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Configuring Windows Server 2003 for LAN Routing (part 3) - Managing General IP Routing Properties & Working with Routing Tables

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3/19/2011 9:39:34 PM

Managing General IP Routing Properties

Certain features provided by the Routing And Remote Access service relate to IP routing in general. You can manage these features only in the General Properties dialog box associated with the General subnode within the IP Routing node in the Routing And Remote Access console.

To open the General Properties dialog box, right-click the General node within IP Routing and then click Properties. The General Properties dialog box contains three tabs: Logging, Preference Levels, and Multicast Scopes. (Because multicasting is not covered on the 70-291 exam, only the first two of these tabs are discussed here.)

Logging Tab

Figure 10 shows the Logging tab. This tab allows you to configure which IP routing events are written to the Event Log. By default, only errors are written to the Event Log, but you can select two higher logging levels: Log Errors And Warnings, and Log The Maximum Amount Of Information. In addition, you can disable logging in IP routing when you select the Disable Event Logging option.

Figure 10. IP Routing log configuration


Preference Levels Tab

Figure 11 shows the Preference Levels tab. IP routing decisions are performed according to routing tables, and this feature allows you to arrange the priority of routes collected from various sources. When two sources provide a conflict in routes, only the route with the higher preference level is added to the routing table. These preference levels therefore supersede any metrics assigned to routes.

Figure 11. IP Routing preference levels


Preference levels are listed in order. The first (top) route source has the highest priority and the lowest rank number (1). The lowest route source has the lowest priority and the highest rank number (120). You can adjust the rank of a route source by clicking a source on the list and using the Move Up button and Move Down button as needed.

Working with Routing Tables

Routers read the destination addresses of received packets and then route those packets according to directions provided by routing tables. Figure 12 shows a routing table used by an IP router.

Figure 12. IP Routing table


The routing table contains entries called routes that provide directions toward destination networks or hosts. Three types of routes exist:

  • Host route This type of route provides a route to a specific destination host or to a broadcast address. In IP routing tables, host routes are distinguished by a 255.255.255.255 network mask.

  • Network route This type of route provides a route to a specific destination network. In IP routing tables, network routes can be distinguished by any subnet mask between 0.0.0.0 and 255.255.255.255.

  • Default route Routing tables contain a single default route. This route is used to forward all packets whose destination address does not match any address listed in the routing table. In IP routing tables, the default route is defined by the 0.0.0.0 address and 0.0.0.0 network mask.

Viewing the IP Routing Table

You can view the IP routing table through the Routing And Remote Access console or through a command prompt.

In the Routing And Remote Access console, expand the IP Routing node, right-click the Static Routes node, and then click Show IP Routing Table. (Figure 12 shows an example output from this operation.)

To view the routing table from a command line, type route print and press Enter. Figure 13 shows an example output.

Figure 13. Routing table viewed from a command prompt


Reading the IP Routing Table

Routers use routing tables to determine where to send packets. When IP packets are sent to an IP router, the router reads the destination address of the packet and compares that destination address to the entries in the routing table. One of these entries is used to determine which interface to use to send the packet and to which hop (gateway) the packet will be sent next.

To assist in this process, each routing table entry includes the five columns described in the following sections, as shown in Figure 13.

Network Destination

This column provides entries that the router compares to the destination address of every received IP packet. A few of these entries are common to most routing tables. For example, the entry 0.0.0.0 represents the default route, used when no other matches are found in the routing table. The entry 127.0.0.0 points to the loopback address of 127.0.0.1, which corresponds to the local machine. Each entry of 224.0.0.0, furthermore, refers to a separate multicast route. Finally, entries with a final octet value of 255 represent a broadcast address. Broadcast addresses include specific subnet broadcast addresses, such as 192.168.1.255, and the limited broadcast address 255.255.255.255, which is general for all networks and routers.

Netmask

The value in this column determines which part of the IP packet’s destination address is compared to the entries in the Network Destination column. This information is important because the largest match determines the route or table entry that is applied to the packet.

For instance, suppose the router whose routing table is shown in Figure 13 receives two packets, the first destined for the address 192.168.1.1 and the second destined for the address 192.168.1.2. Both packets match the third routing table entry because the netmask value of 255.255.255.0 signifies that the first three octets (plus a 0 for the fourth octet) are compared to the table’s network destination value of 192.168.1.0.

However, only the first packet matches the fourth entry because the netmask of 255.255.255.255 signals that all four octets are compared to the table’s network destination value of 192.168.1.1. The fourth entry is thus applied to the first packet because this entry represents the largest match in the routing table. In this manner, the third entry is applied to the second packet because that entry represents the packet’s only match in the routing table aside from the default route.

Gateway

When a particular route or table entry is applied to a packet, the gateway value determines the next address or hop for which that packet is destined. For example, according to the routing table shown in Figure 13, an IP packet with a destination such as 206.73.118.5 (which matches only the default route of 0.0.0.0) would next be forwarded to the gateway address of 207.46.252.3. Note that the gateway value for the default route is the same as the default gateway address configured in TCP/IP properties.

A basic concept to understand for the gateway value is that this address should be distinct from that of the network destination listed in the same routing table entry, even when the network destination itself is within broadcast range of the router. For example, in Figure 13, the sixth entry provides directions to the 207.46.252.0/24 destination—one of two subnets to which the router is directly connected. (The other subnet is 192.168.1.0/24.)

Even though the router is directly connected to the 207.46.252.0/24 subnet, the router receives packets destined for this subnet only through its 192.168.1.1 interface. (Packets originating from the 207.46.252.0/24 subnet do not need the router to reach the same subnet.) From this perspective, you can see that the next hop for packets originating from one of the router’s local subnets is the opposite interface of the router. Therefore, the routing table specifies as the gateway the router’s interface on that destination subnet (207.46.252.88).

Tip

Another way to understand the necessary distinction between the gateway address and the network destination address is to think of each routing table entry as providing directions (an interface and gateway) to a given destination. With this analogy in mind, it makes sense that these entries wouldn’t provide directions merely by repeating the destination queried for. (How do you get to 192.168.1.5? Just go there!) Consequently, when the network destination is within a router’s broadcast range, the router’s routing table provides directions to this destination by pointing not to the destination itself but to the router’s own local address that faces the destination. This gateway value will thus be the same as the interface value. Similarly, even in cases where the destination is one of the router’s own addresses, the routing table specifies a distinct address—the loopback address of 127.0.0.1—as the interface and gateway.


Interface

When a particular route (table entry) is applied to a packet, the interface value specified in that route determines which local network interface is used to forward the packet to the next hop. For example, in Figure 13, an IP packet with a destination of 131.107.23.101 matches only the default route. According to the routing table, such a packet is sent through the interface 207.46.252.88 toward the default gateway address.

Metric

This column indicates the cost of using a route. If separate routes (entries) match an IP packet’s destination address equally, the metric is used to determine which route is applied. Lower metrics have precedence over higher metrics.

For the routing protocol RIP, the metric is determined by the number of hops before the network destination. However, you can use any algorithm to determine the metric if you are configuring a route manually.

Static and Dynamic Routing

For every host and router, IP automatically builds a simple routing table that includes only the essential network destinations. These addresses can occur in eight types and include the default address, the loopback address, the default gateway address, locally configured addresses, local subnet addresses, local subnet broadcast addresses, the limited broadcast address, and multicast addresses for each adapter.

All of these eight routing table entry types describe routes that are connected directly to the IP host or router. This default arrangement might work for simple routing situations, but in a complex network, a router needs to be told which among its many interfaces to use to send packets destined for unknown (non-neighboring) networks.

To allow routing tables to properly forward traffic to hosts that are outside broadcast range, you must choose between one of two alternatives. First, you can add routes to these destinations manually by using the Route Add command or by using the Routing And Remote Access console. This process defines static routing. Second, you can configure a dynamic routing protocol such as RIP or OSPF to allow routers to share routing table information with each other. This process defines dynamic routing.

Table 1 compares some of the basic differences between static and dynamic routing.

Table 1. Comparing Static and Dynamic Routing
Static RoutingDynamic Routing
A feature of IP.A feature of routing protocols such as RIP or OSPF.
Routers do not share routing information.Routers share routing information automatically.
Routing tables are built and sustained manually.Routing tables are built and sustained dynamically.
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