An OSPF ASBR uses AS external LSAs to originate advertisements of a route to another routing domain, such as an RIP domain. These advertisements are
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Flooded in the area in which the ASBR operates
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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:
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If the NSSA includes an ASBR, you can suppress advertising some or all of its summarized external routes into the backbone area.
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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.)
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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 HPE 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.
The AS external LSA reduction feature behavior changes under the following conditions:
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There is one ASBR advertising (originating) a route to the external destination, but one of the following happens:
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A second ASBR comes on-line.
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A second ASBR that is already on-line begins advertising an equivalent route to the same destination.
In either of these cases, the HPE switch with the higher router ID floods the AS external LSAs and the other switch flushes its equivalent AS external LSAs.
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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.
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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.
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:
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Replace type-3 summary LSA injection into a stub area or NSSA with a type-3 default summary route (0.0.0.0/0.)
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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.
The no-summary command does not affect intra-area advertisements, meaning the switch still accepts summary LSAs from OSPF 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.
When you use no-summary, the change takes effect immediately. If you apply the option to a previously configured area, the switch flushes all of the summary LSAs it has generated (as an ABR) from the area.
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NOTE: This feature applies only when the switch is configured as an ABR for a stub area or NSSA. To completely prevent summary LSAs from injection into the area, use |
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To implement the above operation for a stub area or NSSA, enter a command such as the following:
HP Switch(ospf)# area 40 stub 3 no-summary