IOS interface names

George sent me a question that surfaced age-old memories:

I saw the Serial 0/1/0 interface in one of your articles. I understand the Serial 0/1 command as accessing the sub interface of Serial 0 with the 1st interface. But I have never seen the 2nd 0 being used. What is the 2nd "0", and how is it to be used?

In the ancient times when the high-end router was an AGS+, the interface names were kept simple (for example, Serial0). When the Cisco 7000 was introduced with online insertion and removal (OIR) capability, router's life became more complex, as its actual hardware (and thus the interface names) might change while it's running.

To work around this problem, the interface names changed with Cisco 7000 (and broke quite a few of our scripts). Every interface name on that router had two parts: the slot number and the port-in-slot number. Later the same convention was adopted for most modular routers (it doesn’t make sense to have all your interfaces renumbered if you exchange a 2-port WAN card with a 4-port WAN card) and with the addition of WIC (Wan Interface Card) adapters which are effectively module-on-a-module, we’ve got the third layer of interface hierarchy. The Serial2/1/0 interface is thus port#0 on WIC#1 on NM#2.

There are (at least) two other special characters in the interface names:

  • Dot (.) indicates a subinterface. Serial0/1/0.101 is subinterface 101 on WAN interface Serial0/1/0.
  • Colon (:) indicates a channel or channel group on a channelized interface (BRI, E1, T1 …).

If you’re aware of any other interface name format, please take your time to write a comment.

This article is part of You've asked for it series.


  1. Dot (.) also serves to identify an au-4 or au-3 on an multichannel STM-1 port adaptor.
  2. Slot 1:
    Channelized STM-1 (SMI) Port adapter, 1 port


    These are some of the strangest interface identifiers i've seen
  3. The D channel in ISDN is usually referred to as :23 or :15 (depending on whether it is a T1 or E1). But sometimes, it is ":D".
    So, you would see something like "voice-port 3:D"
  4. A couple of interesting cases:

    TwinGig converters play a funny game. My TwinGig-capable Sup 6-E in slot 3 uses the follwing names to describe its pair of tengig interfaces:

    The four Gigabit ports are the ones I would have if twingig converters were installed. They're not. Config elements for all 6 interfaces persist in the configuration, regardless of what hardware is installed.

    This seems to indicate the intention to prevent multiple types of interface (gig and tengig) from landing on the same interface numbers, otherwise the gigabit ports would be numbered from 1 and not from 3.

    But the 2621 router flies in the face of that thinking. It's got s0/0, s0/1, fa0/0 and fa0/1.

    Frankly, I prefer the Sup 6-E logic
  5. Whilst the STM1 card appears strange, it really isn't.


    Is Port 0 of card 1 (as per the article), with JKLM 1122 (google KLM numbering) channel 3 (i.e. the 3rd 64Kb channel of the specified 2Mb circuit of the STM1.)
  6. The show controllers command accepts interface type and number only with space in between, like Serial 0/1, not Serial0/1. It is a good practice to use this format throughout.
  7. Speaking of Twingigs, the 3560E-12D behaves in yet another manner.

    There are 12 X2 interfaces, which gives a possible 24 Twingig interfaces.


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