Developing Routing Strategies

After planning your network infrastructure based on your design model, plan how to implement routing. Figure 1.4 shows the tasks involved in developing a unicast routing strategy. For information about IP multicast routing, see "Planning IP Multicasting" later in this chapter.

Figure 1.4: Developing a Routing Strategy

To plan an effective routing solution for your environment, you must understand the differences between hardware routers and software routers; static routing and dynamic routing; and distance vector routing protocols and link state routing protocols.

Choosing Hardware or Software Routing

A router is a device that holds information about the state of its own network interfaces and contains a list of possible sources and destinations for network traffic. The router directs incoming and outgoing packets based on that information. By projecting network traffic and routing needs based on the number and types of hardware devices and applications used in your environment, you can better decide whether to use a dedicated hardware router, a software-based router, or a combination of both. Generally, dedicated hardware routers handle heavier routing demands best, and less expensive software-based routers are sufficient to handle lighter routing loads.

A software-based routing solution, such as the Windows Server 2003 Routing and Remote Access service, can be ideal on a small, segmented network with relatively light traffic between subnets. Conversely, enterprise network environments that have a large number of network segments and a wide range of performance requirements might need a variety of hardware-based routers to perform different roles throughout the network.

Choosing Static or Dynamic Routing

Routing can be either static or dynamic, depending on how routing information is generated and maintained:

• In static routing, routing information is entered manually by an administrator and remains constant throughout the router's operation.

• In dynamic routing, a router is configured to automatically generate routing information and share the information with neighboring routers.

You must decide where best to implement each type of routing.

Static Routing

In static routing, a network administrator enters static routes in the routing table manually by indicating:

• The network ID, consisting of a destination IP address and a subnet mask.

• The IP address of a neighboring router (the next hop).

• The router interface through which to forward the packets to the destination.

Static routing has significant drawbacks. Because a network administrator defines a static route, errors are more likely than with a dynamically assigned route. A simple typographical error can create chaos on the network. An even greater problem is the inability of a static route to adapt to topology changes. When the topology changes, the administrator might have to make changes to the routing tables on every static router. This does not scale well on a large internetwork.

However, static routing can be effective when used in combination with dynamic routing. Instead of using static routing exclusively, you can use a static route as the redundant backup for a dynamically configured route. In addition, you might use dynamic routing for most paths but configure a few static paths where you want the network traffic to follow a particular route. For example, you might configure routers to force traffic over a given path to a high-bandwidth link.

Dynamic Routing Protocols

Conceptually, the dynamic routing method has two parts: the routing protocol that is used between neighboring routers to convey information about their network environment, and the routing algorithm that determines paths through that network. The protocol defines the method used to share the information externally, whereas the algorithm is the method used to process the information internally.

The routing tables on dynamic routers are updated automatically based on the exchange of routing information with other routers. The most common dynamic routing protocols are:

• Distance vector routing protocols

• Link state routing protocols

Understanding how these protocols work enables you to choose the type of dynamic routing that best suits your network needs.

Distance Vector Routing Protocols

A distance vector routing protocol advertises the number of hops to a network destination (the distance) and the direction in which a packet can reach a network destination (the vector). The distance vector algorithm, also known as the Bellman-Ford algorithm, enables a router to pass route updates to its neighbors at regularly scheduled intervals. Each neighbor then adds its own distance value and forwards the routing information on to its immediate neighbors. The result of this process is a table containing the cumulative distance to each network destination.

Distance vector routing protocols, the earliest dynamic routing protocols, are an improvement over static routing, but have some limitations. When the topology of the internetwork changes, distance vector routing protocols can take several minutes to detect the change and make the appropriate corrections.

One advantage of distance vector routing protocols is simplicity. Distance vector routing protocols are easy to configure and administer. They are well suited for small networks with relatively low performance requirements.

Most distance vector routing protocols use a hop count as a routing metric. A routing metric is a number associated with a route that a router uses to select the best of several matching routes in the IP routing table. The hop count is the number of routers that a packet must cross to reach a destination.

Routing Information Protocol (RIP) is the best known and most widely used of the distance vector routing protocols. RIP version 1 (RIP v1), which is now outmoded, was the first routing protocol accepted as a standard for TCP/IP. RIP version 2 (RIP v2) provides authentication support, multicast announcing, and better support for classless networks. The Windows Server 2003 Routing and Remote Access service supports both RIP v1 and RIP v2 (for IPv4 only).

Using RIP, the maximum hop count from the first router to the destination is 15. Any destination greater than 15 hops away is considered unreachable. This limits the diameter of a RIP internetwork to 15. However, if you place your routers in a hierarchical structure, 15 hops can cover a large number of destinations.

Link State Routing Protocols

Link state routing protocols address some of the limitations of distance vector routing protocols. For example, link state routing protocols provide faster convergence than do distance vector routing protocols. Convergence is the process by which routers update routing tables after a change in network topology — the change is replicated to all routers that need to know about it. Although link state routing protocols are more reliable and require less bandwidth than do distance vector routing protocols, they are also more complex, more memory-intensive, and place a greater load on the CPU.

Unlike distance vector routing protocols, which broadcast updates to all routers at regularly scheduled intervals, link state routing protocols provide updates only when a network link changes state. When such an event occurs, a notification in the form of a link state advertisement is sent throughout the network.

The Windows Server 2003 Routing and Remote Access service supports the Open Shortest Path First (OSPF) protocol, the best known and most widely used link state routing protocol. OSPF is an open standard developed by the Internet Engineering Task Force (IETF) as an alternative to RIP. OSPF compiles a complete topological database of the internetwork. The shortest path first (SPF) algorithm, also known as the Djikstra algorithm, is used to compute the least-cost path to each destination. Whereas RIP calculates cost on the basis of hop count only, OSPF can calculate cost on the basis of metrics such as link speed and reliability in addition to hop count.

Unlike RIP, OSPF can support an internetwork diameter of 65,535 (assuming that each link is assigned a cost of 1). OSPF transmits multicast frames, reducing CPU usage on a LAN. You can hierarchically subdivide OSPF networks into areas, reducing router memory overhead and CPU overhead.

Like RIP v2, OSPF supports variable length subnet masks (VLSM) and noncontiguous subnets. For information about variable length subnet masks and noncontiguous subnets, see "Designing a Structured Address Assignment Model" later in this chapter.

Selecting the Appropriate Routing Protocol

Select a routing protocol based on the following considerations:

• For a small, simple network that is not expected to grow, use a simpler distance vector routing protocol like RIP v2. For a large, complex internetwork, use a newer, more sophisticated link state routing protocol like OSPF.

• Use RIP v2 or OSPF if you need to support variable length subnet masks. Although the outdated RIP v1 is still widely used in private networks, it does not support VLSM and thus is not well suited for enterprise networks. For more information about VLSM, see "Planning Variable Length Subnet Masks" later in this chapter.