From: hcb@gettcomm.com
Date: Tue May 30 2006 - 23:49:11 ART
Quoting CCIEin2006 <ciscocciein2006@gmail.com>:
> Hello folks,
>
> Being that my OSPF experience is limited to small environments with one area
> (area 0) I would like to know how big companies do OSPF.
> For example, when would they decide to create multiple areas?
My general guide is router processor utilization, in particular how long the
processor stays near 100%, which it will do with running the SPF algorithm.
The 100 router limit that Cisco used to recommend isn't really applicable with
fast processors.
In general, the processor load during SPF is a function of
(intra-area links**2) * log(routers)
plus a linear increase with the number of inter-area and external links.
> How do they usually assign areas - by geographic area, by department, by
> function?
I tend to go geographic when possible, which has the advantage of tending to
have a lesser number of telcos at which to point fingers. Still, departmental
can make sense, if much of the traffic stays inside the department. This is an
art, not a science.
For that matter, as the OSPF size grows, I'll often do multiple domains, each
with their own area 0.0.0.0, and link a backbone of backbones with BGP or
floating statics.
> Are virtual links common practice in the real world?
No. I've used GRE or IPIP links more often than virtual links.
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