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.