The Fundamentals of OSPF Routing Design | OSPF Network Design Solutions

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Stub Areas

A stub area in OSPF is an area that carries a default route, and inter-area routes but does not carry any external routes. Stub areas reduce network overhead by placing sections of the network into dead end areas, also known as stub areas. This reduces the routes being advertised across the network.

Because default routing is used, the LSDB is reduced in size, which in turn also reduces the load being placed on the router’s CPU and memory. Routing updates are also reduced because specific link flaps will not be injected across the network; instead, they are confined to the area or they don’t even enter the area, depending on where they occurred.

There are three different types of stub areas: normal stub, totally stubby area (TSA), and NSSAs. Each stub area and the corresponding characteristics will be discussed in the sections that follow.

Stub Area Design Golden Rules

Many stub area design rules are in place because a stub area is designed and configured not to carry external routers. If a situation occurred within a stub area that caused external links to be injected into the area, then its usefulness is ruined. The following are the rules:

• Only a single ABR can be in a stub area, but if there is more than one, then accept non-optimal routing paths.

• No ASBRs can be within a stub area.

• No virtual links area is allowed.

• All routers within any type of stub area must be configured to recognize their location (that is, what area they are in and any specific OSPF settings for that area). If the routers do not all agree on their location, then they will not become neighbors and routing will not take effect.

• The backbone area cannot be configured as a stub area.

Normal Stub Areas

The configuration command area # stub turns on stub area routing. External routes being advertised into OSPF must be done via the summary-address command; this is typically done at the ASBR.

Normal stub areas only block external routes; however, they do allow summary routes. For example, LSA Types 1-4 are allowed and 5-7 are blocked. This is the difference between normal stub areas and the other types of stub areas.

The command that configures an area as stub is as follows:

    area <area-id> stub [no-summary]

The command that configures a default-cost into an area is as follows:

    area area-id default-cost cost

If the cost is not set using the area area-id default-cost cost command, a cost of one will be advertised by the ABR. Figure 5-11 shows a very good example of stub areas. In the examples that follow, the router configuration files will be presented based upon the setup in Figure 5-11.

Figure 5-11 Configuring an OSPF area as a stub area.

Assume that area 2 is to be configured as a stub area. The following examples show the routing table of RTE before and after configuring area 2 as a stub area.

Before Becoming a Stub Area

    RTC#    interface Ethernet 0    ip address 203.250.14.1 255.255.255.0    interface Serial1    ip address 203.250.15.1 255.255.255.252    router ospf 10    network 203.250.15.0 0.0.0.255 area 2    network 203.250.14.0 0.0.0.255 area 0    RTE#sh 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    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    Gateway of last resort is not set    203.250.15.0 255.255.255.252 is subnetted, 1 subnets    C 203.250.15.0 is directly connected, Serial0    O IA 203.250.14.0 [110/74] via 203.250.15.1, 00:06:31, Serial0    128.213.0.0 is variably subnetted, 2 subnets, 2 masks    O E2 128.213.64.0 255.255.192.0    [110/10] via 203.250.15.1, 00:00:29, Serial0    O IA 128.213.63.0 255.255.255.252    [110/84] via 203.250.15.1, 00:03:57, Serial0    131.108.0.0 255.255.255.240 is subnetted, 1 subnets    O 131.108.79.208 [110/74] via 203.250.15.1, 00:00:10, Serial0

RTE has learned the inter-area routes (O IA) 203.250.14.0 and 128.213.63.0, and it has learned the intra-area route (O) 131.108.79.208 and the external route (O E2) 128.213.64.0. If you configure area 2 as stub, you need to do the following:

After Becoming a Stub Area

    RTC#    interface Ethernet 0    ip address 203.250.14.1 255.255.255.0    interface Serial1    ip address 203.250.15.1 255.255.255.252    router ospf 10    network 203.250.15.0 0.0.0.255 area 2    network 203.250.14.0 0.0.0.255 area 0    area 2 stub    RTE#    interface Ethernet0    ip address 203.250.14.2 255.255.255.0    interface Ethernet1    ip address 131.108.79.209 255.255.255.240    interface Serial1    ip address 203.250.15.1 255.255.255.252    router ospf 10    network 203.250.15.0 0.0.0.255 area 2    network 203.250.14.0 0.0.0.255 area 0    network 131.108.0.0 0.0.255.255 area 2    area 2 stub

Notes:
Note that the stub command is configured on RTE also; otherwise, RTE will never become a neighbor to RTC. The default cost was not set, so RTC will advertise 0.0.0.0 to RTE with a metric of 1.

    RTE#sh 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    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    Gateway of last resort is 203.250.15.1 to network 0.0.0.0    203.250.15.0 255.255.255.252 is subnetted, 1 subnets    C 203.250.15.0 is directly connected, Serial0    O IA 203.250.14.0 [110/74] via 203.250.15.1, 00:26:58, Serial0    128.213.0.0 255.255.255.252 is subnetted, 1 subnets    O IA 128.213.63.0 [110/84] via 203.250.15.1, 00:26:59, Serial0    131.108.0.0 255.255.255.240 is subnetted, 1 subnets    O 131.108.79.208 [110/74] via 203.250.15.1, 00:26:59, Serial0    O*IA 0.0.0.0 0.0.0.0 [110/65] via 203.250.15.1, 00:26:59, Serial0

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