Folks,
Section 16.2 states the following
16.2. Calculating the inter-area routes
The inter-area routes are calculated by examining summary-LSAs.
If the router has active attachments to multiple areas, only
backbone summary-LSAs are examined. Routers attached to a
single area examine that area's summary-LSAs. In either case,
the summary-LSAs examined below are all part of a single area's
link state database (call it Area A).
From this it is clear that the an ABR will only examine the summary LSAs
from the backbone area. This is done to avoid the counting to infinity
problem since exchanging routes between areas is done using Distance
Vector mechanisms.
Now some of vendors seems to skip this check i.e examining summary LSAs
from all attached areas. Can any one explain why this would be
considered useful, assuming this is done intentionally.