70-240 in 15 minutes a week: Windows 2000 Routing (Part 2)

By ServerWatch Staff (Send Email)
Posted Apr 2, 2003


by Dan DiNicolo
http://www.2000trainers.com

Welcome to article number 25 in my 70-240 in 15 minutes a week series. This week's article covers the second part of Windows 2000 Routing. This includes a look at OSPF, as well as demand-dial and multicast routing. This article again falls into the Windows 2000 Networking portion of the series.

The material to be covered in this article includes:

- The Open Shortest Path First (OSPF) routing protocol
- Demand-dial routing
- Multicast routing with IGMP



Open Shortest Path First

The OSPF routing protocol is the only other traditional routing protocol included with Windows 2000 outside of RIP versions 1 and 2. Traditionally RIP is used in small networks because it is easy to configure. However, certain scalability issues with RIP (such as the limitation that it only allows up to 15 hops) tend to make it a poor choice for larger networks. Whereas RIP is a distance-vector protocol, OSPF is a link-state protocol, meaning that each router has a database of the network routing topology. While this leads to more effective routing decision-making, it also increased the complexity of setting up an OSPF-based topology. Note that both RIP and OSPF can be run on routers at the same time.

In order to better understand how OSPF works, you need to be familiar with some key concepts. These include the idea of an Autonomous System (AS), areas, backbone areas, and the different types of OSPF routers (these differ in their responsibilities and how they function). The section below outlines these key concepts.

Autonomous System - an AS basically refers to a collection of areas that fall under the same administrative control, and has a backbone area between which different areas communicate directly. 

Area - An OSPF area is a portion of an AS that includes contiguous subnet ranges. One of the main purposes of an OSPF area is route aggregation, which allows routing within an area to be confined to that area and not travel over the backbone. This is also sometimes referred to as route summarization, where routers within an area know only about their area, and a default route to the backbone. This makes OSPF a more efficient routing protocol, since every router does not need to necessarily know the details of other network available. As a general rule, follow the idea that an OSPF area should be comprised of the same systems that make up an Active Directory site. Areas are usually numbed in the format 0.0.0.x, where x usually designates a subnet range (although this is convention, not any requirement).

Backbone Area - the backbone area is the (usually) high-speed area into which all other OSPF areas are connected (these other areas are generally referred to as stub areas. Any traffic moving between different areas should communicate via the backbone area. The backbone area is always designated as area 0.0.0.0 in an OSPF implementation.

Stub Areas - A stub area is an area connected to the backbone area by an Area Boundary Router (ABR). When designing an OSPF-based topology, you should try to connect all stub areas to the backbone instead of connecting them to other stub areas. In a stub area, you can set up a single static route for all traffic destined outside of the area. The diagram below shows what an OSPF setup with a backbone and three stub areas might look like. Article 25 in Dan DiNicolo's 70-240 in 15 minutes a week series covers the second part of the Windows 2000 Routing and Remote Access Service (RRAS). This includes a look at OSPF, as well as demand-dial and multicast routing.

Area Border Router - any router in an OSPF system that borders and interconnects two or more areas (such as the backbone and a stub area) is considered an Area Border Router. Each ASB will carry an individual link state database for each area with which it is interconnected.

Autonomous System Boundary Router (ASBR) - a router than interconnects different Autonomous Systems in an OSPF topology. 

Backbone Router - Any router that interconnects to the backbone area, including ABRs with a backbone connection.

Internal Router - any router that has all interfaces connected within the same area. These routers only carry a single link state database, containing information about the area in which it exists.

Virtual Link - A logical link between the backbone area and an ABR when a physical link between them does not exist. It is usually recommended that you avoid using virtual links where possible, since they can sometimes cause routing problems that can be difficult to troubleshoot. 

While I could dedicate an entire article to OSPF and all of its workings, I'll spare you the majority of the details. The key things to understand are that an OSPF-enabled router speaks to the other routers in its own area directly, exchanging routing information. This ensures that every router within the area has the same link state database as every other router in the area, and changes are flooded within the area as they occur. If a network has been properly designed into a hierarchical VLSM (variable length subnet mask) scheme, routing will be much more efficient and effective, since OSPF usually exchanges less traffic than RIP. Note that OSPF and RIP version 2 both pass subnet mask information in their routing table updates, while RIP version 1 does not. For companies that use VLSM, this is a critical consideration in choosing a routing protocol.

In order to configure a Windows 2000 Server to act as an OSPF router, you need to add the OSPF protocol, as shown below:

Once OSPF has been added, you will need to configure an interface to use OSPF. As such, it is possible to have one or more interfaces use OSPF, while another (like a dial-up interface) might not. After an interface is added, you will be presented with the OSPF properties page, as shown below:

Note that by default, an OSPF interface will be made part of the backbone area. The General tab allows you to configure the network type as well as router priority, cost, and an authentication password. The NBMA Neighbors tab allows you to configure the IP addresses of other OSPF routers on non-broadcast networks (such as Frame Relay, for example). Finally, the Advanced tab allows you to configure OSPF properties such as the Hello interval (how often an OSPF router announces its existence on the network), MTU size and so forth, as shown below.

Note also that by right clicking on the OSPF heading under IP routing you can easily view neighboring routers, the link state database(s) of the system, and more.

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