OSPF Special Area Types

• Standard Area
  • Areas that can accept intra-area, inter-area, and external routes.
  • In other words, the accept link updates, route summaries, and routes from other AS.
• Backbone (transit) Area
  • The central area to which all other areas connect.
  • Labeled as Area 0
  • Has all the properties of a standard area
• Stub Area
  • Does not accept routes belonging to other AS.
  • Theses areas do, however, have inter-area and intra-area routes within the same AS.
  • In order to reach the outside networks, the routers in the stub area use a default route which is injected into the area by the ABR.
  • A typical application of this area would be a situation in which a branch office does not need to know the routes to every other offices, but instead uses a default route to the central office where it can reach other destinations from there.
  • Cannot contain ASBRs (except when the ABR is also the ASBR).
• Totally Stubby Area
  • Only allows intra-area (within the same area) routes and default routes injected to the area.
  • In other words, it does not allow external autonomous system routes or summary routes from other areas.
  • It uses a default route to send packets to networks external to the area.
  • Cannot contain ASBRs (except when the ABR is also the ASBR)
• NSSA
  • Allows the flexibility of importing a few external routes into the area while still trying to retain the stub characteristic.
  • An example would be a router that is connected to an external AS which is running a different routing protocol – RIP, maybe. The router is now considered an ASBR. If that router is configured as an NSSA, the NSSA ASBR can start generating a special LSA type 7. These type-7 LSAs are flooded into the NSSA area. When the LSAs hit the NSSA ABR, they are converted to type-5 LSAs and are then flooded throughout the OSPF domain.

Characteristics that qualifies an area as stub or totally stubby:

• Single point of exit from the stub area. If there are multiple exits, one or more ABRs inject a default route into the stub area and suboptimal routing paths are acceptable.
• All OSPF routers inside the stub area, including ABRs, must be configured as stub routers. Recall that matching area flag is one of the condition for neighbor relationship. When the routers are configured as stub, all stub routers set a flag (the E-bit) in their Hello packets to zero.
• Virtual links cannot be configured within, nor transit, a stub area
• No ASBR is inside the stub area. ASBRs produce type 5 LSAs and type 5 LSAs cannot exist within a stub area.
• The area is not the backbone area (Area 0).

Configure Stub Areas

• Stub Areas reduce the size of the LSDB inside an area, which results in less memory used.
• Since the area also receives less routing updates, the SPF algorithm also runs less frequently
• Type 5 External network LSAs, such as those redistributed from other routing protocols into OSPF, are not permitted to flood into a stub area.
• A single default route (to 0.0.0.0) replaces many external routes.
• To configure, use the following router configuration command to all routers within the area:

area area-id stub

• area-id identifies the stub area. It can be a decimal value or a value in dotted-decimal format, like an IP address.

• By default, the ABR of a stub or totally stubby area advertises a default route with a cost of 1.
• To change the cost default route, use the following router configuration command:

area area-id default-cost cost

• The parameters are:
  • area-id - identifies the stub, totally stubby, or NSSA. Can be a decimal or dotted-decimal value.
  • cost – cost of the route summary. Values range from 0 – 16777215.

Figure 1: Example OSPF Stub Area Configuration

R3 Stub Configuration on Figure 1:

R3(config)#int fa0/0
R3(config-if)#ip address 192.168.1.2 255.255.255.0
R3(config)#int s1/0
R3(config-if)#ip address 192.168.2.1 255.255.255.0

!

R3(config)#router ospf 100
R3(config-router)#network 192.168.1.0 0.0.0.255 area 0
R3(config-router)#network 192.168.2.0 0.0.0.255 area 2
R3(config-router)#area 2 stub

R4 Stub Configuration on Figure 1:

R4(config)#int s1/0
R4(config-if)#ip address 192.168.2.2 255.255.255.0

!

R4(config)#router ospf 100
R4(config-router)#network 192.168.2.0 0.0.0.255 area 2
R4(config-router)#area 2 stub

• area 2 stub of the last line of each configuration defines the stub.
• Each router in the stub area must be configured with the area stub command in order to form neighborhood relationships.

Figure 2A and 2B: Routing Table for Standard Area (No Stub Area)

• In a standard OSPF area configuration, notice that in both routers R3 and R4, the route to the RIP network 10.10.10.0 is advertised as a Type 2 External route (E2)
• Any inter-area routes are advertised as IA.

Figure 3A nd 3B: Routing Table in a Stub Area

• After the area 2 stub command was configured on R3 and R4, notice that R3 (which is the ABR) still shows the route to the RIP network as a type 2 External route (E2). That is to be expected because at the ABR is where the filtering of the external route happens.
• Pay particular attention to the route designate with O*IA. Notice how the previous advertisement of the external route is replaced with this inter-area default route. The ABR filtered the type 5 LSA and injected a default route instead.

Figure 4A and 4B: OSPF Database on Router 4 Showing Standard and Stub Area Differences

• In the first figure above, you can see the OSPF database information of R4, configured in a standard OSPF area.
  • Notice the different kinds of LSAs advertised into it:
    • Type 1 – Router Link States.
    • Type 3 – Summary Net Link States.
    • Type 4 – Summary ASB Link States.
    • Type 5 – AS External Link LSA.
• The second figure, after area 2 stub command was configured show only the Type 1 LSAs and the Summary LSA for the default route (0.0.0.0).

Configure Totally Stubby Area

• A totally stubby area is cisco-proprietary.
• Whereas the stubby area blocks external type 5 LSAs, a totally stubby area blocks type 5 LSAs as well as Type 3 and Type 4 LSAs.
  • Totally stubby areas, therefore, only recognizes intra-area routes and the default route 0.0.0.0.
• The ABR injects the default summary link 0.0.0.0 into the totally stubby area.
  • Each router picks the closest ABR as a gateway to everything outside the area.

• To configure, use the router configuration command:

area area-id stub

• At the ABR only, configure:

area area-id stub no-summary

• The no-summary parameter stops summary LSAs (Type 3 and 4), in addition to external LSAs, from flooding into the totally stubby area.

Example

• Using figure 1 as the topology, the only difference with the configuration of a stub area and totally stubby area is the addition of the no-summary parameter from the area stub command on the ABR.

R3(config-router)#area 2 stub no-summary

• Examine the differences between the routing table R4 configured in a stubby area and a totally stubby area:

Figure 5A and 5B: R4′s Routing Table for Stubby Area and Totally Stubby Area

• In the first figure above the stub area ip route for R4 shows a default route (0.0.0.0) being advertised as well as and Inter-area (IA) route to a network in another area.
• The second figure, only the default route is advertised.
• Next, we compare the OSPF database for a stub area and totally stubby area:

Figure 6A and 6B: R4′s OSPF Database

• Notice on the last screenshot that, other than Type 1 or 2 LSA, there is only a single Type 3 LSA in the OSPF database.
  • This one, single, Type 3 LSA will be the only other LSA that will ever be advertised in a totally stubby area. And it is used to carry the default route information.

Configure NSSA (Not-So-Stubby Areas)

• A non-proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area.
• It is described in RFC 3101.
• The NSSA “bends” a rule of the stub area – the rule that says that there cannot be an ASBR inside of a stub area.
  • It allows an ASBR to be present in the stub, while at the same time performs an essential function of injecting a default route into the NSSA instead of the external routes advertised by ASBRs.
• Redistribution into an NSSA creates a special type of LSA known as Type 7. This only exists in an NSSA area.
  • An NSSA ASBR generates the Type 7 LSA, and an NSSA ABR translates it into a Type 5 LSA, which gets propagated into the OSPF domain.
• To configure an NSSA, use the router configuration command to all routers in the NSSA:

area area-id nssa [no-redistribution] [default-information-originate] [metric metric-value] [metric-type type-value] [no-summary]

• The parameters are:
  • area-id - A decimal or dotted-decimal value that identifies the NSSA.
  • no-redistribution - (Optional) Used when the router is an NSSA ABR and you want the redistribute command to import routes only into the standard areas, but not into the NSSA area.
  • default-information-originate – (Optional) Used to generate a type 7 default LSA into the NSSA area. The keyword takes effect only on an NSSA ABR or an NSSA ASBR.
  • metric metric-value – (Optional) Metric used for generating the default route. Values can be from 0 – 16777214.
  • metric-type type-value - (Optional) OSPF metric type for default routes. Could be one of two values:
    1. Type 1 external route
    2. Type 2 external route
  • no-summary – (Optional) Allows an area to be an NSSA but not have summary routes injected into it. Thus, the area is a totally stubby NSSA.

Figure 7:Example OSPF NSSA Configuration

R2 NSSA OSPF Configuration

R2(config)#router rip
R2(config-router)#redistribute ospf 100 metric 5
R2(config-router)#network 10.0.0.0
!
R2(config)#router ospf 100
R2(config-router)#redistribute rip subnets
R2(config-router)#network 192.168.10.0 0.0.0.255 area 1
R2(config-router)#default-metric 150
R2(config-router)#area 1 nssa

R3 NSSA OSPF Configuration

R3(config)#router ospf 100
R3(config-router)#summary-address 10.0.0.0 255.0.0.0
R3(config-router)#network 172.17.0.0 0.0.255.255 area 0
R3(config-router)#network 192.168.10.0 0.0.0.255 area 1
R3(config-router)#area 1 nssa default-information-originate

• In the example above R2 is the ASBR that redistributes RIP routes into area 1, the NSSA.
• R3 is the NSSA ABR
  • This router converts type 7 LSAs into type 5 LSAs for advertisement into backbone area 0
  • R3 is also configured to summarize (summary-address) the type 5 LSAs that original from the RIP network.
    • 10.0.0.0 networks are summarized to 10.0.0.0/8 and are advertised into area 0
  • To cause R3 to generate an O*N2 default route (O*N2 0.0.0.0/0) into the NSSA, the default-information-originate parameter is used on the area area-id nssa command on R3.

Figure 8: Shows Type 7 LSAs in R3′s OSPF Database

Figure 9: Shows the R4′s Summarized  Type 5 LSA (O E2), originated from the RIP network

Figure 10: R2′s Routing Table Showing the Default Route Originated by the default-information-originate Command

NSSA Totally Stubby Configuration

R3(config)#router ospf 100
R3(config-router)#summary-address 10.0.0.0 255.0.0.0
R3(config-router)#network 172.17.0.0 0.0.255.255 area 0
R3(config-router)#network 192.168.10.0 0.0.0.255 area 1
R3(config-router)#area 1 nssa no-summary

• The no-summary parameter works exactly the same as the totally stubby technique.
• A single default route replaces both inbound external (type 5) LSA ans summary (type 3 and 4) LSAs into the area.

Figure 11: Default Route advertised to R2

• The flooding of Type 3 and Type 4 LSAs into the NSSA is blocked. Instead a default route is injected into the NSSA totally stub area as a type 3 summary LSA.

Resources:

1. What Are OSPF Areas and Virtual Links?
2. OSPF Area Types – Packetlife.net
3. OSPF Not-So-Stubby Area (NSSA)
4. How Does OSPF Generate Default Routes?
5. How OSPF Injects a Default Route into a Normal Area
6. How OSPF Injects a Default Route into a Stub or Totally Stub Area
7. How OSPF Injects a Default Route into a Not So Stubby Area
8. RFC 3101 – The OSPF Not-So-Stubby Area (NSSA) Option

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.