I am reading through documentation which talks about "Serial network interface" something which also shows up when IOS boots.

I never see "Serial network interface" defined anywhere. If my router says 2 Serial network interface(s) what does that mean? Does it mean that those 2 Serial network interfaces can support Ethernet? What about frame relay?

  • 9
    Holy read the manual batman. Mar 18, 2013 at 2:11
  • @SpacemanSpiff And, just for kicks, which manual exactly should batman read?
    – jscott
    Mar 18, 2013 at 2:26
  • A serial network interface is a serial port that acts as a network interface. They can be used to connect two routers back-to-back but are most commonly used to connect to either a modem or a DSU. Mar 18, 2013 at 2:47
  • 2
    Since I upvoted Spiff's comment, but its apparently too vague, here's the basic router configuration manual: cisco.com/en/US/docs/routers/access/1900/software/configuration/…
    – cpt_fink
    Mar 18, 2013 at 5:13

2 Answers 2


While not a duplicate, you should read my answer here: Why do modern routers not require clock settings anymore?

The short answer is that before Ethernet, built-in T1 interfaces, WIC cards, etc., routers connected to each other with serial ports. Usually that serial port connected the router to something like a T1 CSU/DSU or an ISDN adapter. But the port on the router was a serial port.

I am not aware of a solution to convert a serial port on a Cisco router to Ethernet. But serial ports do support most other communications protocols like T-carriers, x.25, frame relay, etc.

  • +1 and I would almost accept this answer, but it's still not clear how it answers my question of if ethernet, frame-relay can be used on a serial port. Put another way, how do you know the list of layer-2 protocols supported by a given interface port?
    – T. Webster
    Mar 18, 2013 at 5:56
  • see my edits. you know what protocols are supported by reading the documentation. it varies from router to router.
    – longneck
    Mar 18, 2013 at 10:54
  • I used to work with a 2500 series (or maybe older) that had something like that, 10mb ethernet to serial connectors, I don't think there would be much use nowadays, native adapters are much better options.
    – NickW
    Mar 18, 2013 at 11:07
  • @NickW are you thinking of AUI? it looks like a serial port, but it's not. en.wikipedia.org/wiki/Attachment_Unit_Interface
    – longneck
    Mar 18, 2013 at 12:19
  • I just may be.. it has been many a year since I used those.
    – NickW
    Mar 18, 2013 at 12:22

SNI is the term telephone company employees use to refer to the aluminium boxes commonly attached to the side of your home. It usually is a single pair of copper telephone cables that can be rewired to accommodate two or more common extension lines within the home. Its a telephone splitter...

  • That doesn't seem related to the question.
    – RalfFriedl
    Sep 13, 2019 at 20:39

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