the VLAN ID 40 option. Additionally, if your router requires you to configure the MTU value, it must be set to 1492.

siggi

ToNIX interesting, do you then get a path mtu exceeding 1500 bytes? My main switch won’t handle jumbo frames, baby or otherwise, unless I toggle a setting. So far I’ve seen no reason to do it.

siggi

WalterDnes yeah, this is the easy, lazy way to do it. The alternative is to bump the MTU on the Ethernet to the ONT to 1508 or better, to accommodate the 8 bytes of PPPoE overhead.

fersingb

Since this thread is about the MTU I’m asking here. Is any of you having issues with the MTU since today? Everything was fine with MTU 1492 on my RB5009 until this morning around 2-3am. This morning I noticed that some services were not reachable anymore and after investigation, lowering my MTU to 1462 “fixed” it.

I see the PPPoE session first gets a MTU of 1492, then it drops to 1480, but even with that value I’m not able to reach the gateway on Ebox’s side, I have to manually force it to 1462 for things to work properly.

GrumpyCanadian

fersingb My brother’s EBOX fibre connection still appears to be problem-free with the MTU set at 1500 bytes.

fersingb

Thanks GrumpyCanadian , after I reached out to ebox’s support, they escalated the issue and got a call back yesterday evening.

For the record, they are moving some infra, and some people might experience some issues. In my case it was back to normal by the end of the afternoon yesterday.

cinergi

Me too! My MTU seems to have gone down to 1462 as well. It’s been causing typical symptoms of MTU issues which is why I thought to check it via ping. It was previously 1500 with no problems (1508 counting the PPPoE overhead). Restarting the PPPoE connection has no effect; it says the MTU is 1500, but it’s not. Ebox is closed now, will call them tomorrow morning if not resolved by then.

cinergi

Update: The Ebox Level 1 support was not useful in this case. I don’t think they really understood the issue. But I then did another reset of my PPPoE session (I had already tried yesterday with no success), and this time when it reconnected the MTU is back to the normal 1500. So the issue seems to have been resolved by Ebox’s network engineering team.

siggi

I’ve been fine with MTU 1500 for a week or so.

Dick

ToNIX I don’t remember which one I specifically adjusted.

I also use Mikrotik
From my configuration export:

/interface ethernet set [ find default-name=ether1 ] mtu=1508 name=GigE-1 rx-flow-control=auto \ tx-flow-control=auto /interface vlan add interface=GigE-1 mtu=1508 name=ebox-ppp vlan-id=40 /ppp profile add name=FTTH use-upnp=no /interface pppoe-client add add-default-route=yes disabled=no interface=ebox-ppp keepalive-timeout=60 \ max-mru=1500 max-mtu=1500 name=PPPOE password={passsword} profile=FTTH \ use-peer-dns=yes user={username}

GrumpyCanadian

Dick I’m not familiar with your particular hardware, but unless its software is quietly making the necessary adjustments, something seems off about those settings. For PPPoE traffic to “fit” through with an MTU or 1500 bytes, the VLAN interface needs an MTU of at least 1508 bytes. Then, for VLAN traffic with an MTU of 1508 bytes to “fit” on an Ethernet interface, its MTU would have to be at least 1512 bytes, to account for VLAN overhead. Is that indirectly what’s happening here?

Dick

GrumpyCanadian Then, for VLAN traffic with an MTU of 1508 bytes to “fit” on an Ethernet interface, its MTU would have to be at least 1512 bytes

The VLAN tag is at L2. The L3 MTU is 1508 (1480+20+8). On that specific interface, the L2MTU is 1600 bytes by default. Lots of room for the VLAN tag 🙂

Here’s a capture on the VLAN (the reply to a 1472 byte ping to test MTU to google.ca):

Frame: 1522 bytes minus the ethernet frame header 14 bytes, leaves 1508 bytes for PPPOE (in fact, 6 bytes for PPPOE and 2 bytes for PPP). The IPV4 total length is 1500 bytes (ICMP header 8 bytes + IP header 20 bytes + 1472 byte payload).