Aug 19

Like most CCIE Lab stories, this one starts with a way to access the consoles remotely – a terminal server. In my case, a Cisco 2811 with a NM-16A module (and a CAB-OCTAL-ASYNC breakout cable for connecting 8 routers).

I have had the opportunity to work with terminal servers many times before (including the actual CCIE lab) – but they were always configured by other people. This time, I had to configure the TS myself. No problem, I say…

TS(config)#int lo0
TS(config-if)#ip add

!--- line numbers may vary depending on module
!--- use "show line" to determine the numbers in use
TS(config-if)#line 66 81
TS(config-line)#transport input all

!--- Optional
TS(config-line)#flowcontrol hardware

!--- Optional, but it makes accessing the devices easier
TS(config)#ip host r1 2066
TS(config)#ip host r2 2067
TS(config)#ip host r3 2068
TS(config)#ip host r4 2069
TS(config)#ip host r5 2070
TS(config)#ip host r6 2071
TS(config)#ip host r7 2072
TS(config)#ip host r8 2073

Looks good, and I can now access the devices by telnetting to the various ports on Yet something is wrong… The lines become busy at random intervals, which makes the terminal server refuse connections. A “clear line” solves the problem, but only for a very short time – just seconds later, the line becomes active again.

TS#sh line
   Tty Line Typ     Tx/Rx    A Modem  Roty AccO AccI  Uses  Noise Overruns  Int

*   1/6   72 TTY   9600/9600  -    -      -    -    -     3   1448  447/1343   -
*   1/7   73 TTY   9600/9600  -    -      -    -    -     5    129   62/188    -

Translating "r7"
Trying r7 (, 2072)...
% Connection refused by remote host

After several hours of frustration (clearing lines just to have them become busy again almost immediately), I realize what I forgot:

TS(config-if)#line 66 81
TS(config-line)#no exec

The explanation? Any kind of noise on the wire was interpreted by the TS as an incoming character. As a result, the TS would activate the line, opening an exec session. A “clear line” would close the session, only to have it opened again by the noise.

As somebody else put it – “‘No exec’ isn’t mandatory, but it will help you keep your sanity!”

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