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Managing TCP/IP Routing

  • Explain the purpose and properties of routing and switching.

Because today’s networks branch out between interconnected offices all over the world, networks may have any number of separate physical network segments connected using routers. Routers are devices that direct data between networks. Essentially, when a router receives data, it must determine the destination for the data and send it there. To accomplish this, the network router uses two key pieces of information: the gateway address and the routing tables.

The Default Gateway

A default gateway is the router’s IP address, which is the pathway to any and all remote networks. To get a packet of information from one network to another, the packet is sent to the default gateway, which helps forward the packet to its destination network. Computers that live on the other side of routers are said to be on remote networks. Without default gateways, Internet communication is not possible because your computer doesn’t have a way to send a packet destined for any other network. On the workstation, it is common for the default gateway option to be configured automatically through DHCP configuration.

Routing Tables

Before a data packet is forwarded, a chart is reviewed to determine the best possible path for the data to reach its destination. This chart is the computer’s routing table. Maintaining an accurate routing table is essential for effective data delivery. Every computer on a TCP/IP network has a routing table stored locally. Figure 3.3 shows the routing table on a Windows 7 system.

Figure 3.3.

Figure 3.3. The routing table on a Windows 7 system.

As shown in Figure 3.3, the information in the routing table includes the following:

  • Destination: The host IP address.
  • Network mask: The subnet mask value for the destination parameter.
  • Gateway: Where the IP address is sent. This may be a gateway server, router, or another system acting as a gateway.
  • Interface: The address of the interface that’s used to send the packet to the destination.
  • Metric: A measurement of the directness of a route. The lower the metric, the faster the route. If multiple routes exist for data to travel, the one with the lowest metric is chosen.

Routing tables play an important role in the network routing process. They are the means by which the data is directed through the network. For this reason, a routing table needs to be two things. It must be up to date and complete. The router can get the information for the routing table in two ways: through static routing or dynamic routing.

Static Routing

In environments that use static routing, routes and route information are manually entered into the routing tables. Not only can this be a time-consuming task, but also errors are more common. In addition, when a change occurs to the network’s layout, or topology, statically configured routers must be manually updated with the changes. Again, this is a time-consuming and potentially error-laden task. For these reasons, static routing is suited to only the smallest environments, with perhaps just one or two routers. A far more practical solution, particularly in larger environments, is to use dynamic routing.

You can add a static route to a routing table using the route add command. To do this, specify the route, the network mask, and the destination IP address of the network card your router will use to get the packet to its destination network.

The syntax for the route add command is as follows:

route add 192.168.2.1 mask (255.255.255.0) 192.168.2.4

Adding a static address is not permanent; in other words, it will most likely be gone when the system reboots. To make it persistent (the route is still in the routing table on boot), you can use the switch with the command.

Dynamic Routing

In a dynamic routing environment, routers use special routing protocols to communicate. The purpose of these protocols is simple: They enable routers to pass on information about themselves to other routers so that other routers can build routing tables. Two types of routing protocols are used: the older distance-vector protocols and the newer link-state protocols.

Distance-Vector Routing

With distance-vector router communications, each router on the network communicates all the routes it knows about to the routers to which it is directly attached. In this way, routers communicate only with their router neighbors and are unaware of other routers that may be on the network.

The communication between distance-vector routers is known as hops. On the network, each router represents one hop, so a network using six routers has five hops between the first and last router.

The tracert command is used in a Windows environment to see how many hops a packet takes to reach a destination. To try this at the command prompt, enter tracert comptia.org. Figure 3.4 shows an example of the output on a Windows 7 workstation.

Figure 3.4.

Figure 3.4. The results of running tracert on a Windows 7 system.

Several distance-vector protocols are in use today, including Routing Information Protocol (RIP and RIPv2), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP):

  • RIP: As mentioned, RIP is a distance-vector routing protocol. RIP is limited to a maximum of 15 hops. One of the downsides of the protocol is that the original specification required router updates to be transmitted every 30 seconds. On smaller networks this is acceptable; however, this can result in a huge traffic load on larger networks. The original RIP specification also did not support router authentication, leaving it vulnerable to attacks.
  • RIPv2: The second version of RIP dealt with the shortcomings of the original design. Authentication was included to enable secure transmissions, also, it changed from a networkwide broadcast discovery method to a multicast method to reduce overall network traffic. However, to maintain compatibility with RIP, RIPv2 still supports a limit of 15 hops.
  • BGP: A routing protocol often associated with the Internet. BGP can be used between gateway hosts on the Internet. BGP examines the routing table, which contains a list of known routers, the addresses they can reach, and a cost metric associated with the path to each router so that the best available route is chosen. BGP communicates between the routers using TCP.
  • EIGRP: A protocol that enables routers to exchange information more efficiently than earlier network protocols. EIGRP uses its neighbors to help determine routing information. Routers configured to use EIGRP keep copies of their neighbors’ routing information and query these tables to help find the best possible route for transmissions to follow. EIGRP uses Diffusing Update Algorithm (DUAL) to determine the best route to a destination.

Distance-vector routing protocols operate by having each router send updates about all the other routers it knows about to the routers directly connected to it. The routers use these updates to compile their routing tables. The updates are sent automatically every 30 or 60 seconds. The interval depends on the routing protocol used. Apart from the periodic updates, routers can also be configured to send a triggered update if a change in the network topology is detected. The process by which routers learn of a change in the network topology is called convergence.

Routing loops can occur on networks with slow convergence. Routing loops occur when the routing tables on the routers are slow to update and a redundant communication cycle is created between routers. Two strategies can combat potential routing loops:

  • Split horizon: Works by preventing the router from advertising a route back to the other router from which it was learned. This prevents two nodes from bouncing packets back and forth between them, creating a loop.
  • Poison reverse (also called split horizon with poison reverse): Dictates that the route is advertised back on the interface from which it was learned, but it has a hop count of infinity, which tells the node that the route is unreachable.

Although distance-vector protocols can maintain routing tables, they have three problems:

  • The periodic update system can make the update process slow.
  • The periodic updates can create large amounts of network traffic—much of the time unnecessarily, because the network’s topology should rarely change.
  • Perhaps the most significant problem is that because the routers know about only the next hop in the journey, incorrect information can be propagated between routers, creating routing loops.

Link-State Routing

A router that uses a link-state protocol differs from a router that uses a distance-vector protocol because it builds a map of the entire network and then holds that map in memory. On a network that uses a link-state protocol, routers send link-state advertisements (LSAs) that contain information about the networks to which they connect. The LSAs are sent to every router on the network, thus enabling the routers to build their network maps.

When the network maps on each router are complete, the routers update each other at a given time, just like with a distance-vector protocol; however, the updates occur much less frequently with link-state protocols than with distance-vector protocols. The only other circumstance under which updates are sent is if a change in the topology is detected, at which point the routers use LSAs to detect the change and update their routing tables. This mechanism, combined with the fact that routers hold maps of the entire network, makes convergence on a link-state-based network quickly occur.

Although it might seem like link-state protocols are an obvious choice over distance-vector protocols, routers on a link-state-based network require more powerful hardware and more RAM than those on a distance-vector-based network. Not only do the routing tables need to be calculated, but they must also be stored. A router that uses distance-vector protocols need only maintain a small database of the routes accessible by the routers to which it is directly connected. A router that uses link-state protocols must maintain a database of all the routers in the entire network.

Link-state protocols include the following:

  • Open Shortest Path First (OSPF): A link-state routing protocol based on the SPF (Shortest Path First) algorithm to find the least-cost path to any destination in the network. In operation, each router using OSPF sends a list of its neighbors to other routers on the network. From this information, routers can determine the network design and the shortest path for data to travel.
  • Intermediate System-to-Intermediate System (IS-IS): A link-state protocol that discovers the shortest path for data to travel using the shortest path first (SPF) algorithm. IS-IS routers distribute topology information to other routers, enabling them to make the best path decisions.

So what’s the difference between the two? OSPF (a network layer protocol) is more often used in medium to large enterprise networks because of its special tunneling features. IS-IS is more often used in large ISP networks because of its stability features and that it can support more routers.

Routing Metrics

Following are a number of metrics related to routing that you should know for the exam:

  • Hop counts are the number of hops necessary to reach a node. A hop count of infinity means the route is unreachable.
  • The Maximum Transmission Unit (MTU) defines the largest data unit that can be passed without fragmentation.
  • Bandwidth specifies the maximum packet size permitted for Internet transmission.
  • Costs are the numbers associated with traveling from point A to point B (often hops). The lower the total costs (the less links in the route), the more that route should be favored.
  • Latency is the amount of time it takes for a packet to travel from one location to another.

Cram Quiz

  1. Which of the following best describes the function of the default gateway?

    • circle.jpg A. It provides the route for destinations outside the local network.
    • circle.jpg B. It enables a single Internet connection to be used by several users.
    • circle.jpg C. It identifies the local subnet and formulates a routing table.
    • circle.jpg D. It is used to communicate in a multiple-platform environment.
  2. What is the term used for the number of hops necessary to reach a node?
    • circle.jpg A. Jump list
    • circle.jpg B. Link stops
    • circle.jpg C. Connections
    • circle.jpg D. Hop count

Cram Quiz Answers

  1. A. The default gateway enables systems on one local subnet to access those on another. Answer B does not accurately describe the role of the default gateway. Answers C and D don’t describe the main function of a default gateway, which is to provide the route for destinations outside the local network.
  2. D. The hop count is the number of hops necessary to reach a node.
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