topic Re: RFC4638 MTU 1500 not working in Broadband

RFC4638 MTU 1500 not working

v3locity — Wed, 22 May 2024 23:11:36 GMT

I've just migrated from Zen to BT Business FTTP 900/110.

On Zen (over the same Openreach network), I was able to setup my connection (PPPOE) with MTU/MRU 1500 using pfSense as the Openreach network supports baby jumbo frames (RFC 4638), avoiding the need for any TCP MSS clamping, which is optimal.

Now on BT Business, even with the included hub, my connection has MTU 1492. Using the exact same settings on pfSense (as I had with Zen) doesn't work, as the BT equipment seems to be negotiating 1492 instead -- logs attached. Could this be a mis-configuration on the BT side? As far as I know BT/Openreach fully supports MTU 1500 over PPPoE.

I have tried contacting technical support over the phone and chat but no one seems to be able to understand what I am talking about. As far as they’re concerned, I have a working Internet connection.

Please can someone from BT/Openreach advise or comment? Would really appreciate some help!

Many thanks!

PPPoE logs -

May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] Link: OPEN event
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: Open event
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: state change Initial --> Starting
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: LayerStart
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] PPPoE: Set PPP-Max-Payload to '1500'
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] PPPoE: Connecting to ''
May 22 23:13:52 ldnrouter01 ppp[91300]: PPPoE: rec'd ACNAME "acc-aln2.l-per"
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] PPPoE: rec'd PPP-Max-Payload '1500'
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] PPPoE: connection successful
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] Link: UP event
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: Up event
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: state change Starting --> Req-Sent
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: SendConfigReq #1
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] PROTOCOMP
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x2d124624
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: rec'd Configure Request #250 (Req-Sent)
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] AUTHPROTO CHAP MD5
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x39beceb2
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: SendConfigAck #250
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] AUTHPROTO CHAP MD5
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x39beceb2
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: state change Req-Sent --> Ack-Sent
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: rec'd Configure Reject #1 (Ack-Sent)
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] PROTOCOMP
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: SendConfigReq #2
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x2d124624
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: rec'd Configure Ack #2 (Ack-Sent)
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x2d124624
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: state change Ack-Sent --> Opened
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: auth: peer wants CHAP, I want nothing
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: LayerUp
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] CHAP: rec'd CHALLENGE #1 len: 64
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] Name: "acc-aln2.l-per"
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] CHAP: Using authname "xxxxxxx@hg7.btclick.com"
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] CHAP: sending RESPONSE #1 len: 44
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: rec'd Configure Request #1 (Opened)
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1492
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] AUTHPROTO CHAP MD5
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x10259efe
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: LayerDown
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: SendConfigReq #3
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x2d124624
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: SendConfigAck #1
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1492
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] AUTHPROTO CHAP MD5
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x10259efe
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: state change Opened --> Ack-Sent
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: rec'd Configure Ack #3 (Ack-Sent)
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MRU 1500
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] MAGICNUM 0x2d124624
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: state change Ack-Sent --> Opened
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: auth: peer wants CHAP, I want nothing
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] LCP: LayerUp
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] CHAP: rec'd CHALLENGE #2 len: 29
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] Name: "COLLNS04"
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] CHAP: Using authname "xxxxxxx@hg7.btclick.com"
May 22 23:13:52 ldnrouter01 ppp[91300]: [wan_link0] CHAP: sending RESPONSE #2 len: 44
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan_link0] CHAP: rec'd SUCCESS #2 len: 4
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan_link0] LCP: authorization successful
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan_link0] Link: Matched action 'bundle "wan" ""'
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan_link0] Link: Join bundle "wan"
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] Bundle: Status update: up 1 link, total bandwidth 64000 bps
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: Open event
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: state change Initial --> Starting
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: LayerStart
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: Open event
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: state change Initial --> Starting
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: LayerStart
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: Up event
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: state change Starting --> Req-Sent
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: SendConfigReq #1
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPADDR 0.0.0.0
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] COMPPROTO VJCOMP, 16 comp. channels, no comp-cid
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: Up event
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: state change Starting --> Req-Sent
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: SendConfigReq #1
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: rec'd Configure Reject #1 (Req-Sent)
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] COMPPROTO VJCOMP, 16 comp. channels, no comp-cid
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: SendConfigReq #2
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPADDR 0.0.0.0
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: rec'd Configure Ack #1 (Req-Sent)
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPV6CP: state change Req-Sent --> Ack-Rcvd
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: rec'd Configure Nak #2 (Req-Sent)
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPADDR 81.134.xxx.xxx
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] 81.134.xxx.xxx is OK
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: SendConfigReq #3
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPADDR 81.134.xxx.xxx
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: rec'd Configure Ack #3 (Req-Sent)
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPADDR 81.134.xxx.xxx
May 22 23:13:53 ldnrouter01 ppp[91300]: [wan] IPCP: state change Req-Sent --> Ack-Rcvd
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPCP: rec'd Configure Request #2 (Ack-Rcvd)
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPADDR 81.139.176.1
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] 81.139.176.1 is OK
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPCP: SendConfigAck #2
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPADDR 81.139.176.1
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPCP: state change Ack-Rcvd --> Opened
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPCP: LayerUp
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] 81.134.xxx.xxx -> 81.139.176.1
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IFACE: Up event
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IFACE: Rename interface ng0 to pppoe0
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IFACE: Add description "WAN"
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPV6CP: rec'd Configure Request #2 (Ack-Rcvd)
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPV6CP: SendConfigAck #2
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPV6CP: state change Ack-Rcvd --> Opened
May 22 23:13:55 ldnrouter01 ppp[91300]: [wan] IPV6CP: LayerUp

Re: RFC4638 MTU 1500 not working

BethM — Thu, 23 May 2024 11:40:55 GMT

Hi v3locity

So I can query into this for you, what hardware are you using?

Re: RFC4638 MTU 1500 not working

v3locity — Thu, 23 May 2024 12:28:00 GMT

Hi BethM

I am using the supplied ONT which is a Nokia single port.

Currently I am using pfSense plus running on a Supermicro server but the result is exactly the same when using the supplied BT Business Hub in default configuration.

In addition to the PPPoE logs I am able to supply pcap files during the establishment of the PPPoE session.

Thank you!

Re: RFC4638 MTU 1500 not working

BethM — Fri, 24 May 2024 14:21:18 GMT

Hi v3locity

Thanks for clarifying, I'm now asking around and will come back to you once I have an answer.

Re: RFC4638 MTU 1500 not working

BethM — Fri, 24 May 2024 14:33:37 GMT

Hi v3locity

'1492 is the max MTU size on our fibre network. This is due to PPPoE having an extra 8 bits needed for it’s own encapsulation.

1500 was the standard on ADSL products that used PPPoA.

Unsure why it would have worked before but as far as BT network we have always been 1492 on any fibre product.'

Re: RFC4638 MTU 1500 not working

v3locity — Fri, 24 May 2024 14:54:00 GMT

Hi BethM,

Thanks for looking into this.

Your findings are quite surprising, as Openreach definitely supports MTU 1500 payload over PPPoE, so MTU 1508 (RFC 4638 or baby jumbo frames) on the Ethernet layer. I've used this on Zen FTTP over Openreach for the last few years. Are you saying that while Openreach supports it, BT does not?

There are several threads which suggests that it is possible within the BT network:

MTU 1500 working 🙂

Advice for MTU of 1500 on Bt Business FTTH/FTTP

Many thanks!

Re: RFC4638 MTU 1500 not working

chris6273 — Sun, 26 May 2024 21:32:10 GMT

Back in the days of ADSL and PPPoA, an MTU of 1458 was common but I found 1430 bytes always worked best.

Unless it's something particular with BT Business, Openreach supports RFC 4638 on their FTTC/FTTP platforms - I successfully run the following settings on my two internet connections:

BT Retail (FTTC w/ MikroTik hEX v3 & HG612)
- MikroTik hEX - WAN Interface: 1508 bytes
- MikroTik hEX - PPPoE Interface: 1500 bytes

EE (FTTP w/ MikroTik RB4001 & Nokia ONT)
- MikroTik RB4001 - WAN Interface: 1508 bytes
- MikroTik RB4001 - PPPoE Interface: 1500 bytes

Both use PPPoE sessions.

Could you double check the MTU settings on the physical Ethernet interfaces, both on your kit and the BT Business Hub?

Re: RFC4638 MTU 1500 not working

v3locity — Sun, 26 May 2024 22:35:00 GMT

Hi chris6273,

Thanks for your reply, good to have another example of this working on BT Retail.

Yes, fully aware that Openreach supports RFC 4638 on their network, as mentioned, it was working as expected on Zen before my migration to BT Business. This is using the exact kit and fibre line as before.

Not sure how to check interface MTU on BT Business Hub, but on pfSense I have checked that the interface MTU is indeed 1508 when the WAN (PPPoE) interface was set to MTU 1500.

Thank you!

Re: RFC4638 MTU 1500 not working

v3locity — Sat, 08 Jun 2024 22:31:10 GMT

Hi BethM,

Wondering if you were able to check and provide further updates?

As per various forum posts and member feedback this should be possible.

Many thanks.

Re: RFC4638 MTU 1500 not working

AdamC — Tue, 11 Jun 2024 14:01:21 GMT

Hi,

We have not had any new updates on this as yet. I will check in with Beth and update you with any new information we have.

Thank you

Adam