STATIC ROUTING

INTRODUCTION
Routing is at the core of every data network, moving information across an internetwork from source to destination. Routers are the devices responsible for the transfer of packets from one network to the next.
Routers learn about remote networks either dynamically using routing protocols or manually using static routes. In many cases routers use a combination of both dynamic routing protocols and static routes. This chapter focuses on static routing.
Static routes are very common and do not require the same amount of processing and overhead as we will see with dynamic routing protocols
ROLE OF ROUTER
The router is a special-purpose computer that plays a key role in the operation of any data network. Routers are primarily responsible for interconnecting networks by:
Determining the best path to send packets
Forwarding packets toward their destination
Routers perform packet forwarding by learning about remote networks and maintaining routing information. The router is the junction or intersection that connects multiple IP networks. The routers primary forwarding decision is based on Layer 3 information, the destination IP address.
The router's routing table is used to find the best match between the destination IP of a packet and a network address in the routing table. The routing table will ultimately determine the exit interface to forward the packet and the router will encapsulate that packet in the appropriated data page link frame for that outgoing interface
Introducing the Topology
3 1800 series routers connected via WAN links
Each router connected to a LAN represented by a switch and a PC
Introducing the Topology
The figure shows the topology used in this chapter. The topology consists of three routers, labeled R1, R2, and R3. Routers R1 and R2 are connected through one WAN link, and routers R2 and R3 are connected through another WAN link. Each router is connected to a different Ethernet LAN, represented by a switch and a PC.
Each router in this example is a Cisco 1841. A Cisco 1841 router has the following interfaces:
Two Fast Ethernet interfaces: Fast Ethernet 0/0 and Fast Ethernet 0/1
Two serial interfaces: Serial 0/0/0 and Serial0/0/1
Router Connections
Connecting a router to a network requires a router interface connector to be coupled with a cable connector. Cisco routers support many different connector types.
Serial Connectors
For WAN connections, Cisco routers support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 standards for serial connections, as shown. Memorizing these connection types is not important. Just know that a router has a DB-60 port that can support five different cabling standards. Because five different cable types are supported with this port, the port is sometimes called a five-in-one serial port. The other end of the serial cable is fitted with a connector that is appropriate to one of the five possible standards.
Note: The documentation for the device to which you want to connect should indicate the standard for that device.
Newer routers support the smart serial interface that allows for more data to be forwarded across fewer cable pins. The serial end of the smart serial cable is a 26-pin connector. It is much smaller than the DB-60 connector used to connect to a five-in-one serial port. These transition cables support the same five serial standards and are available in either DTE or DCE configurations.
These cable designations are only important to you when configuring your lab equipment to simulate a "real-world" environment. In a production setting, the cable type is determined for you by the WAN service you are using.
Ethernet Connectors
A different connector is used in an Ethernet-based LAN environment. An RJ-45 connector for the unshielded twisted-pair (UTP) cable is the most common connector used to connect LAN interfaces. At each end of an RJ-45 cable, you should be able to see eight colored strips, or pins. An Ethernet cable uses pins 1, 2, 3, and 6 for transmitting and receiving data.
Two types of cables can be used with Ethernet LAN interfaces:
A straight-through, or patch cable, with the order of the colored pins the same on each end of the cable
A crossover cable, with pin 1 connected to pin 3, and pin 2 connected to pin 6
Straight-through cables are used for:
Switch-to-router
Switch-to-PC
Hub-to-PC
Hub-to-server
Crossover cables are used for:
Switch-to-switch
PC-to-PC
Switch-to-hub
Hub-to-hub
Router-to-router
Router-to-server
EXAMINING ROUTER INTERFACE
the show ip route command is used to display the routing table. Initially, the routing table is empty if no interfaces have been configured.
As you can see in the routing table for R1, no interfaces have been configured with an IP address and subnet mask.
Static routes and dynamic routes will not be added to the routing table until the appropriate local interfaces, also known as the exit interfaces, have been configured on the router. This procedure will be examined more closely in later chapters.
Interfaces and their Status
The status of each interface can be examined by using several commands.
The show interfaces command shows the status and gives a detailed description for all interfaces on the router. As you can see, the output from the command can be rather lengthy. To view the same information, but for a specific interface, such as FastEthernet 0/0, use the show interfaces command with a parameter that specifies the interface. For example:
R1#show interfaces fastethernet 0/0
FastEthernet0/0 is administratively down, line protocol is down
Notice that the interface is administratively down and the line protocol is down. Administratively down means that the interface is currently in the shutdown mode, or turned off. Line protocol is down means, in this case, that the interface is not receiving a carrier signal from a switch or the hub. This condition may also be due to the fact that the interface is in shutdown mode.
You will notice that the show interfaces command does not show any IP addresses on R1's interfaces. The reason for this is because we have not yet configured IP addresses on any of the interfaces.
Additional Commands for Examining Interface Status
The show ip interface brief command can be used to see a portion of the interface information in a condensed format.
The show running-config command displays the current configuration file that the router is using. Configuration commands are temporarily stored in the running configuration file and implemented immediately by the router. Using this command is another way to verify the status of an interface such as FastEthernet 0/0.
R1#show running-config
<some output omitted>
interface FastEthernet0/0
no ip address
shutdown
<some output omitted>
However, using show running-config is not necessarily the best way to verify interface configurations. Use the show ip interface brief command to quickly verify that interfaces are up and up (administratively up and line protocol is up).