8.5 Introduction to Routing

Basic configuration only requires the network administrator to enable the directly connected networks within the dynamic routing protocol. The dynamic routing protocol will automatically do as follows:

- Discover remote networks - Maintain up-to-date routing information - Choose the best path to destination networks - Attempt to find a new best path if the current path is no longer available

What does a code of "O" indicate next to a route in the routing table? - a directly connected route - a route with an administrative distance of 0 - a gateway of last resort - a route learned dynamically from OSPF

- a route learned dynamically from OSPF

A directly connected route is automatically created when a router interface is configured with IP address information and is activated. The router adds two route entries with the codes _ (i.e., the connected network) and _ (i.e., the local interface IP address of the connected network). The route entries also identify the exit interface to use to reach the network. The two directly connected networks in this example are _____ and _____. Routers R1 and R2 are also using the OSPF dynamic routing protocol to exchange router information. In the example routing table, R1 has a route entry for the _____ network that it learned dynamically from router R2 via the OSPF routing protocol.

C L 192.168.10.0/24 209.165.200.224/30 10.1.1.0/24

Static routing has the following characteristics:

- A static route must be configured manually. - The administrator needs to reconfigure a static route if there is a change in the topology and the static route is no longer viable. - A static route is appropriate for a small network and when there are few or no redundant links.

Common route sources (codes) include these:

- L - Directly connected local interface IP address - C - Directly connected network - S - Static route was manually configured by an administrator - O - OSPF - D - EIGRP

Which is a characteristic of static routes? - They are manually configured. - They are advertised to directly connected neighbors. - They are appropriate when there are many redundant links. - They automatically adjust to a change in network topology.

- They are manually configured.

True or False? A router can be configured with a combination of both static routes and a dynamic routing protocol.

- True

This type of route is also known as a gateway of last resort. - static route - remote route - default route - directly connected route

- default route

What is the command used on a Cisco IOS router to view the routing table? - netstart -r - route print - show ip route - show routing table

- show ip route

The routing table stores three types of route entries:

1) Directly-connected networks- In the figure, the directly-connected networks in the R1 IPv4 routing table would be 192.168.10.0/24 and 209.165.200.224/30. 2) Remote networks- In the figure, the remote network in the R1 IPv4 routing table would be 10.1.1.0/24. 3) Default route- In the figure, the R1 IPv4 routing table would most likely include a default route to forward all packets to router R2.

A router can learn about remote networks in one of two ways:

1) Manually - Remote networks are manually entered into the route table using static routes. 2) Dynamically - Remote routes are automatically learned using a dynamic routing protocol.

Dynamic routing

A dynamic routing protocol allows the routers to automatically learn about remote networks, including a default route, from other routers. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator. If there is a change in the network topology, routers share this information using the dynamic routing protocol and automatically update their routing tables. Dynamic routing protocols include OSPF and Enhanced Interior Gateway Routing Protocol (EIGRP). The figure shows an example of routers R1 and R2 automatically sharing network information using the routing protocol OSPF.

It [IS / IS NOT] common for some routers to use a combination of both static routes and a dynamic routing protocol.

IS

If there is a change in the network topology, the static route [IS / IS NOT] automatically updated and must be manually reconfigured.

IS NOT

Default route

Like a host, most routers also include a default route entry, a gateway of last resort. The default route is used when there is no better (longer) match in the IP routing table. A default route has a network address of all zeroes. For example, the IPv4 network address is 0.0.0.0. A static route entry in the routing table begins with a code of S*, as highlighted in the example.

Static routes

Static routes are route entries that are manually configured. The figure shows an example of a static route that was manually configured on router R1. The static route includes the remote network address and the IP address of the next hop router.

Routing table

The router examines the destination IP address of the packet and searches its routing table to determine where to forward the packet. The routing table contains a list of all known network addresses (prefixes) and where to forward the packet. These entries are known as route entries or routes. The router will forward the packet using the best (longest) matching route entry.

show ip route

The show ip route privileged EXEC mode command is used to view the IPv4 routing table on a Cisco IOS router. The example shows the IPv4 routing table of router R1. At the beginning of each routing table entry is a code that is used to identify the type of route or how the route was learned.

Directly-connected networks

These network route entries are active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each router interface is connected to a different network segment.

Remote networks

These network route entries are connected to other routers. Routers learn about remote networks either by being explicitly configured by an administrator or by exchanging route information using a dynamic routing protocol.