Setting PPPoE MTU 1500 Virgin Plus (Bell) Canada

Installing and Using OpenWrt Network and Wireless Configuration
1

Hello,
Any suggestion on settings/setup to make MTU 1500 working for PPPoE WAN interface?
I'm using SFP media converter, physical wan & VLAN wan.35 interfaces are set to 1508 sucessfully.
Everything appears to be working and mru 1500 offered during first phase of LCP negotiations, but then overriden by 1492 later on. Below is pppd log.
Thanks!

Fri Dec  2 20:01:34 2022 daemon.notice pppd[8896]: Connect: pppoe-wan <--> wan.35
Fri Dec  2 20:01:34 2022 daemon.debug pppd[8896]: sent [LCP ConfReq id=0x1 <magic 0x4f92137f>]
Fri Dec  2 20:01:34 2022 daemon.debug pppd[8896]: rcvd [LCP ConfAck id=0x1 <magic 0x4f92137f>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: rcvd [LCP ConfReq id=0xcd <mru 1500> <auth pap> <magic 0x372b2f5f>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [LCP ConfAck id=0xcd <mru 1500> <auth pap> <magic 0x372b2f5f>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [LCP EchoReq id=0x0 magic=0x4f92137f]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [PAP AuthReq id=0x1 user="v2***@virginmobile.ca" password=<hidden>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: rcvd [LCP EchoRep id=0x0 magic=0x372b2f5f]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: rcvd [LCP ConfReq id=0x30 <mru 1492> <auth pap> <magic 0x1336bf0c>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [LCP ConfReq id=0x2 <magic 0x81c0833b>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [LCP ConfAck id=0x30 <mru 1492> <auth pap> <magic 0x1336bf0c>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: rcvd [LCP ConfAck id=0x2 <magic 0x81c0833b>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [LCP EchoReq id=0x0 magic=0x81c0833b]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: sent [PAP AuthReq id=0x2 user="v2***@virginmobile.ca" password=<hidden>]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: rcvd [LCP EchoRep id=0x0 magic=0x1336bf0c]
Fri Dec  2 20:01:37 2022 daemon.debug pppd[8896]: rcvd [PAP AuthAck id=0x2 ""]
Fri Dec  2 20:01:37 2022 daemon.notice pppd[8896]: PAP authentication succeeded
2

Several threads about it, search for PPPoE mtu.

3

frollic,
I have checked all info I could find. Based on all info I could gather setting physical & VLAN interface to 1508 and PPPoE mtu to 1500 all is needed. This was done, and it almost works i.e. provider does offer mru 1500 when MRU negotiatiation starts and openwrt accepts the profile, but then provider offers 1492 and openwrt accepts that as well and this is the profile used - see pppd log.
Is there any way to reject 2nd 1492 offer?

1 Like
4

Your ISP needs to support that in the first place, does Virgin Plus allow that?

5

moeller0,
It appears it is supported if mru 1500 is offered by Virgin as the first option during mru negotiations unless I interpret pppd log incorrectly.

6

You need to set physical wan MTU to 1512, not 1508.

7

patrakov,
Tried that i.e. 1512, 1520, same result.
Looking at logs LCP EchoReq/LCP EchoRep are successful for mru 1500 profile, which suggests there is no problem with physical interface. I also tried to manually set pppoe-wan to mtu 1500 after it is up and can successfully ping sites with 1472 packet size unfragmented (1500 MTU), but ping from eny external source only works up to 1464 unfragmented (1492 MTU).
Is there any setting to reject second offer for 1492 profile?

8

There is a quirk in OpenWRT: PPPoE always negotiates MTU 8 less than asked. So set the MTU on pppoe-wan to 1508 (that's in addition to setting it to 1512 on the real wan interface and to 1508 on the vlan).

9

patrakov,
Tried setting as suggested - wan 1512, wan.35 1508, pppoe-wan 1508. Same result - [PPP-max-payload 05 dc] @ pppoe discovery phase, then first offer of mru 1500 @ LCP mru negotiation phase, then 2nd offer of mru 1492, which gets accepted - see log in my first post. OpenWRT is version 22.03.
Anything else to try? I'm thinking if it's possible to reject 2nd offer of mru 1492 it should work.