2-12 area area-id stub no-summary

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2-12 area area-id stub no-summary

Syntax Description:

  • area-id ” OSPF area ID. This value can be entered as a decimal number in the range of 0 to 4,294,967,295 or in IP address form in the range 0.0.0.0 to 255.255.255.255.

Purpose: Use of the no-summary keyword on the stub area's ABR creates a totally stubby area. In a totally stubby area, both external and inter-area OSPF routes are blocked from being advertised into the area. The backbone or Area 0 cannot be configured as a totally stubby area. An ASBR cannot be part of a totally stubby area, since the purpose of an ASBR is to inject external routes into OSPF. A totally stubby area cannot be used as the transit area for a virtual link.

Initial Cisco IOS Software Release: 10.0

Configuration Example: Configuring an OSPF Totally Stubby Area

In Figure 2-14, Router C is redistributing connected interfaces into OSPF. These redistributed routes are OSPF external routes.

Figure 2-14. An OSPF Totally Stubby Area

graphics/02fig14.gif

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/1   ip address 10.1.1.1 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 1  _______________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   !   interface Serial1   ip address 10.1.1.5 255.255.255.252   clock rate 64000   !   router ospf 1   network 10.1.1.4 0.0.0.3 area 0   network 10.1.1.0 0.0.0.3 area 1  _______________________________________________________________________ Router C  interface Loopback0   ip address 3.3.3.3 255.255.255.255   !   interface Serial0   ip address 10.1.1.6 255.255.255.252   !   router ospf 1   redistribute connected subnets   network 10.1.1.4 0.0.0.15 area 0  

Before configuring Area 1 as a totally stubby area, verify that the redistributed routes and OSPF inter-area routes are being advertised into Area 1.

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF,  IA - OSPF inter area  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1,  E2 - OSPF external type 2  , E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      1.0.0.0/32 is subnetted, 1 subnets C       1.1.1.1 is directly connected, Loopback0  3.0.0.0/32 is subnetted, 1 subnets   O E2    3.3.3.3 [110/20] via 10.1.1.2, 00:00:02, Serial0/1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1  O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:00:02, Serial0/1  

There is one OSPF external route and one OSPF inter-area route in the routing table on Router A. Configure Area 1 as a stub area by modifying the configurations on Routers A and B as shown in the following listing.

 Router A  router ospf 1    area 1 stub    network 10.1.1.0 0.0.0.3 area 1  _______________________________________________________________________ Router B  router ospf 1    area 1 stub    network 10.1.1.4 0.0.0.3 area 0   network 10.1.1.0 0.0.0.3 area 1  
Verification

Verify that the external OSPF routes have been blocked from being advertised into the stub area. Also verify that the ABR is injecting a default route into the stub area.

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF,  IA - OSPF inter area  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR  Gateway of last resort is 10.1.1.2 to network 0.0.0.0  1.0.0.0/32 is subnetted, 1 subnets C       1.1.1.1 is directly connected, Loopback0      10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial1  O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:00:09, Serial0/1   O*IA 0.0.0.0/0 [110/65] via 10.1.1.2, 00:00:09, Serial0/1  

Modify the configuration on Router B, the ABR, to create a totally stubby area.

 Router B  router ospf 1    area 1 stub no-summary    network 10.1.1.4 0.0.0.3 area 0   network 10.1.1.0 0.0.0.3 area 1  

Verify that the OSPF inter-area routes are no longer being advertised by the ABR into the stub area.

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is 10.1.1.2 to network 0.0.0.0      1.0.0.0/32 is subnetted, 1 subnets C       1.1.1.1 is directly connected, Loopback0      10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1 O*IA 0.0.0.0/0 [110/65] via 10.1.1.2, 00:00:17, Serial0/1 

You can also verify that Router B is blocking OSPF inter-area routes or summary LSAs by using the command show ip ospf.

 rtrB#  show ip ospf  Routing Process "ospf 1" with ID 2.2.2.2  Supports only single TOS(TOS0) routes  It is an area border router  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 3. Checksum Sum 0x14946  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 2. 1 normal 1 stub 0 nssa  Area BACKBONE(0)         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 17 times         Area ranges are         Number of LSA 4. Checksum Sum 0x204BF         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0  Area 1  Number of interfaces in this area is 1  It is a stub area, no summary LSA in this area   generates stub default route with cost 1  Area has no authentication         SPF algorithm executed 25 times         Area ranges are         Number of LSA 7. Checksum Sum 0x424A1         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 
Troubleshooting
Step 1. Verify that there is a neighbor relationship between the OSPF routers by using the show ip ospf neighbor command.

Step 2. Ensure that every router in the stub area and the ABR to the stub area have the area configured as a stub by using the router configuration command area area-id stub.

Step 3. When creating a totally stubby area, verify that the keyword no-summary has been used on the ABR to the stub area.

Step 4. An ASBR should not be part of a totally stubby area.

Step 5. A totally stubby area cannot be used as the transit area for a virtual link.

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Cisco[r] OSPF Command and Configuration Handbook
Cisco OSPF Command and Configuration Handbook (paperback)
ISBN: 1587055406
EAN: 2147483647
Year: 2002
Pages: 236

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