Configuring OSPF Route Summarization
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Configuring Route Summarization |
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Router(config-router)# |
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area area-id range address mask [advertise | not-advertise] [cost cost] |
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• Consolidates interarea routes on an ABR Router(config-router)# |
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summary-address ip-address mask [not-advertise] [tag tag] |
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• Consolidates external routes, usually on an ASBR |
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©2006— — 1 |
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OSPF does not perform automatic summarization on major network boundaries. To configure manual interarea route summarization on an ABR, use the following procedure:
1. Configure OSPF.
2. Use the area range command to instruct the ABR to summarize routes for a specific area before injecting them into a different area via the backbone as type 3 summary LSAs.
Cisco IOS software creates a summary route to interface null0 when manual summarization is configured, to prevent routing loops. For example, if the summarizing router receives a packet to an unknown subnet that is part of the summarized range, the packet matches the summary route based on the longest match. The packet is forwarded to the null0 interface (in other words, it is dropped), which prevents the router from forwarding the packet to a default route and possibly creating a routing loop.
© 2006 Cisco Systems, Inc. Configuring OSPF 3-117
area range Parameters
The table describes the parameters for the area range command.
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Parameter |
Description |
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area-id |
Identifies the area subject to route summarization. |
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address |
Summary address that is designated for a range of addresses. |
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mask |
IP subnet mask that is used for the summary route. |
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advertise |
(Optional) Sets the address range status to advertise and generates a type 3 summary LSA. |
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not-advertise |
(Optional) Sets the address range status to DoNotAdvertise. The type 3 summary LSA is suppressed, and the component networks remain hidden from other networks. |
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cost |
(Optional) Metric or cost for this summary route, which is used during OSPF SPF calculation to determine the shortest paths to the destination. The value can be 0 to 16777215. |
To configure manual route summarization on an ASBR to summarize external routes, complete the following steps:
1. Configure OSPF.
2. Use the summary-address command to instruct the ASBR (or the ABR) to summarize external routes before injecting them into the OSPF domain as type 5 external LSA.
summary-address Parameters
The table describes the parameters of the summary-address command.
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Parameter |
Description |
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ip-address |
Summary address that is designated for a range of addresses |
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mask |
IP subnet mask that is used for the summary route |
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not-advertise |
(Optional) Used to suppress routes that match the prefix and mask pair |
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tag |
(Optional) Tag value that can be used as a match value for controlling redistribution via route maps |
3-118 Building Scalable Cisco Internetworks (BSCI) v3.0 © 2006 Cisco Systems, Inc.
Route Summarization Configuration Example at ABR
Routerl(config)# router ospf 100
Routerl(config-router)#network 172.16.32.1 0.0.0.0 area 1 Routerl(config-router)#network 172.16.96.1 0.0.0.0 area 0 Routerl(config-router)#area 0 range 172.16.96.0 255.255.224.0 Routerl(config-router)tarea 1 range 172.16.32.0 255.255.224.0
Routerl(config)# router ospf 100
Routerl(config-router)#network 172.16.32.1 0.0.0.0 area 1 Routerl(config-router)#network 172.16.96.1 0.0.0.0 area 0 Routerl(config-router)#area 0 range 172.16.96.0 255.255.224.0 Routerl(config-router)tarea 1 range 172.16.32.0 255.255.224.0
Router2(config)# router ospf 100
Router2(config-router)#network 172.16.64.1 0.0.0.0 area 2 Router2(config-router)#network 172.16.127.1 0.0.0.0 area 0 Router2(config-router)#area 0 range 172.16.96.0 255.255.224.0^ Router2(config-router)tarea 2 range 172.16.64.0 255.255.224.0 ?
Continue reading here: Specifies the authentication type for an area was in Cisco IOS software before 120
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