IS-IS reference

Randy Bush wrote:

The backbone should not take _any_ routing information from the leaf

there are cases where one has to:

o multi-homed customers who need to speak bgp.

o pops where one's local complexity is sufficiently high,

In these cases one should be prepared to include portions of these
networks into the backbone, as treat them accordingly, from an
administrative point of view. Letting more than one group to
play with routing within the backbone is a recipe for disaster.

In case with multi-homed customers it may not be possible, so
some very paranoid filtering is in order.