2-11 area area-id stub

2-11 area area-id stub

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: In Figure 2-11, Area 1 has one exit point. Routers in Area 1 do not need to know the specific external routes that are being redistributed into OSPF by the ASBR. Therefore, Area 1 can be configured as a stub area and the ABR will advertise a default route into Area 1.

Figure 2-11. An OSPF Stub Area with One Exit

In Figure 2-12, Area 1 has two exit points. If routing to the external networks that have been redistributed into OSPF by the ASBR can be sub-optimal, then Area 1 can be configured as a stub area. The routers in Area 1 will be receiving a default route advertisement from both ABRs. All routers in a stub area, including the ABR, must be configured with the command area area-id stub. Inter-area OSPF routes will be advertised into a stub area but external OSPF routes will be blocked. The ABR for the stub area will inject a default route into the stub area. The backbone or Area 0 cannot be configured as a stub area. An ASBR cannot be part of a stub area since the purpose of an ASBR is to inject external routes into OSPF. A stub area cannot be used as the transit area for a virtual link.

Figure 2-12. An OSPF Stub Area with Multiple Exits

Initial Cisco IOS Software Release: 10.0

Configuration Example: Configuring an OSPF Stub Area

In Figure 2-13, Router C is redistributing connected interfaces into OSPF. These redistributed routes are OSPF external routes.

Figure 2-13. An OSPF Stub Area

 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 stub area, verify that the redistributed routes are being advertised as OSPF external routes.

 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, 00:00:25, Serial0/1  10.0.0.0/30 is subnetted, 3 subnets C       10.1.1.8 is directly connected, Serial0/0 C       10.1.1.0 is directly connected, Serial0/1  O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:00:25, 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 10.1.1.0 0.0.0.3 area 1   network 10.1.1.4 0.0.0.3 area 0  

Verification

Verify that OSPF inter-area routes are being advertised into the stub area. Also, verify that external OSPF routes have been blocked from being advertised into the stub area and 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, 3 subnets C       10.1.1.8 is directly connected, Serial0/0 C       10.1.1.0 is directly connected, Serial0/1  O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:00:04, Serial0/1   O*IA 0.0.0.0/0 [110/65] via 10.1.1.2, 00:00:04, Serial0/1  

Verify that Area 1 is a stub area by using the command show ip ospf on Routers A and B.

 rtrA#  show ip ospf  Routing Process "ospf 1" with ID 1.1.1.1  Supports only single TOS(TOS0) routes  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 0. Checksum Sum 0x0  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) (Inactive)         Number of interfaces in this area is 0         Area has no authentication         SPF algorithm executed 2 times         Area ranges are         Number of LSA 1. Checksum Sum 0x51E9         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  Area has no authentication         SPF algorithm executed 6 times         Area ranges are         Number of LSA 9. Checksum Sum 0x38619         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 ______________________________________________________________________ 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 0x14F43  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 3  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 15 times         Area ranges are         Number of LSA 4. Checksum Sum 0x20ABC         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   generates stub default route with cost 1  Area has no authentication         SPF algorithm executed 18 times         Area ranges are         Number of LSA 8. Checksum Sum 0x34E69         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 using the router configuration command area area-id stub.

Step 3. An ASBR should not be part of a stub area.

Step 4. A stub area cannot be used as the transit area for a virtual link.