10.2.5 This page will describe the functions of a routing table.
Routers use routing protocols to build and maintain routing tables that contain route information. This aids in the process of path determination. Routing protocols fill routing tables with a variety of route information. This information varies based on the routing protocol used. Routing tables contain the information necessary to forward data packets across connected networks. Layer 3 devices interconnect broadcast domains or LANs. A hierarchical address scheme is required for data transfers.
Routers keep track of the following information in their routing tables:
• Protocol type – Identifies the type of routing protocol that created each entry.
• Next-hop associations – Tell a router that a destination is either directly connected to the router or that it can be reached through another router called the next-hop on the way to the destination. When a router receives a packet, it checks the destination address and attempts to match this address with a routing table entry.
• Routing metric – Different routing protocols use different routing metrics. Routing metrics are used to determine the desirability of a route. For example, RIP uses hop count as its only routing metric. IGRP uses bandwidth, load, delay, and reliability metrics to create a composite metric value.
• Outbound interfaces – The interface that the data must be sent out of to reach the final destination.
Routers communicate with one another to maintain their routing tables through the transmission of routing update messages. Some routing protocols transmit update messages periodically. Other protocols send them only when there are changes in the network topology. Some protocols transmit the entire routing table in each update message and some transmit only routes that have changed. Routers analyze the routing updates from directly-connected routers to build and maintain their routing tables.
The next page will explain routing algorithms and metrics.
10.2.6 This page will define algorithms and metrics as they relate to routers.
An algorithm is a detailed solution to a problem. Different routing protocols use different algorithms to choose the port to which a packet should be sent. Routing algorithms depend on metrics to make these decisions.
Routing protocols often have one or more of the following design goals:
• Optimization – This is the capability of a routing algorithm to select the best route. The route will depend on the metrics and metric weights used in the calculation. For example, one algorithm may use both hop count and delay metrics, but may consider delay metrics as more important in the calculation.
• Simplicity and low overhead – The simpler the algorithm, the more efficiently it will be processed by the CPU and memory in the router. This is important so that the network can scale to large proportions, such as the Internet.
• Robustness and stability – A routing algorithm should perform correctly when confronted by unusual or unforeseen circumstances, such as hardware failures, high load conditions, and implementation errors.
• Flexibility – A routing algorithm should quickly adapt to a variety of network changes. These changes include router availability, router memory, changes in bandwidth, and network delay.
• Rapid convergence – Convergence is the process of agreement by all routers on available routes. When a network event causes changes in router availability, updates are needed to reestablish network connectivity. Routing algorithms that converge slowly can cause data to be undeliverable.
Routing algorithms use different metrics to determine the best route. Each routing algorithm interprets what is best in its own way. A routing algorithm generates a number called a metric value for each path through a network. Sophisticated routing algorithms base route selection on multiple metrics that are combined in a composite metric value.
Typically, smaller metric values indicate preferred paths.
Metrics can be based on a single characteristic of a path, or can be calculated based on several characteristics. The following metrics are most commonly used by routing protocols:
• Bandwidth – Bandwidth is the data capacity of a link. Normally, a 10-Mbps Ethernet link is preferable to a 64-kbps leased line.
• Delay – Delay is the length of time required to move a packet along each link from a source to a destination. Delay depends on the bandwidth of intermediate links, the amount of data that can be temporarily stored at each router, network congestion, and physical distance.
• Load – Load is the amount of activity on a network resource such as a router or a link.
• Reliability – Reliability is usually a reference to the error rate of each network link.
• Hop count – Hop count is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of four indicates that data would have to pass through four routers to reach its destination. If multiple paths are available to a destination, the path with the least number of hops is preferred.
• Ticks – The delay on a data link using IBM PC clock ticks. One tick is approximately 1/18 second.
• Cost – Cost is an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator.
The next page will discuss two types of routing protocols.
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