Setting Up Stub Areas

Problem

You want to optimize OSPF performance in an area that connects only to the OSPF backbone.

Solution

Configure the area as a stub area:

	[edit protocols ospf]
	aviva@RouterE# set area 0.0.0.3 interface t1-0/0/3.0
	aviva@RouterE# set area 0.0.0.3 stub

Discussion

When some areas of your network have no external connections, you can make them into stub areas. This reduces the amount of OSPF protocol traffic that is flooded through the area, which improves performance on the router by decreasing the size of the OSPF routing database and thus decreasing the amount of CPU needed to perform the SPF calculations. Another common reason to set up a stub area is to allow legacy routers that don have enough memory or CPU horsepower to participate in the OSPF network. A stub area receives OSPF routing-database information from all the other areas in the network. However, instead of receiving all AS external advertisements, the stub area gets only a default summary (0.0.0.0/0) from ABRs to reach external destinations and gets summaries for destinations in other OSPF areas.

You configure each router in the stub area as a stub router by including the stub statement when configuring the area. In our recipe, Area 0.0.0.3 is a stub area (see Figure 12-3). Configure the stub area on each router that is part of this area.

Check that the stub area is up and running:

	aviva@RouterE> show ospf interface detail
	Interface State Area DR ID BDR ID Nbrs
	t1-0/0/3.0 PtToPt 0.0.0.3 0.0.0.0 0.0.0.0 1
	Type: P2P, Address: 0.0.0.0, Mask: 0.0.0.0, MTU: 1500, Cost: 65
	 Adj count: 1
	Hello: 10, Dead: 40, ReXmit: 5, Stub
	Auth type: None

Figure 12-3. OSPF stub area topology

Type 4 (ASBR Summary) and Type 5 (AS External) LSAs are not flooded into stub areas. When Area 0.0.0.3 is a regular area, not a stub area, the routers in that area receive Type 4 and Type 5 LSAs. Here are the entries in RouterEs link-state database:

	aviva@RouterE> show ospf database summary
	Area 0.0.0.3:
	 3 Router LSAs
	 1 Network LSAs
	 9 Summary LSAs
	 1 ASBRSum LSAs
	Externals:
	 2 Extern LSAs
	Interface lo0.0:
	Interface t1-0/0/3.0:
	Interface t1-0/0/3.0:

The output shows one ASBR Summary and two External LSAs. After you configure the area as a stub, these routes are no longer sent from Area 0.0.0.0 so are not in the routers link-state database:

	aviva@RouterE> show ospf database summary
	Area 0.0.0.3:
	 3 Router LSAs
	 1 Network LSAs
	 9 Summary LSAs
	Externals:
	Interface lo0.0:
	Interface t1-0/0/3.0:
	Interface t1-0/0/3.0:

You can further reduce the number of LSA packets flooded through the stub area by configuring the ABR so that it does not flood Type 3 (Network Summary) LSAs to the routers in the stub area:

	[edit protocols ospf]
	aviva@RouterJ# set area 0.0.0.3 stub no-summaries

The routers in the stub area no longer receive the Type 3 LSAs:

	aviva@RouterE> show ospf database summary
	Area 0.0.0.3:
	 3 Router LSAs
	 1 Network LSAs <-- No Type 3 LSAs are listed after the Type 2 LSAs
	Externals:
	Interface t1-0/0/3.0:
	Interface t1-0/0/3.0: