Serial WAN Interface gets auto IP?

[ValleyITPC]

I'm building a test lab and just linked two 2811's together running IOS file:

c2800nm-adventerprisek9-mz.151-4.M4.bin. 

One has  a WIC-2T, the other a WIC-1T.  Cable is a 26pin Smart Seraial to DB60 crossover for a back to back config. 

I forget which end is the DCE but anyway. 

I configured only the IP (10.1.6.1/30) + no shut on one router.  The other router has a cleared config.  I console in to that one, set hostname, username, a few items for line cons 0, and then did a sh run.  It seems my serial int on this router has automatically decided it will be 10.1.6.2/30 and is in an Up state already. 

That's handy but kind of freaky.  So my questions are:

What exactly is the behavior here?  If one end detects the other not configured, does it push that config, or does the router unconfigured detect the other end as up and decide to generate compatible settings?  What if I had used a /24 (or something) instead of /30?  Does it matter if the first interface configured is DCE or DTE? 

the other serial cable on the WIC-2T is running to a WIC-1T on an 1841 running IOS 12.3 and this auto-config didn't happen .

Thanks for any insight. 

[ValleyITPC]

Ok, so the answer is SLARP.  LOL if that isn't a name to make you think of something nasty I don't know what is.  Did some mild reading about this after spotting it in the Method column when doing a sh ip int br on the other 2811.  I don't know much about SLARP, other than what wikipedia says but it's ARP for serial lines and I guess it also populates the IP too.  Still appreciate any further insights on the technology from anybody but at least the basic answer is SLARP. 

SLARP:  just had to say it one more time.

[Jeff Van Houten · Accepted solution]

You're correct. By default fresh interfaces on serial lines that detect a /30 address at the remote end will automatically configure the corresponding address.

Sent from Cisco Technical Support iPad App

[ValleyITPC]

Thanks Jeff.  SLARP, it's what's for dinner.