What is a CTunnel interface?

You might have noticed that your IOS release supports a ctunnel interface (hint: your image has to support CLNS) and wondered what it could do. Well, it’s a GRE tunnel between a pair of NSAPs, so you can transport IP traffic across your well-engineered CLNS network without ever exposing the core routers to the dangers of IP.

But wait, it gets better: starting with IOS releases 12.3(7)T and 12.2(33)SRA, you can transport IPv6 across the ctunnel interface. Unfortunately, they haven’t implemented MPLS over GRE over CLNS yet (the mpls ip command is present, but does not work).

It looks like there's at least one potentially very large-scale application that could use this feature.


  1. Speaking of "strange" interfaces, it is possible to define a virtual-template interface with a type (ethernet, serial, tunnel) - for example: "interface virtual-template 1 type ethernet" (it's even possible to create sub-interfaces with encapsulation dot1q). By default it is serial. Do you know how to use the "ethernet" type ?
  2. Interesting ... not yet ;)
  3. Hardware serial intefaces using HDLC encapsulation can be used as parent for subinterfaces. Do you know how to use the subinterfaces and how they are demultiplexed at other end?
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