IPv6 Configuration Guide K/KA/KB.15.15

Virtual links are not allowed for NSSAs.
Reducing AS-external-LSAs and inter-area-prefix-LSAs
An OSPFv3 ASBR uses AS-external-LSAs to originate advertisements of a route to another routing
domain. These advertisements are:
• Flooded in the area in which the ASBR operates.
• Injected into the backbone area and then propagated to any other OSPFv3 areas (except
stub and NSSA areas) within the local OSPFv3 AS. If the AS includes an NSSA, there are
two additional options:
• If the NSSA includes an ASBR, you can suppress advertising some or all of its summarized
external routes into the backbone area. See “Configuring ranges on an ABR to reduce
advertising to the backbone” (page 218).
• Replace all inter-area-prefix-LSAs and all external routes from the backbone area with the
default route (::/0).
Algorithm for AS-external-LSA reduction
The AS-external-LSA reduction feature behavior changes under the following conditions:
• There is one ASBR advertising (originating) a route to the external destination, but one of the
following happens:
• A second ASBR comes online.
• A second ASBR that is already online begins advertising an equivalent route to the same
destination.
In either case above, the routing switch with the higher router ID floods the AS-external-LSAs
and the other routing-switch flushes its equivalent AS-external-LSAs.
• One of the ASBRs starts advertising a route that is no longer equivalent to the route the other
ASBR is advertising. In this case, the ASBRs each flood AS-external-LSAs. Since the LSAs either
no longer have the same cost or no longer have the same next-hop router, the LSAs are no
longer equivalent, and the LSA reduction feature no longer applies.
• The ASBR with the higher router ID becomes unavailable or is reconfigured so that it is no
longer an ASBR. In this case, the other ASBR floods the AS-external-LSAs.
About replacing inter-area-prefix-LSAs and type-7-external-LSA default routes with an
AS-external-LSA default route
By default, a routing switch operating as an ABR for a stub area or NSSA injects non-default,
inter-area routes (inter-areaprefix-LSAs) into the stub areas and NSSAs. For NSSAs, the routing
switch also injects a type-7-LSA default external route. You can further reduce LSA traffic into these
areas by using no-summary. This command option configures the routing switch to:
• Replace injection of inter-area-prefix-LSAs into a stub area or NSSA with an inter-area-prefix-LSA
default summary route (::/0).
• Replace injection of all external routes into an NSSA with an inter-area-prefix-LSA default
route.
You can enable this behavior when you first configure the stub area or NSSA, or at a later time.
The no-summary command does not affect intra-area advertisements, meaning the switch still
accepts summary LSAs from OSPFv3 neighbors within its area and floods them to other neighbors.
The switch can form adjacencies with other routers regardless of whether summarization is enabled
or disabled for areas on each switch.
254 OSPFv3 Routing