• Flooded in the area in which the ASBR operates
• Injected into the backbone area and then propagated to any other OSPF areas (except stub areas) within the
local OSPF 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.
◦ Replace all type-3 summary LSAs and the default external route from the backbone area with the default
summary route (0.0.0.0/0.)
In some cases, multiple ASBRs in an AS can originate equivalent external LSAs. The LSAs are equivalent when
they have the same cost, the same next hop, and the same destination. In such cases, the switch optimizes
OSPF by eliminating duplicate AS external LSAs. That is, the ASBR with the highest router ID floods the AS
external LSAs for the external domain into the OSPF AS, while the other ASBRs flush the equivalent AS external
LSAs from their databases. As a result, the overall volume of route advertisement traffic within the AS is reduced
and the switches that flush the duplicate AS external LSAs have more memory for other OSPF data.
This enhancement implements the portion of RFC 2328 that describes AS external LSA reduction. This
enhancement is enabled by default, requires no configuration, and cannot be disabled.
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 on-line.
◦ A second ASBR that is already on-line begins advertising an equivalent route to the same destination.
In either of these cases, the switch with the higher router ID floods the AS external LSAs and the other
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.
Replacing type-3summary LSAs and type-7 default external LSAs with a
type-3 default route LSA
By default, a routing switch operating as an ABR for a stub area or NSSA injects non-default, summary routes
(LSA type 3) into the stub areas and NSSAs. For NSSAs, the routing switch also injects a type-7 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 type-3 summary LSA injection into a stub area or NSSA with a type-3 default summary route
(0.0.0.0/0.)
• Disable injection of the type-7 default external route into an NSSA.
You can enable this behavior when you first configure the stub area or NSSA, or at a later time. For the full
command to use, see
Configuring a stub or NSSA area
on page 200.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
