I think I now understand X.25 CUGs (closed user groups)

(IOS interface-level CUG configuration appeared to be completely backwards... until I realized it is available only on DCE mode interfaces, which reminded me how *highly* asymmetric the whole thing is. There's the "network" (DCE) and there's the "customer" (DTE). So calls exiting a DCE interface are called "incoming calls" because they're incoming to the customer, despite physically being outgoing... and the translation from network CUGs to local CUGs (instead of the other way around) makes sense because the router *is* "the network".)

tracert is the network equivalent of a tsundere

"i'm not *really* sending that packet to you, it just *happens* that its TTL is set to expire in your hands and you'll be forced to reply to me. but I won't care if you don't. (but I'll try twice more just in case you change your mind)"

okay so it seems the Cisco and Yost RJ45 pinouts *are* subtly different (one passes through DSR towards the DTE, the other lacks DSR but has the DCD signal instead)

...which might be why I'm having troubles getting Linux agetty to hang up/log out when Cisco loses the telnet connection, or vice versa.