2-6 area area-id nssa no-redistribution

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2-6 area area-id nssa no-redistribution

Syntax Description:

  • area-id ” OSPF area ID. This value can be entered as a decimal number in the range of 1 to 4,294,967,295 or in IP address form in the range 0.0.0.1 to 255.255.255.255. Area 0 can be entered but Area 0 cannot be configured as an NSSA.

Purpose: In a stub or totally stubby area, the ABR to the stub area will prevent OSPF external routes (type 5) from being advertised into the stub area. This implies that an ASBR cannot be part of a stubby or totally stubby area because an ASBR generates OSPF external type 5 routes. Situations arise where you want to create a stubby or totally stubby area relative to OSPF but also want to advertise redistributed routes from an ASBR across the area. An OSPF area that has these properties is an NSSA. In Figure 2-6, you want Router B, the ABR, to block OSPF external routes from Area 1. You also want the routes redistributed by Router C, the ASBR, to be allowed into the area. Additionally, Router B is an ASBR for the EIGRP routes received from Router E. You want the EIGRP routes to be redistributed into Area 0 but you do not want them advertised into Area 1, the NSSA. If you configure Area 1 as an NSSA, then the external OSPF routes that Router B receives from Area 0 will be blocked from Area 1. The redistributed routes from the ASBR (Router C) will be sent as OSPF type 7 routes. Router B will convert these type 7 routes to OSPF type 5 routes and advertise them into Area 0. If you use the no-redistribute keyword on Router B, then the EIGRP routes will not be converted to OSPF type 7 routes. This will prevent them from entering Area 1 but will allow them to be advertised into Area 0. Normally, routes redistributed into OSPF are type 5 routes. An ASBR that has been configured as an NSSA will generate type 7 routes instead of type 5 routes.

Figure 2-6. An OSPF ABR/ASBR Can Control the Redistribution of Routes into an NSSA

graphics/02fig06.gif

Initial Cisco IOS Software Release: 11.2

Configuration Example: Preventing Redistributed Routes from Entering an OSPF NSSA

In Figure 2-6, Routers C and D are running RIP Version 2. Router D is advertising the networks 156.26.32.0/24 and 156.26.33.0/24 to Router C via RIP. Router C will redistribute the RIP routes, including the 10.1.1.8/30 network, into OSPF. Routers B and E are running EIGRP. Router B will redistribute the EIGRP routes into OSPF. Since Area 1 has been defined as an NSSA, the redistributed RIP and EIGRP routes will be advertised into Area 1 as OSPF type 7 routes. Initially, you will configure Area 1 as a normal OSPF area in order to see the routes that are advertised.

 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   redistribute connected subnets   network 10.1.1.0 0.0.0.3 area 0  _______________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Ethernet0   ip address 172.16.1.1 255.255.255.0   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   !   interface Serial1   ip address 10.1.1.5 255.255.255.252   lockrate 64000   !   router eigrp 1   network 172.16.0.0   !   router ospf 1   redistribute eigrp 1 subnets   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 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   !   interface Serial1   ip address 10.1.1.10 255.255.255.252   clockrate 64000   !   router ospf 1   redistribute rip subnets   network 10.1.1.4 0.0.0.3 area 1   !   router rip   version 2   passive-interface Serial0   network 10.0.0.0  _______________________________________________________________________ Router D  interface Loopback0   ip address 156.26.32.1 255.255.255.0   !   interface Loopback1   ip address 156.26.33.1 255.255.255.0   !   interface Serial0/0   ip address 10.1.1.9 255.255.255.252   !   router rip   version 2   network 10.0.0.0   network 156.26.0.0   no auto-summary  _______________________________________________________________________ Router E  interface Loopback0   ip address 142.1.4.4 255.255.255.255   !   interface Ethernet0/0   ip address 172.16.1.2 255.255.255.0   !   router eigrp 1   network 142.1.0.0   network 172.16.0.0   no-auto-summary  

Router A is advertising Loopback0 as an OSPF type 5 external route since this route was injected into OSPF through redistribution. Routers A and B are also learning the redistributed RIP routes as external type 5 OSPF routes. Routers A and C are learning the EIGRP routes that were redistributed by Router B.

 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      156.26.0.0/24 is subnetted, 2 subnets  O E2    156.26.32.0 [110/20] via 10.1.1.2, 00:02:29, Serial0/1   O E2    156.26.33.0 [110/20] via 10.1.1.2, 00:02:29, Serial0/1  172.16.0.0/24 is subnetted, 1 subnets  O E2    172.16.1.0 [110/20] via 10.1.1.2, 00:01:28, Serial0/1  142.1.0.0/32 is subnetted, 1 subnets  O E2    142.1.4.4 [110/20] via 10.1.1.2, 00:01:28, Serial0/1  10.0.0.0/30 is subnetted, 3 subnets  O E2    10.1.1.8 [110/20] via 10.1.1.2, 00:02:29, Serial0/1  C       10.1.1.0 is directly connected, Serial0/1  O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:02:30, Serial0/1  _______________________________________________________________________ rtrB#  show ip route  Codes:  C - conne  cted, 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 ty  pe 1, N2 - OSPF NSSA exter  nal 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  O E2    1.1.1.1 [110/20] via 10.1.1.1, 00:09:03, Serial0  2.0.0.0/32 is subnetted, 1 subnets C       2.2.2.2 is directly connected, Loopback0      156.26.0.0/24 is subnetted, 2 subnets  O E2    156.26.32.0 [110/20] via 10.1.1.6, 00:09:03, Serial1   O E2    156.26.33.0 [110/20] via 10.1.1.6, 00:09:03, Serial1  172.16.0.0/24 is subnetted, 1 subnets C       172.16.1.0 is directly connected, Ethernet0      142.1.0.0/32 is subnetted, 1 subnets  D       142.1.4.4 [90/409600] via 172.16.1.2, 00:09:41, Ethernet0  10.0.0.0/30 is subnetted, 3 subnets  O E2    10.1.1.8 [110/20] via 10.1.1.6, 00:09:03, Serial1  C       10.1.1.0 is directly connected, Serial0 C       10.1.1.4 is directly connected, Serial1 

Router C is learning the networks redistributed by Routers A and B as an OSPF external type 5 route.

 rtrC#  show ip route  Codes: C - connected, S - static, I - IGRP, R - R  IP, M - mobile, B  - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external ty  pe 1, N2 - OSPF NSSA exter  nal 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  O E2    1.1.1.1 [110/20] via 10.1.1.5, 00:12:02, Serial0  3.0.0.0/24 is subnetted, 1 subnets C       3.3.3.0 is directly connected, Loopback0      156.26.0.0/24 is subnetted, 2 subnets R       156.26.32.0 [120/1] via 10.1.1.9, 00:00:27, Serial1 R       156.26.33.0 [120/1] via 10.1.1.9, 00:00:27, Serial1      172.16.0.0/24 is subnetted, 1 subnets  O E2    172.16.1.0 [110/20] via 10.1.1.5, 00:10:43, Serial0  142.1.0.0/32 is subnetted, 1 subnets  O E2    142.1.4.4 [110/20] via 10.1.1.5, 00:10:43, Serial0  10.0.0.0/30 is subnetted, 3 subnets C       10.1.1.8 is directly connected, Serial1 O IA    10.1.1.0 [110/128] via 10.1.1.5, 00:12:03, Serial0 C       10.1.1.4 is directly connected, Serial0 

Now modify the OSPF configurations on Routers B and C in order to create the NSSA.

 Router B  router ospf 1    area 1 nssa    redistribute eigrp 1 subnets   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 1  _______________________________________________________________________ Router C  router ospf 1    area 1 nssa    redistribute rip subnets   network 10.1.1.4 0.0.0.3 area 1  
Verification

Verify that Area 1 has been configured as an NSSA.

 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 and autonomous system boundary router  Redistributing External Routes from,     eigrp 1, includes subnets in redistribution  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 7. Checksum Sum 0x3F1B4  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 2. 1 normal 0 stub 1 nssa  Area BACKBONE(0)         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 10 times         Area ranges are         Number of LSA 3. Checksum Sum 0x275D9         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 NSSA area   Perform type-7/type-5 LSA translation  Area has no authentication         SPF algorithm executed 23 times         Area ranges are         Number of LSA 9. Checksum Sum 0x4AE6A         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 

Now inspect the routing tables on Routers A, B, and C to view the effect of configuring Area 1 as an NSSA.

 rtrB#  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 O E2    1.1.1.1 [110/20] via 10.1.1.1, 00:02:19, Serial0      2.0.0.0/32 is subnetted, 1 subnets C       2.2.2.2 is directly connected, Loopback0      156.26.0.0/24 is subnetted, 2 subnets  O N2    156.26.32.0 [110/20] via 10.1.1.6, 00:02:20, Serial1   O N2    156.26.33.0 [110/20] via 10.1.1.6, 00:02:20, Serial1  172.16.0.0/24 is subnetted, 1 subnets C       172.16.1.0 is directly connected, Ethernet0      142.1.0.0/32 is subnetted, 1 subnets D       142.1.4.4 [90/409600] via 172.16.1.2, 00:18:28, Ethernet0      10.0.0.0/30 is subnetted, 3 subnets  O N2    10.1.1.8 [110/20] via 10.1.1.6, 00:02:20, Serial1  C       10.1.1.0 is directly connected, Serial0 C       10.1.1.4 is directly connected, Serial1 _______________________________________________________________________ rtrC#  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      3.0.0.0/24 is subnetted, 1 subnets C       3.3.3.0 is directly connected, Loopback0      156.26.0.0/24 is subnetted, 2 subnets R       156.26.32.0 [120/1] via 10.1.1.9, 00:00:06, Serial1 R       156.26.33.0 [120/1] via 10.1.1.9, 00:00:06, Serial1      172.16.0.0/24 is subnetted, 1 subnets  O N2    172.16.1.0 [110/20] via 10.1.1.5, 00:04:52, Serial0  142.1.0.0/32 is subnetted, 1 subnets  O N2    142.1.4.4 [110/20] via 10.1.1.5, 00:04:53, Serial0  10.0.0.0/30 is subnetted, 3 subnets C       10.1.1.8 is directly connected, Serial1 O IA    10.1.1.0 [110/128] via 10.1.1.5, 00:04:53, Serial0 C       10.1.1.4 is directly connected, Serial0 

The redistributed RIP and EIGRP routes have been converted from OSPF E2 routes to OSPF N2 routes. This means that the redistributed RIP and EIGRP routes are now being advertised as type 7 routes instead of type 5 routes. On Router A, these routes should still be OSPF type 5 routes. Router B is converting the external routes learned via Router C from type 7 to type 5. Router B is also advertising the redistributed EIGRP routes as type 5 into Area 0 and type 7 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      156.26.0.0/24 is subnetted, 2 subnets  O E2    156.26.32.0 [110/20] via 10.1.1.2, 00:06:14, Serial0/1   O E2    156.26.33.0 [110/20] via 10.1.1.2, 00:06:14, Serial0/1  172.16.0.0/24 is subnetted, 1 subnets  O E2    172.16.1.0 [110/20] via 10.1.1.2, 00:06:46, Serial0/1  142.1.0.0/32 is subnetted, 1 subnets  O E2    142.1.4.4 [110/20] via 10.1.1.2, 00:06:46, Serial0/1  10.0.0.0/30 is subnetted, 3 subnets  O E2    10.1.1.8 [110/20] via 10.1.1.2, 00:06:14, Serial0/1  C       10.1.1.0 is directly connected, Serial0/1 

The 1.1.1.1 route that Router A was advertising as an OSPF external type 5 route has been blocked from entering the NSSA area by Router B, but the inter-area routes have been permitted. Also, notice that unlike a stub or totally stubby area there is no default route advertised by the ABR or ASBR.

Finally, modify the configuration on Router B to prevent the redistributed EIGRP routes from being advertised into Area 1, but still allow them to be advertised into Area 0.

 Router B  router ospf 1    area 1 nssa no-redistribution    redistribute eigrp 1 subnets   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 1  

Verify that Router B is no longer advertising the redistributed EIGRP routes into Area 1.

 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 and autonomous system boundary router  Redistributing External Routes from,     eigrp 1, includes subnets in redistribution  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 7. Checksum Sum 0x3EFB5  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 2. 1 normal 0 stub 1 nssa  Area BACKBONE(0)         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 11 times         Area ranges are         Number of LSA 3. Checksum Sum 0x26FDC         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 NSSA area, no redistribution into this area  Perform type-7/type-5 LSA translation         Area has no authentication         SPF algorithm executed 26 times         Area ranges are         Number of LSA 7. Checksum Sum 0x31A46         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 _______________________________________________________________________ rtrC#  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      3.0.0.0/24 is subnetted, 1 subnets C       3.3.3.0 is directly connected, Loopback0      156.26.0.0/24 is subnetted, 2 subnets R       156.26.32.0 [120/1] via 10.1.1.9, 00:00:02, Serial1 R       156.26.33.0 [120/1] via 10.1.1.9, 00:00:02, Serial1      10.0.0.0/30 is subnetted, 3 subnets C       10.1.1.8 is directly connected, Serial1 O IA    10.1.1.0 [110/128] via 10.1.1.5, 00:04:37, Serial0 C       10.1.1.4 is directly connected, Serial0 

Verify that the redistributed EIGRP routes are being advertised into Area 0.

 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      156.26.0.0/24 is subnetted, 2 subnets O E2    156.26.32.0 [110/20] via 10.1.1.2, 00:02:45, Serial0/1 O E2    156.26.33.0 [110/20] via 10.1.1.2, 00:02:45, Serial0/1      172.16.0.0/24 is subnetted, 1 subnets  O E2    172.16.1.0 [110/20] via 10.1.1.2, 00:14:41, Serial0/1  142.1.0.0/32 is subnetted, 1 subnets  O E2    142.1.4.4 [110/20] via 10.1.1.2, 00:14:41, Serial0/1  10.0.0.0/30 is subnetted, 3 subnets O E2    10.1.1.8 [110/20] via 10.1.1.2, 00:02:45, Serial0/1 C       10.1.1.0 is directly connected, Serial0/1 O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:14:42, Serial0/1 
Troubleshooting
Step 1. Verify that there is a neighbor relationship between the OSPF routers by using the show ip ospf neighbor command.

Step 2. Verify that every router in the NSSA has been configured with the command area x nssa.

Step 3. The no-redistribute keyword should only be used on the ASBR performing the route redistribution.

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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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