Wednesday, April 4, 2012

The OSPF Totally Stubby Area

The Open Shortest Path First routing protocol is a critical piece of knowledge for any networking professional working in an enterprise environment. Most major networking certifications covering routing and switching including the Cisco Certified Network Associate (CCNA), Cisco Certified Network Professional (CCNP), and Cisco Certified Internetwork Expert (CCIE) extensively test OSPF knowledge and skills. OSPF is the most popular dynamic routing protocol used in complex enterprise networks. This example will demonstrate the concepts and configuration involved with setting up a totally stubby OSPF area. This lab is built using Dynamips/GNS3 utilizing Cisco c3725 routers.

The OSPF implementation in Cisco IOS recognizes 5 main classifications of areas:
Let's start out with a simple true/false question:

True/false: The "totally stubby" area exists as part of the OSPFv2 standards.

The answer is false. The totally stubby area is not part of the RFCs that describe the OSPFv2 protocol; however many vendors including Microsoft, Cisco, Juniper, and others have implemented it and consider it a standard feature, even though it is not part of the Internet Engineering Task Force (IETF) standards. The same is true for the "totally NSSA," but I'll describe that area type and its configuration in a separate post. So, before we get started, you should already have knowledge of the OSPF stub area and the fact that it does not create nor propagate type 4 (ASBR Summary) or type 5 (external LSAs) and internal routers to stub areas have inter-area default routes that are originated by the area border routers (ABRs).

Totally stubby areas also filter type 3 (network summary) LSAs. In this case, routers that are totally internal to the totally stubby area have default routes for all networks that are outside of the area, including those that are in the same OSPF routing domain. Within the area, type 1 and type 2 LSAs are still propagated to describe the detailed topology of the totally stubby area, and a single type 3 LSA for the default route exists in the area. Area border routers still create type 3 LSAs describing the networks that are reachable in the totally stubby area and propagate these into the backbone area. Let's look at a similar topology to the stub area example, but in this case area 1 will be a totally stubby area:



Once again, the only interesting pieces of the configuration are on the ABR. In addition to the area stub command, the no-summary option needs to be specified for area 1:

router ospf 1
 log-adjacency-changes
 area 1 stub no-summary
 network 10.0.1.0 0.0.0.255 area 0
 network 192.168.0.0 0.0.0.255 area 1
!

The routing table on Area1 shows the routes that are internal to area 1 and shows the default route propagated by the ABR:

Area1#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
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.0.1 to network 0.0.0.0

C    192.168.0.0/24 is directly connected, Serial0/0
C    192.168.1.0/24 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/65] via 192.168.0.1, 00:07:38, Serial0/0

The database is also simple, in this case we have no broadcast networks, so there are no type 2 LSAs. There are type 1 LSAs describing the routers and their link states and the single type-3 LSA for the default route:

Area1#show ip ospf database

            OSPF Router with ID (192.168.1.1) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
192.168.0.1     192.168.0.1     646         0x80000027 0x00A4C6 2
192.168.1.1     192.168.1.1     634         0x80000023 0x00BA38 3

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         192.168.0.1     651         0x80000025 0x005758

Looking on Area0, we still see the type 3 LSAs created by ABR for the routes in area 1 and we see the routes for area 1 present in the routing table.

Area0#show ip ospf database

            OSPF Router with ID (10.0.1.1) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         786         0x80000022 0x005206 2
10.0.1.1        10.0.1.1        785         0x80000029 0x008D9C 4
192.168.0.1     192.168.0.1     741         0x80000025 0x00F78E 2

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.0     192.168.0.1     741         0x8000001F 0x000CFE
192.168.1.0     192.168.0.1     726         0x80000001 0x0047DF

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
1.1.1.0         1.1.1.1         795         0x80000024 0x0082A3 0
10.0.0.0        1.1.1.1         794         0x80000002 0x0068D8 0

Area0#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
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/24 is subnetted, 1 subnets
O E2    1.1.1.0 [110/100] via 10.0.0.1, 00:13:53, Serial0/0
     10.0.0.0/24 is subnetted, 2 subnets
C       10.0.0.0 is directly connected, Serial0/0
C       10.0.1.0 is directly connected, Serial0/1
O IA 192.168.0.0/24 [110/128] via 10.0.1.2, 00:13:12, Serial0/1
O IA 192.168.1.0/24 [110/129] via 10.0.1.2, 00:13:02, Serial0/1

Totally stubby areas in OSPF are useful when there is no reason to have anything besides a default route propagated into an area. This can be used effectively for hub and spoke topologies or areas where redundant ABRs have equal cost paths to other parts of the network.

See Also
The Road to the CCIE

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