Selecting the Appropriate Area Types

2017-11-03 09:05:04

Problem

You want to limit the number of routes and entries in the Link State database to conserve router resources and ensure good convergence properties.

Solution

In the Introduction to this chapter, we talked about the various types of OSPF areas. You can configure these different types areas by using the appropriate keywords on the area command.

For a Stubby Area, use the stub keyword:

Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router ospf 55 Router1(config-router)#area 100 stub Router1(config-router)#exit Router1(config)#end Router1#

To configure a Totally Stubby Area, combine the stub and no-summary keywords on the ABR router:

Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router ospf 55 Router1(config-router)#area 100 stub no-summary Router1(config-router)#exit Router1(config)#end Router1#

Only the ABRs needs the no-summary keyword, because they are the only routers that will be doing the route summarization. The other routers in a Totally Stubby Area need only be configured using the stub keyword, as in the previous example.

For Not So Stubby Areas (NSSA), you need to specify the nssa keyword. In this case we have also included the default-information-originate option so that the router can summarize external routes to a single default route:

Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router ospf 55 Router1(config-router)#area 100 nssa default-information-originate Router1(config-router)#exit Router1(config)#end Router1#

In the Introduction we also discussed an interesting variant, called the Totally Stubby, Not So Stubby Area. You can configure it as follows on the ABR:

Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router ospf 55 Router1(config-router)#area 100 nssa no-summary Router1(config-router)#exit Router1(config)#end Router1#

Once again, you can simply configure the other routers in this area with the nssa keyword.

Discussion

In all of the configuration examples, we showed the configuration for just one router. It is important to remember that the routers in an area have to agree on the area type. However, it is mostly the ABR that cares about the area type, because it has to decide what kinds of information to forward from other parts of the network. Every router in a stub area must be configured as stub, although only the ABR needs to worry about the difference between stub and totally stub. Every router in an NSSA area must be configured to support NSSA type 7 LSA messages. But, once again, only the ABR cares about the difference between NSSA and totally stub NSSA.

If you have any ASBR devices that inject routes from other autonomous systems in this area, then they also need to use the area type information. The other routers are only really concerned with forwarding the LSAs around the area.

When you look at the routing table, you can see there are several kinds of routes:

Router3#show ip route ospf O E1 192.168.10.0/24 [110/3611] via 172.20.1.1, 00:00:20, Serial0.1 172.16.0.0/24 is subnetted, 1 subnets O IA 172.16.2.0 [110/3581] via 172.20.1.1, 00:00:20, Serial0.1 172.20.0.0/16 is variably subnetted, 5 subnets, 3 masks O 172.20.220.1/32 [110/11] via 172.20.10.2, 00:00:20, Ethernet0 O 172.20.200.1/32 [110/11] via 172.20.10.2, 00:00:20, Ethernet0 172.22.0.0/24 is subnetted, 1 subnets O IA 172.22.1.0 [110/3581] via 172.20.1.1, 00:00:20, Serial0.1 172.25.0.0/16 is variably subnetted, 3 subnets, 2 masks O IA 172.25.25.6/32 [110/3582] via 172.20.1.1, 00:00:20, Serial0.1 O IA 172.25.25.1/32 [110/3572] via 172.20.1.1, 00:00:20, Serial0.1 O IA 172.25.1.0/24 [110/3581] via 172.20.1.1, 00:00:20, Serial0.1 172.24.0.0/24 is subnetted, 1 subnets O E1 172.24.1.0 [110/3611] via 172.20.1.1, 00:00:20, Serial0.1 10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks O IA 10.2.2.2/32 [110/5134] via 172.20.1.1, 00:00:20, Serial0.1 O E1 10.2.2.0/30 [110/3606] via 172.20.1.1, 00:00:20, Serial0.1 O IA 10.1.1.0/30 [110/5133] via 172.20.1.1, 00:00:20, Serial0.1 O E1 10.100.1.0/24 [110/3611] via 172.20.1.1, 00:00:20, Serial0.1 O E2 192.168.50.0/24 [110/20] via 172.20.1.1, 00:00:20, Serial0.1 Router3#

This output shows only routes that were learned via OSPF, which is indicated by the O at the start of each line. In this routing table, there are several external routes. Most of them are Type 1 externals, which are labeled E1. There is also one Type 2 external route, which is labeled E2 in the output. Some of the other routes have IA beside them, which indicates they are interarea routes, meaning that they did not originate in this area. The remaining routes, such as 172.20.220.1/32, represent networks in this area.

This routing table shows no summarization. For example, the 10.0.0.0/8 classful network has several distinct subnets, including both 10.2.2.0/30 and its subnet 10.2.2.2/32. It's also useful to look at the OSPF database on this router to see how different routes are categorized:

Router3#show ip ospf database OSPF Router with ID (172.25.25.2) (Process ID 44) Router Link States (Area 100) Link ID ADV Router Age Seq# Checksum Link count 172.20.220.1 172.20.220.1 47 0x80000004 0xB352 3 172.25.25.1 172.25.25.1 89 0x80000067 0xE771 2 172.25.25.2 172.25.25.2 47 0x80000065 0x4C66 4 Net Link States (Area 100) Link ID ADV Router Age Seq# Checksum 172.20.10.1 172.25.25.2 42 0x80000002 0xF11B Summary Net Link States (Area 100) Link ID ADV Router Age Seq# Checksum 10.1.1.0 172.25.25.1 173 0x80000002 0x86AC 10.2.2.2 172.25.25.1 173 0x80000002 0x77B3 172.16.2.0 172.25.25.1 173 0x80000002 0xBFD3 172.22.1.0 172.25.25.1 173 0x80000002 0x820C 172.25.1.0 172.25.25.1 173 0x80000002 0x5E2D 172.25.25.1 172.25.25.1 173 0x80000002 0xF08A 172.25.25.6 172.25.25.1 173 0x80000002 0x2349 Summary ASB Link States (Area 100) Link ID ADV Router Age Seq# Checksum 172.25.1.7 172.25.25.1 173 0x80000001 0xC78 172.25.25.1 172.25.25.1 173 0x80000001 0xBCDF Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 10.2.2.0 172.25.25.1 1138 0x8000000A 0x9588 0 10.100.1.0 172.25.25.1 1138 0x80000009 0x4A6B 0 172.24.1.0 172.25.25.1 1138 0x80000009 0x9BC3 0 192.168.10.0 172.25.25.1 1138 0x80000009 0x6C45 0 192.168.50.0 172.25.1.7 428 0x80000002 0xFF36 0 Router3#

This represents a relatively small OSPF network. But it's a useful example because there are some instances of each type of route.

When we configure this area to be a Stubby Area, the ABR prevents External routes from being propagated into the area and replaces them with a default route. ASBRs are not permitted in stubby areas:

Router3#show ip route ospf 172.16.0.0/24 is subnetted, 1 subnets O IA 172.16.2.0 [110/3581] via 172.20.1.1, 00:00:07, Serial0.1 172.20.0.0/16 is variably subnetted, 5 subnets, 3 masks O 172.20.220.1/32 [110/11] via 172.20.10.2, 00:00:20, Ethernet0 O 172.20.200.1/32 [110/11] via 172.20.10.2, 00:00:20, Ethernet0 172.22.0.0/24 is subnetted, 1 subnets O IA 172.22.1.0 [110/3581] via 172.20.1.1, 00:00:07, Serial0.1 172.25.0.0/16 is variably subnetted, 3 subnets, 2 masks O IA 172.25.25.6/32 [110/3582] via 172.20.1.1, 00:00:07, Serial0.1 O IA 172.25.25.1/32 [110/3572] via 172.20.1.1, 00:00:07, Serial0.1 O IA 172.25.1.0/24 [110/3581] via 172.20.1.1, 00:00:07, Serial0.1 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks O IA 10.2.2.2/32 [110/5134] via 172.20.1.1, 00:00:07, Serial0.1 O IA 10.1.1.0/30 [110/5133] via 172.20.1.1, 00:00:07, Serial0.1 O*IA 0.0.0.0/0 [110/3572] via 172.20.1.1, 00:00:07, Serial0.1 Router3#

As you can see, all of the external routes are gone, but the tnterarea routes remain. Looking at the OSPF database, you can see that there is considerably less information for the router to keep track of:

Totally stubby areas prevent external routes like ordinary stub areas. But the ABRs for totally stubby areas also prevent interarea routes from being propagated into the area, replacing them with a single default route instead. The default route is the only summary route allowed. ASBRs are not permitted in stub or Totally Stubby areas:

Router3#show ip route ospf 172.20.0.0/16 is variably subnetted, 5 subnets, 3 masks O 172.20.220.1/32 [110/11] via 172.20.10.2, 00:00:15, Ethernet0 O 172.20.200.1/32 [110/11] via 172.20.10.2, 00:00:15, Ethernet0 O*IA 0.0.0.0/0 [110/3572] via 172.20.1.1, 00:00:15, Serial0.1 Router3#

The Totally Stubby Area has radically reduced the size of the routing table, as well as the OSPF database:

There are two main differences between stubby and NSSA areas. The first is that the NSSA prevents the ABR from propagating external routes throughout the area, but does not replace them with a single default route. The second is that the NSSA may contain ASBRs, which use Type 7 LSAs to carry information about external routes. These Type 7 LSAs are flooded throughout the NSSA area. When they reach the ABR, they are translated into Type 5 LSAs and forwarded to the rest of the OSPF network:

Router3#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router3(config)#ip route 192.168.88.0 255.255.255.0 172.20.10.2 Router3(config)#router ospf 44 Router3(config-router)#redistribute static subnet Router3(config-router)#area 100 nssa default-information-originate Router3(config-router)#exit Router3(config)#end Router3#

With the default-information-originate option, the ABR will forward a default route to summarize all external routes that originate outside of the area:

Router3#show ip route ospf 172.16.0.0/24 is subnetted, 1 subnets O IA 172.16.2.0 [110/3581] via 172.20.1.1, 00:07:43, Serial0.1 172.20.0.0/16 is variably subnetted, 5 subnets, 3 masks O 172.20.220.1/32 [110/11] via 172.20.10.2, 00:07:43, Ethernet0 O 172.20.200.1/32 [110/11] via 172.20.10.2, 00:07:43, Ethernet0 172.22.0.0/24 is subnetted, 1 subnets O IA 172.22.1.0 [110/3581] via 172.20.1.1, 00:07:43, Serial0.1 172.25.0.0/16 is variably subnetted, 3 subnets, 2 masks O IA 172.25.25.6/32 [110/3582] via 172.20.1.1, 00:07:43, Serial0.1 O IA 172.25.25.1/32 [110/3572] via 172.20.1.1, 00:07:43, Serial0.1 O IA 172.25.1.0/24 [110/3581] via 172.20.1.1, 00:07:43, Serial0.1 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks O IA 10.2.2.2/32 [110/5134] via 172.20.1.1, 00:07:43, Serial0.1 O IA 10.1.1.0/30 [110/5133] via 172.20.1.1, 00:07:43, Serial0.1 O*N2 0.0.0.0/0 [110/1] via 172.20.1.1, 00:07:43, Serial0.1 Router3#

The OSPF database for an NSSA area shows information about Type 7 LSAs:

Router3#show ip ospf database OSPF Router with ID (172.25.25.2) (Process ID 44) Router Link States (Area 100) Link ID ADV Router Age Seq# Checksum Link count 172.20.220.1 172.20.220.1 973 0x80000008 0x51AA 3 172.25.25.1 172.25.25.1 502 0x80000072 0x77D0 2 172.25.25.2 172.25.25.2 968 0x8000006E 0xE5BB 4 Net Link States (Area 100) Link ID ADV Router Age Seq# Checksum 172.20.10.1 172.25.25.2 967 0x80000004 0x9371 Summary Net Link States (Area 100) Link ID ADV Router Age Seq# Checksum 10.1.1.0 172.25.25.1 1124 0x80000003 0x2A02 10.2.2.2 172.25.25.1 1124 0x80000003 0x1B09 172.16.2.0 172.25.25.1 1124 0x80000003 0x6329 172.22.1.0 172.25.25.1 1124 0x80000003 0x2661 172.25.1.0 172.25.25.1 1124 0x80000003 0x282 172.25.25.1 172.25.25.1 1124 0x80000003 0x94DF 172.25.25.6 172.25.25.1 1124 0x80000003 0xC69E Type-7 AS External Link States (Area 100) Link ID ADV Router Age Seq# Checksum Tag 0.0.0.0 172.25.25.1 508 0x80000001 0xF31B 0 10.2.2.0 172.25.25.1 1123 0x80000001 0x2FE7 0 10.100.1.0 172.25.25.1 1123 0x80000001 0xE90B 0 172.24.1.0 172.25.25.1 1123 0x80000001 0x3B63 0 192.168.10.0 172.25.25.1 1123 0x80000001 0xCE4 0 192.168.88.0 172.25.25.2 974 0x80000001 0x3AEC 0 Router3#

It is also interesting to look at the routing table entry for one of these Type 7 routes on another router within the same area:

Router1#show ip route 192.168.88.0 Routing entry for 192.168.88.0/24 Known via "ospf 55", metric 20, type NSSA extern 2, forward metric 1572 Last update from 172.20.1.2 on Serial0/0.2, 00:08:56 ago Routing Descriptor Blocks: * 172.20.1.2, from 172.25.25.2, 00:08:56 ago, via Serial0/0.2 Route metric is 20, traffic share count is 1 Router1#

The aptly named Totally Stubby Not So Stubby Area is similar to an NSSA area, but it also acts like a Totally Stubby area in preventing the ABR from advertising Inter Area routes and replaces them with a single summary route, the default route. The ABR for a Totally Stubby NSSA will create a default route by default:

Router3#show ip route ospf 172.20.0.0/16 is variably subnetted, 5 subnets, 3 masks O 172.20.220.1/32 [110/11] via 172.20.10.2, 00:00:47, Ethernet0 O 172.20.200.1/32 [110/11] via 172.20.10.2, 00:00:47, Ethernet0 O*IA 0.0.0.0/0 [110/3572] via 172.20.1.1, 00:00:47, Serial0.1 Router3#

Despite the confusing name, this is an extremely useful type of area. In many large OSPF networks, most of the routes in the routing table are interarea routes. But you can also put an ASBR in this type of area and use it as a transit area to connect to external networks:

Like an NSSA area, the ABR translates Type 7 to Type 5 LSAs:

Router1#show ip ospf Routing Process "ospf 55" with ID 172.25.25.1 Area 100 Number of interfaces in this area is 1 It is a NSSA area Perform type-7/type-5 LSA translation Area has no authentication SPF algorithm executed 75 times Area ranges are Number of LSA 13. Checksum Sum 0x6B01B Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Router1#

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