[c-nsp] Rationale for ISIS default origination behavior

[Mark Tinka]

On Monday, January 21, 2013 09:11:09 PM David Freedman 
wrote:

> I doubt many people make use of an IS-IS default (as I'm
> sure the L1/ATT behaviour is also seen as an annoyance
> in modern IP networks), many networks I've seen running
> IS-IS and BGP tend to do all their routing in the iBGP
> and keep IS-IS for pure infrastructure prefixes
> (loopbacks and sometimes transfer networks).

Indeed.

In multi-level networks, an L1/L2 router will set the 
Attached Bit (i.e., announce a default route). This is sub-
optimal as IGP's are meant to provide more specificity 
toward destinations.

We disable the Attached Bit ('ignore-attached-bit' - a 
hidden command in IOS) on all IS-IS instances.

> In your scenario, the default can be brought from your
> external peers into your iBGP, this would seem quite
> sensible since you would be avoiding redistribution
> and/or conditional default advertisement (which you can
> achieve with IS-IS through the use of route-maps).

Right.

Mark.
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