AT&T put in a 150 Mbps fiber line at our business (not my home). They claim it uses the PPP protocol (I am told this is non-Ethernet), but is terminated in multi-mode fiber using 1000BASE SX (an Ethernet standard). Note that their documents (the OSM, Order Status Management site) say it is PPP, not PPPoE.

My questions are: - is PPP (not PPPoE) actually incompatible with Ethernet? - will a Cisco router with an SFP in a Gig-e port be able to handle this, or else what is an example of a "wic" that can handle PPP over fiber? (Googling has not helped so far.)

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    It’s pppoe, the dude who told you ppp doesn’t have a clue Jun 11, 2018 at 16:47
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    PPP by itself runs over a serial line. Common modern applications require PPPoE or PPPoA (over ATM). A GigE port (on Cisco or any brand router) would not be able to do serial PPP; it would have to be PPPoE (on a router that supports PPPoE and is configured for it on that port.) I would be very surprised to learn that there is equipment that uses SFPs to terminate serial lines. Jun 11, 2018 at 20:00
  • Mike, it is the AT&T OSM online document (Order Status Management) that says "Protocol: PPP". But, the same document also says the circuit is terminated on ports 5&6, but it is actually on ports 4&5 - and only port 5 is labeled. The circuit actually seems to be something like PPPoE as we do get link light lit. So far, everyone on the AT&T side has been not technical enough to help with this.
    – Beel
    Jun 14, 2018 at 14:51
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    – Ron Maupin
    Dec 25, 2018 at 8:40

1 Answer 1


Multi-mode is surprising, usually WAN over fiber comes on single-mode (1000BASE-LX10 or -BX10 for p2p or some PON flavor). If you're not sure check the jacket color, MMF is usually orange, aqua or violet, SMF yellow.

To terminate the PPP end you'd need a router speaking that protocol (likely PPPoE). However, they might as well be using a proprietary protocol - a 1000BASE-SX transceiver terminates any signal up to 1.25 Gbit/s using 850 nm light, it doesn't care what it is. It could theoretically be a serial link actually running PPP - this would be entirely incompatible with Ethernet.

If it's real (Ethernet) 1000BASE-SX a router should be able to terminate it. Anything else might get complicated...

If you don't mind an interruption you could plug the fiber into a (non-production) 1000BASE-SX port. If the link comes up it's Ethernet (most probably running PPPoE).

  • The fiber at the demarc is aqua, meaning 50 micro-meter multimode, I believe. AT&T has single-mode into the building but hands off multi-mode at the demarc, perhaps because the link is 150 Mbps, not Gigabit speed. The official AT&T online document (the OSM - Order Status Management page) says it is PPP but I do get link on my router, so I bet it is PPPoE.
    – Beel
    Jun 14, 2018 at 14:42
  • They might actively convert the SMF protocol to 1000BASE-SX in the ONT, you can't just change from SMF to MMF (without severe penalty). Since the router's Ethernet port goes up it can't be plain PPP which can't run over Ethernet.
    – Zac67
    Jun 14, 2018 at 16:56

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