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What is networking and how does it work? What is a router, routing table and the main routing protocols? We've got the answers.
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Mindaugas Kubilius Network Administrator Mindaugas is a Network Administrator at IPXO with more than 15 years of experience in the IT field. He specializes in building and maintaining various network infrastructures, as well as presenting top-notch engineering solutions to the public. After work, Mindaugas spends his time in nature.
This section discusses the best path to send packets, load balancing, and the concept of administrative distance. Routing Decisions (1.2.2.1)A primary function of a router is to determine the best path to use to send packets. To determine the best path, the router searches its routing table for a network address that matches the destination IP address of the packet. The routing table search results in one of three path determinations:
The logic flowchart in Figure 1-29 illustrates the router packet-forwarding decision process.
Figure 1-29 Packet Forwarding Decision Process Best Path (1.2.2.2)Determining the best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or shortest path to reach that network. Whenever multiple paths to the same network exist, each path uses a different exit interface on the router to reach that network. The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. A metric is the quantitative value used to measure the distance to a given network. The best path to a network is the path with the lowest metric. Dynamic routing protocols typically use their own rules and metrics to build and update routing tables. The routing algorithm generates a value, or a metric, for each path through the network. Metrics can be based on either a single characteristic or several characteristics of a path. Some routing protocols can base route selection on multiple metrics, combining them into a single metric. The following lists some dynamic protocols and the metrics they use:
The animation in the online course highlights how the path may be different depending on the metric being used. Load Balancing (1.2.2.3)What happens if a routing table has two or more paths with identical metrics to the same destination network? When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally. This is called equal cost load balancing. The routing table contains the single destination network, but has multiple exit interfaces, one for each equal cost path. The router forwards packets using the multiple exit interfaces listed in the routing table. If configured correctly, load balancing can increase the effectiveness and performance of the network. Equal cost load balancing can be configured to use both dynamic routing protocols and static routes. By default, Cisco routers can load balance up to four equal cost paths. The maximum number of equal cost paths depends on the routing protocol and IOS version. EIGRP supports equal cost load balancing and is also the only routing protocol to support unequal cost load balancing. Unequal cost load balancing is when a router distributes traffic over network interfaces, even those that are different distances from the destination address. The animation in the online course provides an example of equal cost load balancing. Administrative Distance (1.2.2.4)It is possible for a router to be configured with multiple routing protocols and static routes. If this occurs, the routing table may have more than one route source for the same destination network. For example, if both RIP and EIGRP are configured on a router, both routing protocols may learn of the same destination network. However, each routing protocol may decide on a different path to reach the destination based on that routing protocol’s metrics. RIP chooses a path based on hop count, whereas EIGRP chooses a path based on its composite metric. How does the router know which route to use? Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The AD represents the “trustworthiness” of the route; the lower the AD, the more trustworthy the route source. For example, a static route has an AD of 1, whereas an EIGRP-discovered route has an AD of 90. Given two separate routes to the same destination, the router chooses the route with the lowest AD. When a router has the choice of a static route and an EIGRP route, the static route takes precedence. Similarly, a directly connected route with an AD of 0 takes precedence over a static route with an AD of 1. Table 1-5 lists various routing protocols and their associated ADs. Table 1-5 Default Administrative Distances
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This chapter introduced the router. The main purpose of a router is to connect multiple networks and forward packets from one network to the next. This means that a router typically has multiple interfaces. Each interface is a member or host on a different IP network. Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The routing table is a list of networks known by the router. The routing table includes network addresses for its own interfaces, which are the directly connected networks, as well as network addresses for remote networks. A remote network is a network that can only be reached by forwarding the packet to another router. Remote networks are added to the routing table in one of two ways: either by the network administrator manually configuring static routes or by implementing a dynamic routing protocol. Static routes do not have as much overhead as dynamic routing protocols; however, static routes can require more maintenance if the topology is constantly changing or is unstable. Dynamic routing protocols automatically adjust to changes without any intervention from the network administrator. Dynamic routing protocols require more CPU processing and also use a certain amount of link capacity for routing updates and messages. In many cases, a routing table will contain both static and dynamic routes. Routers make their primary forwarding decision at Layer 3, the network layer. However, router interfaces participate in Layers 1, 2, and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link frame and encoded into bits at Layer 1. Router interfaces participate in Layer 2 processes associated with their encapsulation. For example, an Ethernet interface on a router participates in the ARP process like other hosts on that LAN. The Cisco IP routing table is not a flat database. The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets. Components of the IPv6 routing table are very similar to the IPv4 routing table. For instance, it is populated using directly connected interfaces, static routes, and dynamically learned routes. Page 3
The following activities provide practice with the topics introduced in this chapter. The Labs and Class Activities are available in the companion Routing Protocols Lab Manual (978-1-58713-322-0). The Packet Tracer Activities PKA files are found in the online course. Class Activities
Class Activity 1.0.1.2: Do We Really Need a Map? Class Activity 1.4.1.1: We Really Could Use a Map! Labs
Lab 1.1.1.9: Mapping the Internet Lab 1.1.4.6: Configuring Basic Router Settings with IOS CLI Lab 1.1.4.7: Configuring Basic Router Settings with CCP Packet Tracer Activities
Packet Tracer Activity 1.1.1.8: Using Traceroute to Discover the Network Packet Tracer Activity 1.1.2.9: Documenting the Network Packet Tracer Activity 1.1.3.5: Configuring IPv4 and IPv6 Interfaces Packet Tracer Activity 1.1.4.5: Configuring and Verifying a Small Network Packet Tracer Activity 1.3.2.5: Investigating Directly Connected Routes 12. Check Your Understanding Questions | Next Section Previous Section |
What is the term for a device that determines the best path for data to take across a network?
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