Cisco 7604 Configuration Guide - Page 157

Chapter 8 Configuring IP Routing and DHCP Services Configuring OSPF hostname(config-router)# area 17 default-cost 20 Configuring OSPF NSSA The OSPF implementation of an NSSA is similar to an OSPF stub area. NSSA does not flood type 5 external LSAs from the core into the area, but it can import autonomous system external routes in a limited way within the area. NSSA imports type 7 autonomous system external routes within an NSSA area by redistribution. These type 7 LSAs are translated into type 5 LSAs by NSSA ABRs, which are flooded throughout the whole routing domain. Summarization and filtering are supported during the translation. You can simplify administration if you are an ISP or a network administrator that must connect a central site using OSPF to a remote site that is using a different routing protocol using NSSA. Before the implementation of NSSA, the connection between the corporate site border router and the remote router could not be run as an OSPF stub area because routes for the remote site could not be redistributed into the stub area, and two routing protocols needed to be maintained. A simple protocol such as RIP was usually run and handled the redistribution. With NSSA, you can extend OSPF to cover the remote connection by defining the area between the corporate router and the remote router as an NSSA. To specify area parameters for your network as needed to configure OSPF NSSA, perform the following steps: Step 1 Step 2 If you have not already done so, enter the router configuration mode for the OSPF process you want to configure by entering the following command: hostname(config)# router ospf process_id Enter any of the following commands: • To define an NSSA area, enter the following command: hostname(config-router)# area area-id nssa [no-redistribution] [default-information-originate] • To summarize groups of addresses, enter the following command: hostname(config-router)# summary address ip_address mask [not-advertise] [tag tag] This command helps reduce the size of the routing table. Using this command for OSPF causes an OSPF ASBR to advertise one external route as an aggregate for all redistributed routes that are covered by the address. OSPF does not support summary-address 0.0.0.0 0.0.0.0. In the following example, the summary address 10.1.0.0 includes address 10.1.1.0, 10.1.2.0, 10.1.3.0, and so on. Only the address 10.1.0.0 is advertised in an external link-state advertisement. hostname(config-router)# summary-address 10.1.1.0 255.255.0.0 Before you use this feature, consider these guidelines: - You can set a type 7 default route that can be used to reach external destinations. When configured, the router generates a type 7 default into the NSSA or the NSSA area boundary router. OL-20748-01 Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Configuration Guide using ASDM 8-15