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This chapter is from the book

This chapter is from the book

Path Determination (1.2.2)

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:

  • Directly connected network: If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the interfaces of the router, that packet is forwarded directly to the destination device. This means that the destination IP address of the packet is a host address on the same network as the interface of the router.
  • Remote network: If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.
  • No route determined: If the destination IP address of the packet does not belong to either a connected or remote network, the router determines if there is a Gateway of Last Resort available. A Gateway of Last Resort is set when a default route is configured on a router. If there is a default route, the packet is forwarded to the Gateway of Last Resort. If the router does not have a default route, then the packet is discarded. If the packet is discarded, the router sends an ICMP Unreachable message to the source IP address of the packet.

The logic flowchart in Figure 1-29 illustrates the router packet-forwarding decision process.

Figure 1-29

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:

  • Routing Information Protocol (RIP): Hop count
  • Open Shortest Path First (OSPF): Cisco routers use a cost based on cumulative bandwidth from source to destination
  • Enhanced Interior Gateway Routing Protocol (EIGRP): Bandwidth, delay, load, reliability

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

Route Source

Administrative Distance

Connected

0

Static

1

EIGRP summary route

5

External BGP

20

Internal EIGRP

90

IGRP

100

OSPF

110

IS-IS

115

RIP

120

External EIGRP

170

Internal BGP

200

Unknown

255

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