Configuring OSPF Stub Areas

Stub Area’s are another way of building a scalable OSPF network. Stub Area’s support multiple features that can reduce router resources and simplify configurations. This lab will discuss and demonstrate the configuration and verification of OSPF stub areas.

Real World Application & Core Knowledge

In Lab 9-1 – Configuring Basic OSPF you learned about Stub area’s and their purpose and how they operate. In this lab you will review those concepts and put the technology to use.

Think of Stub area’s in OSPF a way to simplify an area so that a particular area does not need the full routing table; in which case this conserves router resources. Show below is a list of the different type of stub area’s and their properties;

Stub Area – An area that has a single exit point and blocks type 5/7 LSA types and receives type 3/4 LSA’s with a default route (0.0.0.0/0) This type of stub area is an IETF standard. To configure an Area as a stub you’d execute the area # stub in OSPF router configuration mode on the ABR.

Not-So-Stubby-Area (NSSA) – This area allows a stub area to have characteristics of a stub and non stub. External routes advertised into the OSPF autonomous system by am NSSA advertising an LSA type 7 which is translated at the ABR to type 5 and forwarded into the OSPF backbone. This type of stub area is an IETF standard. To configure an area as a NSSA you’d execute the area # nssa in OSPF router configuration mode on the ABR.

Totally Stubby Area – Permits type 1 and 2 LSA’s while blocking types 3*/4/5/7 LSA’s. *TSA’s receive a single type 3 LSA containing a default route to the ABR. This type of stub area is an extension to OSPF created by Cisco. To configure an area as a totally stubby area you’d execute the area # stub no-summary in OSPF router configuration mode on the ABR.

Totally NSSA – Is an area that permits LSA’s 1, 2 and 7 while blocking 3 4 and 5. This stub area receives a default route from the ABR using a type 3 LSA. This type of stub area is an extension to OSPF created by Cisco. To configure an area as a not so totally stubby area area you’d execute the area # nssa no-summary in OSPF router configuration mode on the ABR.

When configuring an area stub type the command is executed on the ABR, however when you specify an area as a stub on the ABR, all routers in that area must have be configured as a stub by using the area # stub

In this lab you will configure and verify area 3 as a stub area and totally stubby area.

Familiarize yourself with the following new command(s);

Command Description
area # stub This command is executed in OSPF configuration mode to configure a specific area in OSPF as a stub. All routers in a stub area must have the stub area flag set. This means that all routers in the area must be configure with this command if the ABR has the area configured as a stub.
area # nssa This command is executed in OSPF configuration mode on the ABR to specify an area as a not so stubby area. This type of area allows for redistributed routes as a type 7 lsa which is translated to a type 5 at the ABR before being propagated through out the autonomous system.
area # stub no-summary This command is executed in OSPF configuration mode on the ABR to specify an area as a totally stubby area which only allows type 1 and 2 LSA’s and a single type 3 LSA (default route) from the ABR. all other LSA’s are blocked.
area # nssa no-summary This command is executed in OSPF configuration mode on the ABR in OSPF configuration mode to specify a specific area as a not so totally stubby area which blocks types 3 4 and 5 LSA’s but allows a single type 3 LSA as the default route and type 7 LSA’s internal to the area.

The following logical topology shown below is used in labs found through out Section 9 – Configuring OSPF;

Lab Prerequisites

  • If you are using GNS3 than load the Free CCNA Workbook GNS3 topology than start devices; R1, R2, R3, R4, R5 and SW1.
  • Establish a console session with devices R1, R2, R3, R4, R5 and SW1 than load the initial configurations provided below by copying the config from the textbox and pasting it into the respected routers console.

 

Lab Objectives

  • Configure Area 3 as a stub area. Afterward; verify the routing table on R3.
  • Configure Area 3 as a totally stubby area. Afterward; verify the routing table on R3.

Lab Instruction

Objective 1. – Configure Area 3 as a stub area. Afterward; verify the routing table on R3.

To complete this objective you must specify Area 3 as a stub area on the ABR and all routers that participate in area 3 as shown below;

R2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#router ospf 1
R2(config-router)#area 3 stub
R2(config-router)#end
R2#
%SYS-5-CONFIG_I: Configured from console by console
%OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0/2 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
R2#

When configuring the are Area Border Router you’ll notice the neighbor relationship will drop as shown above due to the stub flag not matching in the hello packets. However when you configure the neighboring router in area 3 (R3) as a stub area the neighbor relationship will rebuild as shown below;

R3#
%OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0/1 from FULL to DOWN, Neighbor Down: Dead timer expired
R3#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#router ospf 1
R3(config-router)#area 3 stub
R3(config-router)#end
R3#
%SYS-5-CONFIG_I: Configured from console by console
%OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0/1 from LOADING
to FULL, Loading Done
R3#

Shown below is the routing table of R3 to verify that R3 is indeed receiving the correct default route from the ABR.

R3#show ip route
Codes: C - connected, S - static, 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
       * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 10.90.23.1 to network 0.0.0.0

     10.0.0.0/8 is variably subnetted, 10 subnets, 4 masks
O IA    10.90.50.1/32 [110/129] via 10.90.23.1, 00:02:40, Serial0/1
O IA    10.90.40.1/32 [110/129] via 10.90.23.1, 00:02:40, Serial0/1
C       10.90.23.0/30 is directly connected, Serial0/1
C       10.90.23.1/32 is directly connected, Serial0/1
C       10.90.30.0/24 is directly connected, Loopback0
O IA    10.90.145.0/24 [110/129] via 10.90.23.1, 00:02:40, Serial0/1
O IA    10.90.45.0/30 [110/192] via 10.90.23.1, 00:02:40, Serial0/1
O IA    10.90.20.1/32 [110/65] via 10.90.23.1, 00:02:40, Serial0/1
O IA    10.90.10.1/32 [110/129] via 10.90.23.1, 00:02:40, Serial0/1
O IA    10.90.245.0/29 [110/128] via 10.90.23.1, 00:02:40, Serial0/1
O*IA 0.0.0.0/0 [110/65] via 10.90.23.1, 00:02:40, Serial0/1
R3#

You can also verify that Area 3 is a stub area by using the show ip ospf command on R2 or R3 as this will explicitly tell you rather or not a specific area is a stub area as shown below;

R3#show ip ospf
 Routing Process "ospf 1" with ID 3.3.3.3
 Start time: 00:03:08.388, Time elapsed: 01:33:52.844
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 5000 msecs
 Minimum hold time between two consecutive SPFs 10000 msecs
 Maximum wait time between two consecutive SPFs 10000 msecs
 Incremental-SPF disabled
 Minimum LSA interval 5 secs
 Minimum LSA arrival 1000 msecs
 LSA group pacing timer 240 secs
 Interface flood pacing timer 33 msecs
 Retransmission pacing timer 66 msecs
 Number of external LSA 0. Checksum Sum 0x000000
 Number of opaque AS LSA 0. Checksum Sum 0x000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 0 normal 1 stub 0 nssa
 Number of areas transit capable is 0
 External flood list length 0
 IETF NSF helper support enabled
 Cisco NSF helper support enabled
    Area 3
        Number of interfaces in this area is 2 (1 loopback)
        It is a stub area
        Area has no authentication
        SPF algorithm last executed 00:08:19.176 ago
        SPF algorithm executed 6 times
        Area ranges are
        Number of LSA 10. Checksum Sum 0x04EE8E
        Number of opaque link LSA 0. Checksum Sum 0x000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0

R3#

Objective 2. – Configure Area 3 as a totally stubby area. Afterward; verify the routing table on R3.

To configure Area 3 as a totally stubby area you only need to change the stub type on the ABR now as R3 already has area 3 specified as a stub due to the previous objective.

R2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#router ospf 1
R2(config-router)#area 3 stub no-summary
R2(config-router)#end
R2#

Shown below is the routing table of R3, you’ll notice that R3 is now only receiving a default route from R2 (The ABR) as the ABR is treating area 3 as a totally stubby area, it is only advertising a single type 3 LSA which is the default route shown in the routing table as O*IA 0.0.0.0/0 and blocking type 3 and 4 LSA’s which are the other area routes which would typically be O*IA routes as shown in verification of Objective 1.

R3#show ip route
Codes: C - connected, S - static, 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
       * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 10.90.23.1 to network 0.0.0.0

     10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
C       10.90.23.0/30 is directly connected, Serial0/1
C       10.90.23.1/32 is directly connected, Serial0/1
C       10.90.30.0/24 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/65] via 10.90.23.1, 00:00:29, Serial0/1
R3#
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