During the routing process, the routing decisions of hosts and routers are aided by a database of routes known as the routing table. The routing table is not exclusive to a router. Depending on the routable protocol, hosts may also have a routing table that may be used to decide the best router for the packet to be forwarded. IP hosts have a routing table. IPX hosts do not have a routing table.
The types of possible entries in a routing table include:
Network Route . A route to a specific Network ID in the internetwork.
Host Route . A route to a specific internetwork address (Network ID and Host ID). Instead of making a routing decision based on just the network ID, the routing decision is based on the combination of network ID and host ID. Host routes allow intelligent routing decisions to be made for each internetwork address. Host routes are typically used to create custom routes to control or optimize specific types of internetwork traffic.
Default Route . A route that is used when no other routes for the destination are found in the routing table. For example, if a router or end system cannot find a network route or host route for the destination, the default route is used. Rather than being configured with routes for all the Network IDs in the internetwork, the default route is used to simplify the configuration of end systems or routers.
In many router implementations including the Windows 2000 Routing and Remote Access service, there is a routing table and a forwarding table. The routing table is used to store all the routes from all possible sources. The forwarding table is what is used by the routable protocol when forwarding the packet. For example, for a Windows 2000 router, the Routing and Remote Access service maintains the IP routing table using a component called the Route Table Manager. The IP forwarding table is contained within the TCP/IP protocol. The Route Table Manager updates the IP forwarding table based on incoming route information from multiple sources. The contents of the routing table do not necessarily match the contents of the forwarding table. For the purposes of discussion in this introductory chapter, the routing table and the forwarding table are the same.
Routing Table Structure
As illustrated in Figure 1.5, entries in the routing table usually consist of the following fields:
Network ID The Network ID field contains the identification number for a network route or an internetwork address for a host route.
Forwarding Address The Forwarding Address field contains the address to which the packet is to be forwarded. The forwarding address can be a network interface card address or an internetwork address. For network IDs to which the end system or router is directly attached, the Forwarding Address field may be blank.
Interface The Interface field indicates the network interface that is used when forwarding packets to the network ID. This is a port number or other type of logical identifier. For example, the interface for a 3COM EtherLink III network interface card may be referred to as ELNK3 in the routing table.
Metric The Metric field indicates the cost of a route. If multiple routes exist to a given destination network ID, the metric is used to decide which route is to be taken. The route with the lowest metric is the preferred route. Some routing algorithms only store a single route to any Network ID in the routing table even when multiple routes exist. In this case, the metric is used by the router to decide which route to store in the routing table.
Metrics can indicate different ways of expressing a route preference:
Hop Count . A common metric. Indicates the number of routers (hops) in the path to the network ID.
Delay . A measure of time that is required for the packet to reach the network ID. Delay is used to indicate the speed of the path—local area networks (LAN) links have a low delay, wide area network (WAN) links have a high delay—or a congested condition of a path.
Throughput . The effective amount of data that can be sent along the path per second. Throughput is not necessarily a reflection of the bit rate of the link, as a very busy Ethernet link may have a lower throughput than an unutilized 64-Kbps WAN link.
Reliability . A measure of the path constancy. Some types of links are more prone to link failures than others. For example, with WAN links, leased lines are more reliable than dial-up lines.
Lifetime The Lifetime field indicates the lifetime that the route is considered valid. When routes are learned through the exchange of information with other routers, this is an additional field that is used. Learned routes have a finite lifetime. To keep a learned route in the routing table, the route must be refreshed through a periodic process. If a learned route's lifetime expires, it is removed from the routing table. The timing out of learned routes provides a way for routers to reconfigure themselves when the topology of an internetwork changes due to a downed link or a downed router.
Figure 1.5 Routing Table Structure
The Lifetime field is typically not visible in routing tables.
This list of fields is a representative list in the routing tables. Actual fields in the routing tables for different routable protocols may vary. For information about the IP routing table, see "Introduction to TCP/IP" in the TCP/IP Core Networking Guide . For information about the IPX routing table, see "IPX Routing" in this book.
Locality of the Routing Table
All the routing decisions made by the end system or the router are based on information in a local routing table that physically resides in the random access memory (RAM) of the system making the routing decision. There is no single, holistic view of the internetwork that is being gathered by a server and downloaded to each end system and router so that all users have the same view of the internetwork and all traffic flows along predictable pathways.
Each router in a path between a source and destination makes a local routing decision based on its local routing table. The path taken from the source to the destination may not be the same as the path for response packets from the destination back to the source. If the information in the local routing tables of the end systems or routers is incorrect due to misconfiguration or changing network conditions, then routing problems can result. Troubleshooting routing problems may involve the analysis of the routing tables of the end systems (source and destination) and all the routers forwarding packets between them.
For information about the operation and troubleshooting of IP routing, see "Unicast IP Routing" in this book. For information about the operation and troubleshooting of IPX routing, see "IPX Routing" in this book.