OSPF Areas

OSPF provides two levels of hierarchy throughout an area. An area is a 32-bit number that can be defined either in an IP address format of "Area 0.0.0.0" or as a decimal number format, such as "Area 0." Area 0 is a backbone area, which is required if more than one area is configured. All areas must be connected to Area 0; otherwise , virtual links are needed, as shown in Figure 8-18.

Figure 8-18. Using a Virtual Link Where an Area Is Not Attached to the Backbone

Example 8-7 shows the configuration required for a virtual link between Router E and Router B. Area 2 is the transit area between Routers E and B. Router E will form a virtual link with Router B's router ID, and vice versa. It is recommended that you use a loopback IP address as a router ID because loopback links always stay up; therefore, the virtual link will stay up.

Example 8-7 Configuring the Virtual Link Between Routers E and B

 RouterE#  router ospf 1    area 2 virtual-link 141.108.1.1   _____________________________________________________________________________________ RouterB#  router ospf area 2 virtual-link 141.108.1.21    area 2 virtual-link 141.108.1.21   

A virtual link itself is not a bad thing. The bad design would include an area that is not connected to Area 0, as shown in Figure 8-18, and then patching it up with a virtual link. Virtual links can be very useful in several scenarios. Figure 8-19 shows an example in which a virtual link can be used as a backup and for redundancy ‚ in case the link between routers A and B goes down, the Area 3 connectivity will not be broken. Also, if the link between Routers C and D goes down, the backbone remains contiguous through the virtual link.

Figure 8-19. Using a Virtual Link as a Backup

Example 8-8 shows the configuration of Routers A, B, C, and D. Routers A and D form a virtual link between each other with transit Area 2, and Router C and D form a virtual link with transit Area 1 between them. The virtual link between Routers A and B is to back up Area 3 connectivity; the virtual link between routers C and D is to back up Area 0 if the link between Routers E and F fails.

Example 8-8 Configuring the Virtual Link Between Routers A, B, C, and D

 RouterA#  router ospf 1   area 2 virtual-link 141.108.1.2  _____________________________________________________________________________________ RouterB#  router ospf 1   area 2 virtual-link 141.108.1.1  _____________________________________________________________________________________ RouterC#  router ospf 1   area 1 virtual-link 141.108.1.4  _____________________________________________________________________________________ RouterD#  router ospf 1   area 1 virtual-link 141.108.1.3  

Figure 8-20 shows another example in which a virtual link can be useful for optimal routing. If the link between Routers B and C is put in Area 1, Area 0 will suffer from suboptimal routing. If that link is put into Area 0, area 1 will suffer from suboptimal routing. So, the solution is to put that link in Area 1 and then configure a virtual link between Routers B and C. This way, it will carry both the backbone and Area 1 traffic.

Figure 8-20. Using a Virtual Link for Path Optimization

Example 8-9 shows the configuration that is required to form a virtual link between Routers B and C. This virtual link is needed for path optimization.

Example 8-9 Configuring Routers B and C to Form a Virtual Link for Path Optimization

 RouterB#  router ospf 1   area 1 virtual-link 141.108.1.3  _____________________________________________________________________________________ RouterC#  router ospf 1   area 1 virtual-link 141.108.1.2  

OSPF has several types of areas, which can be defined according to the needs of a network:

• Normal area

• Stub area

• Totally stubby area

• Not-so-stubby area (NSSA)

• Totally not-so-stubby area

The sections that follow cover the different OSPF areas in greater detail.

Normal Areas

When the area is defined by default, it is considered a normal or regular area. Normal areas have the following characteristics:

• Summary LSAs from other areas are injected.

• External LSAs are injected.

• External default LSAs can be injected.

In Figure 8-21, Area 1 and Area 2 are normal areas. IGRP routes are redistributed into Area 1, and RIP routes are redistributed into Area 2.

Figure 8-21. OSPF Normal Area Example

Stub Areas

In stub areas, no external LSAs are allowed. Recall the Options field in OSPF Hello packet. One of the bits in that option field is the E bit. In cases of stub areas, the E bit is clear, indicating that the area is incapable of importing any external LSAs.

Stub areas have the following characteristics:

• Summary LSAs from other areas are injected.

• The default route is injected as a summary route.

• External LSAs are not injected.

In Figure 8-22, Area 1 is defined as a stub area. No redistribution can take place at Routers I, H, or G because no Type 5 LSAs are allowed by stub areas. Also, RIP routes that are injected at Router E as OSPF externals are blocked at Router F; however, Area 1 still receives the summary route created for Area 2 by Router F (ABR). A default summary LSA also will be injected by the ABR (Router F) into Area 1. This means that if Routers I, H, or G need to send a packet to external destination, they will always forward the packet to the nearest ABR, which is Router F in this case.

Figure 8-22. Stub Area Example

Example 8-10 shows the configuration required to make Area 1 a stub area. This stub configuration must be done on all the routers in Area 1.

Example 8-10 Configuring Area 1 as a Stub Area

 RouterF#  router ospf 1   area 1 stub  

Totally Stubby Areas

Totally stubby areas are the most restricted form of area. Routers in this type of area rely on only the injection of a default summary route from the ABR. No other external or summary information is included in the routing table. This is an extension to the stub area, so all the characteristics are still true for this area. This area has the following characteristics:

• No summary LSAs are allowed.

• No external LSAs are allowed.

• The default route is injected as a summary route.

In Figure 8-13, Area 1 will not receive any summary route or any external routes. The only routes that Area 1 will have are the intra-area (marked with O in the routing table) routes for Area 1 and the default route injected by the ABR, which is marked with O IA.

Example 8-11 shows the configuration required on the ABR to make Area 1 a totally stubby area. Note that the difference between the stubby area and the totally stubby area is that no summary LSA is generated into a totally stubby area. Because summary LSA generation takes place only at the ABR, the configuration change needs to happen only at the ABR. All other routers that are configured with a stub option do not require any change in the con-figuration. The keyword no-summary here means to avoid sending any summary LSAs in Area 1.

Example 8-11 Configuring the ABR (Router F) to Make Area 1 Totally Stubby

 RouterF#  router ospf 1   area 1 stub no-summary  

Not-So-Stubby Areas

This is also an extension of the stub area. Suppose in Figure 8-12 that Area 1 is defined as a stub area and there is a requirement of redistribution of an IGRP route into that area. If Area 1 were defined as stub, this would not be possible. To redistribute an IGRP route into Area 1, Area 1 must be changed into an NSSA. When Area 1 is changed into an NSSA, it will allow redistribution and then IGRP routes can be redistributed into the NSSA area as Type 7 LSAs.

NSSAs were created to inject external routes from stub areas into the OSPF domain. In the NSSA, when the ASBR injects a route into the AS, it generates a Type 7 LSA. The ABR translates this LSA to a Type 5 LSA, which is propagated to the rest of the autono-mous system. The Type 7 LSA flooding scope is within the NSSA area.

NSSA is supported starting in Cisco IOS Software Release 11.2. NSSAs have the following characteristics:

• Type 7 LSAs carry external information within an NSSA.

• Type 7 LSAs are converted into Type 5 LSAs at the NSSA ABR.

• No external LSA are allowed.

• Summary LSAs are injected.

Because this is an extension of a stub area, RIP routes are not injected into Area 1 as OSPF external routes; IGRP routes, however, get converted into Type 7 LSAs.

Example 8-12 shows the configuration example for an NSSA area. The keyword nssa must be typed on all the routers that are part of Area 1, as shown Figure 8-21

Example 8-12 Configuring an NSSA on All the Routers in the NSSA Area

 RouterF#  router ospf 1    area 1 nssa   

Type 7 LSAs

The packet format for Type 7 LSA is very similar to that of Type 5. The three main differences are as follows :

• The Type field contains the value of 7 instead of 5, indicating its Type 7 LSA.

• The forwarding address is calculated as follows:

  If the route has a next-hop address (not true for connected routes), try to use it. This is possible only if the route is an OSPF internal route. Everything that was explained in the Type 5 forwarding address selection also holds true for Type 7 LSAs. If any of the conditions explained in the Type 5 forwarding address selection criteria is not true, the next hop will not be used as a forwarding address. The following two rules apply in that case:

  - Use one of the loopback addresses (if it's up and OSPF is running) in the area that is announcing LSAs.

  - If no loopback addresses are configured, use the address of the first interface in that area.

• The P bit is explained in the following example.

NSSA LSA Example

Example 8-13 shows the output of the NSSA LSA from Figure 8-23. Router I is the NSSA ASBR doing redistribution of IGRP into OSPF.

Figure 8-23. Network Diagram Where Type 7 LSAs Are Originated

Type 7 LSAs are generated into Area 1 and then are translated into Type 5 LSAs by the NSSA ABR, which is Router F.

Example 8-13 NSSA LSA Output

 RouterI#  show ip ospf database nssa-external 10.10.10.0  LS age: 36  Options: (No TOS-capability,  Type 7/5 translation  , DC)   LS Type: AS External Link  Link State ID: 10.10.10.0 (External Network Number)   Advertising Router: 141.108.1.21  LS Seq Number: 80000001   Checksum: 0x4309   Length: 36   Network Mask: /24         Metric Type: 2 (Larger than any link state path)         TOS: 0         Metric: 20         Forward Address: 141.108.1.21         External Route Tag: 0 

The output of the NSSA LSA resembles that of the external LSA output, except that there are a few important things to remember regarding the P bit in this output:

• The P bit is used to tell the NSSA ABR whether to translate Type 7 LSAs into Type 5 LSAs. This bit was already mentioned in the Option field that was discussed in the "Hello Packets" section earlier.

• No Type 7/5 translation means bit P = 0.

• Type 7/5 translation means bit P = 1.

• If bit P = 0, the NSSA ABR must not translate this LSA into a Type 5 LSA. This happens when the NSSA ASBR is also an NSSA ABR.

• If bit P = 1, the NSSA ABR (if multiple NSSA ABRs exist, the one with the lowest router ID) must translate this Type 7 LSA into a Type 5 LSA.

P stands for propagation. Basically, this bit is used for propagation control. The ABR makes the decision based on the value of this bit.

NSSA Configuration Example

Example 8-14 shows a configuration example for defining an NSSA area. This configu-ration must be present on all routers that are in Area 1, as shown in Figure 8-23.

Example 8-14 Configuring an NSSA

 RouterF#  router ospf 1   area 1 nssa  

After defining Area 1 as an NSSA in Figure 8-23, it will have the following characteristics:

• No Type 5 LSAs are allowed in Area 1. This means that no RIP routes are allowed in Area 1.

• All IGRP routes are redistributed as Type 7 routes. This Type 7 route can exist only within NSSA.

• All Type 7 LSAs are translated into Type 5 LSAs by the NSSA ABR and are leaked into the OSPF domain as Type 5 LSAs.

Totally Not-So-Stubby Areas

This type of area is an extension to the NSSA. If only one exit point exists, this is the most recommended form of NSSA area type. In Figure 8-23, if Area 1 is defined as a totally NSSA, the following is true:

• No RIP routes will be injected into Area 1 because those are external routes.

• No summary LSA from other areas will be injected into Area 1 because of the definition of a totally NSSA.

• The default summary LSA will be generated by the ABR, Router F.

Totally NSSAs have the following characteristics:

• No summary LSAs are allowed.

• No external LSAs are allowed.

• The default route is injected as a summary route.

• Type 7 LSAs are converted into Type 5 LSAs at the NSSA ABR.

Example 8-15 shows the configuration required on the NSSA ABR. As in case of totally stubby areas, the no-summary command is needed only on the ABR because the summary LSA generation is done on the ABR.

Example 8-15 Configuration on the NSSA ABR, Router F, for Totally NSSA Area

 RouterF#  router ospf 1   area 1 nssa no-summary  

Filtering in NSSA

In some situations, there is no need to inject external routes into the NSSA as Type 7 routes. This situation usually occurs when an ASBR is also an NSSA ABR.

When redistribution takes place in this scenario, the router generates Type 5 LSAs as well as Type 7 LSAs. In Figure 8-24, Area 1 is configured using the no-redistribution option.

Figure 8-24. Scenarios Where NSSA Filtering Can Be Used

This means that all IGRP routes are redistributed into Area 0, but no Type 7 LSAs will be generated for Area 1. Example 8-16 shows the configuration that prevents NSSA ABR Router A from generating Type 7 LSAs for IGRP routes.

Example 8-16 Configuration to Filter Type 7 in NSSA

 RouterA#  router ospf 1   area 1 nssa no-redistribution  

Configure the no-redistribution command on an NSSA ABR that's also an ASBR.

Another case of filtering occurs when you need to prevent the Type 7 LSAs from being translated outside the NSSA. In other words, you want to control which Type 7 LSAs get translated into Type 5 LSAs. For example, Figure 8-24 shows a RIP-learned route 141.108.10.0/24 that's being injected into the OSPF NSSA Area 1. You don't want this route to be leaked into the rest of the OSPF areas. Example 8-17 shows the configu-ration that will prevent RIP routes from being translated into Type 5 LSAs. This configuration can be used on either Router A or Router B.

Example 8-17 Configuration to Control Type 7 to Type 5 Conversion

 RouterA#  router ospf 1   summary-address 141.108.10.0 255.255.255.0 not-advertise  

This summary-address configuration generates a Type 7 LSA that won't be translated into a Type 5 LSA by the NSSA ABR.

Default Routes in NSSA

There are two ways to have a default route in an NSSA:

• When you configure an area as an NSSA, the NSSA ABR doesn't generate a default summary route, by default.

• In the case of a stub area or an NSSA, totally stubby area, the NSSA ABR generates a default summary route.

Default Summary Route

By defining an area as an NSSA, totally stubby area, the NSSA ABR generates a default summary route. As mentioned earlier, if the NSSA area were not defined as a totally stubby area, a default summary route would not be generated by the NSSA ABR. Example 8-18 shows how to send a default summary route in an NSSA by configuring an NSSA, totally stubby area. This is done by applying the no-summary option on the NSSA ABR.

Example 8-18 Configuration to Generate the Default Summary Route into an NSSA Area

 RouterA#  router ospf 1   area 1 nssa no-summary  

Default Type 7

Example 8-19 shows the configuration that generates a Type 7 default route. You can configure this command on any NSSA ASBR or NSSA ABR, with the following rules:

• NSSA ASBR can generate a default only when it has a default route in its routing table.

• NSSA ABR can generate a default route with or without a default route in its own routing table.

Example 8-19 Configuration for Originating Type 7 Default into an NSSA Area

 RouterA#  router ospf 1   area 1 nssa default-information-originate