Totally Not-so-stubby Areas (TNSSAs)
Totally Not-so-stubby Areas (TNSSAs) are an extension of NSSAs. Like NSSAs, Type 5 LSAs are not allowed into a TNSSA. However, unlike NSSAs, Summary LSAs are not allowed into a TNSSA. In addition to this, when a TNSSA is configured, the default route is injected into the area as a Type 7 LSA. TNSSAs have the following characteristics:
- Type 7 LSAs are converted into Type 5 LSAs at the NSSA ABR
- They do not allow Network Summary LSAs
- They do not allow External LSAs
- The default route is injected as a Summary LSA
The no-summary keyword is used to configure an NSSA. This configuration prevents Network Summary LSAs from being advertised into the area. By default, when an Area is configured as an NSSA, Network Summary LSAs are still flooded into the area. Referencing the topology illustrated in Figure 3-22, assuming that area 1 is still configured as an NSSA; the Type 3 LSAs will still be flooded by R2 (ABR) into this area and can be seen on R1 as follows:
| R1#show ip ospf databaseOSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum Type-7 AS External Link States (Area 1) Link ID ADV Router Age Seq# Checksum Tag |
To configure area 1 as a TNSSA and prevent Summary LSAs from being advertised into the area, R2 (ABR) would be configured as follows:
| R2(config)#router ospf 2 R2(config-router)#area 1 nssa no-summary R2(config-router)#exit |
When this configuration is implemented, R2 generates and advertises a default route into the area as a Type 3 LSA. All other Summary LSAs are not flooded into the area. This can be validated by checking the LSDB on R1 as illustrated in the following output:
| R1#show ip ospf databaseOSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum Type-7 AS External Link States (Area 1) Link ID ADV Router Age Seq# Checksum Tag |
The Link State ID for the default route will be set to 0.0.0.0. In addition to this, the route uses a default metric value of 1 as shown in the following output on R1:
| R1#show ip ospf database summary 0.0.0.0OSPF Router with ID (1.1.1.1) (Process ID 1)
Summary Net Link States (Area 1) Routing Bit Set on this LSA |
NOTE: The same entry is also installed into the LSDB on R2 for area 1. In addition to this, it is important to remember that while the default route generated when the area [area ID] nssa default-information-originaterouter configuration command is used in NSSA configuration will be a Type 2 External NSSA route, the default route flooded into the area when a TNSSA is configured will be marked as an inter-area route, not an external route as illustrated below on R1:
| R1#show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via “ospf 1”, distance 110, metric 65, candidate default path, type inter area Last update from 10.0.0.2 on Serial0/0, 00:05:57 ago Routing Descriptor Blocks: * 10.0.0.2, from 2.2.2.2, 00:05:57 ago, via Serial0/0 Route metric is 65, traffic share count is 1 |
Notice that this time the route has a metric of 65. This metric is calculated by adding the metric advertised by R2 (1) and the cost of the link between R1 and R2 (64). To determine the default link cost, use the show ip ospf interface [name] command as follows:
| R1#show ip ospf interface serial 0/0 Serial0/0 is up, line protocol is up Internet Address 10.0.0.1/24, Area 1 Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40 Hello due in 00:00:06 Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 4, maximum is 4 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 Suppress hello for 0 neighbor(s) |
NOTE: OSPF metrics are described in detail later in this chapter.
Stub Areas
Stub areas are somewhat similar to NSSAs with the major exception being that external routes (Type 5 or Type 7) are not allowed into Stub areas. It is important to understand that Stub functionality in OSPF and EIGRP is not at all similar. In OSPF, the configuration of an area as a Stub area reduces the size of the routing table and the OSPF database for the routers within the Stub area by preventing external LSAs from being advertised into such areas without any further configuration. Stub areas have the following characteristics:
- The default route is injected into Stub area by the ABR as a Type 3 LSA
- Type 3 LSAs from other areas are permitted into these areas
- External route LSAs, i.e. Type 4 and Type 5 LSAs are not allowed
NOTE: Because Type 5 LSAs are not allowed into Stub areas, this negates the need to advertise Type 4 LSAs, which advertise the ASBR. Therefore Type 4 LSAs are essentially also filtered from Stub areas. While we know that Type 5 LSAs are used for external routes, Type 4 LSAs should also be implied, because, as was stated earlier in this chapter, Type 1, Type 2, and Type 3 LSAs are used for internal route calculation, while Type 4, Type 5, and Type 7 LSAs are used for external route calculation.
In Cisco IOS software, Stub areas are configured using the area [area ID] stub [no-summary] router configuration command. The no-summary keyword prevents Summary LSAs from being flooded into the area. If this keyword is used, the area becomes a Totally Stubby Area (TSA). TSAs are described in the following section.
The topology shown in Figure 3-20 will be used to illustrate Stub area configuration:
Fig. 3-20. Configuring Stub Areas
Referencing the topology illustrated in Figure 3-20, area 1 is configured as a Stub area by issuing the area 1 stubrouter configuration commands on R1 and R2 as follows:
| R1(config)#router ospf 1 R1(config-router)#area 1 stub R1(config-router)#exit |
| R2(config)#router ospf 2 R2(config-router)#area 1 stub R2(config-router)#exit |
Following this configuration, the ABR (R2) generates a default route into the Stub area. This is generated as a Type 3 LSA and is flooded only into the Stub area as illustrated below:
| R2#show ip ospf database summary 0.0.0.0OSPF Router with ID (2.2.2.2) (Process ID 2)
Summary Net Link States (Area 1) LS age: 199 |
By default, the route is assigned a metric of 1. In addition to this, the route is generated as an inter-area route as shown in the output of the show ip route command on R1:
| R1#show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via “ospf 1”, distance 110, metric 65, candidate default path, type inter area Last update from 10.0.0.2 on Serial0/0, 00:04:47 ago Routing Descriptor Blocks: * 10.0.0.2, from 2.2.2.2, 00:04:47 ago, via Serial0/0 Route metric is 65, traffic share count is 1 |
This external route is used by routers in the Stub area to reach external destinations because Type 5 LSAs are not flooded into Stub areas. However, Type 3 LSAs are still permitted. This can be validated by looking at the LSDB on R1:
| R1#show ip ospf databaseOSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum |
If, for example, router R4 was configured as an ASBR by redistributing routes into OSPF, while the Type 4 and Type 5 LSAs would be propagated to R2, neither would be flooded into the Stub area. The LSDB of R2 would show the following entries:
| R2#show ip ospf database asbr-summaryOSPF Router with ID (2.2.2.2) (Process ID 2)
Summary ASB Link States (Area 0) Routing Bit Set on this LSA R2#show ip ospf database external OSPF Router with ID (2.2.2.2) (Process ID 2) Type-5 AS External Link States Routing Bit Set on this LSA |
However, neither would be flooded into area 1 because of the Stub configuration:
| R1#show ip ospf databaseOSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum
|
Totally Stubby Areas
Totally Stubby Areas (TSAs) are an extension of Stub areas. However, unlike Stub areas, TSAs further reduce the size of the LSDB on routers in the TSA by restricting Type 3 LSAs in addition to the external LSAs. TSAs are typically configured on routers that have a single ingress and egress point into the network, for example in a traditional hub-and-spoke network. The area routers forward all external traffic to the ABR. The ABR is also the exit point for all backbone and inter-Area traffic to the TSA. TSAs have the following characteristics:
- The default route is injected into Stub areas as a Type 3 Network Summary LSA
- Type 3, 4, and 5 LSAs from other areas are not permitted into these areas
Totally Stubby areas are configured by issuing the area [area ID] stub no-summary command on the ABR. Referencing the topology illustrated in Figure 3-19, area 1 would be configured as a Totally Stubby area by configuring R2 (ABR) as follows:
| R2(config)#router ospf 2 R2(config-router)#area 1 stub no-summary R2(config-router)#exit |
This configuration results in a single Type 3 LSA (default route) being flooded into area 1; however, all other Type 3 LSAs are suppressed:
| R1#show ip ospf databaseOSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum |
The default route is advertises as an inter-area route with a default metric of 1. The cumulative metric is calculated on R1 by adding the default metric (1) to the cost of the link between R1 and R2 (64). This is illustrated in the following output:
| R1#show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via “ospf 1”, distance 110, metric 65, candidate default path, type inter area Last update from 10.0.0.2 on Serial0/0, 00:09:55 ago Routing Descriptor Blocks: * 10.0.0.2, from 2.2.2.2, 00:09:55 ago, via Serial0/0 Route metric is 65, traffic share count is 1 |
Now that we have a solid understanding of the configuration of the different types of areas that are described in this section, it is important to understand where and when to use such configurations. NSSAs should be used on stub routers, e.g. in the spoke routers in hub-and-spoke networks when you want to import (redistribute) external routes into OSPF. These routes may be static routes, connected routes, or even routes from another routing protocol.
Because external routing information (Type 4 and Type 5) is not allowed into the NSSA, this reduces the size of the LSDB. In addition to this, the TNSSA prevents the flooding of Network Summary LSAs into the TNSSA further reducing the size of the LSDB.
Stub areas should be configured when connecting to stub routers. Unlike NSSAs, Stub areas do not allow any external routes. While Stub areas reduce the overall size of the LSDB for routers in those Areas, they do have some limitations, which you are expected to be familiar with. These limitations are as follows:
- You cannot include an ASBR in a Stub Area because external routes are not allowed
- All routers in the Stub Area must be configured as Stub routers
- You cannot configure a Virtual Link across a Stub area
- The OSPF backbone Area (Area 0) cannot be configured as a Stub area