[AusNOG] Oddity

[Karl Auer]

This is weird. Or at least, I think it's weird.

Client has a phone tail (J11) coming into a little Netcomm NB6 ADSL2+
router/modem. The Netcomm is on, and the ADSL light is lit up. The
Netcomm is not attached by any means to anything else (and is not a
wireless router). Specifically, there is no Ethernet connection to the
Netcomm. Just power and an RJ11.

Client has another link to the Internet. I'm not sure what it is, but
since he can definitely get to the Internet and the Netcomm is not
locally networked with anything, it seems to me I can be reasonably sure
it's not the Netcomm, right? Hm. Read on.

There's a Cisco 800 series router sitting there too; it's showing DSL
"CD" on, and RX/TX is showing lots of activity. Another RJ11 tail runs
into the Cisco, and this has an old piece of cardboard tied to it saying
"New ADSL connection" and a phone number (yes - THAT sort of client).
I'm guessing that's the Internet connection, but it's only a guess. It
can't be the Netcomm, so it must be the Cisco, right?

Well, maybe not. The client has a symmetrical DSL link from Telstra to
an office in another town. Maybe the Cisco is handling that? In which
case what's handling the Internet connection?

Anyway, none of that is weird, that's just general ignorance. No; what's
weird is that if I turn off the Netcomm, the client loses Internet
connectivity. The Cisco's CD light stays on.

About the only explanation I can think of is that somehow the external
IP address of the Netcomm is involved in routing traffic to the client.
I can't imagine how, though. I just can't believe that the Netcomm IS
the Internet connection - with no Ethernet connected, no PPP light
showing and (obviously) no Ethernet activity.

I'm not seeking help here, I'm just fascinated by the fact that this
Netcomm, not networked locally at all, can somehow affect the local
network's Internet connectivity on (presumably) a completely other ADSL
link. I didn't think that was possible, but clearly it is.

Obviously I'm about to learn something :-)

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)
http://www.biplane.com.au/kauer
http://twitter.com/kauer389

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