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Basic Router Configuration and Operation

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Basic Router Configuration and Operation

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

Basic Router Configuration and Operation

IP Configuration Commands

Command Configuration Mode
ip address ip address mask [secondary] Interface mode
ip host name [tcp-port-number] address1

interface} [distance] [name name]
ip name-server server-address1 [[server- address2]…server
ip domain-lookup
ip routing
hostname name
Line configuration mode
password value
Line configuration mode
keepalive [seconds]
Interface subcommand
exec-timeout minutes [seconds]
Line subcommand
logging synchronous
Line subcommand

IP Exec Commands

Command Function
Show hosts Lists all host names and
corresponding IP
show interfaces [type number]
Lists interface statistics,
including IP address
show ip interface [type number]
Provides a detailed view
of IP parameter settings
per interface
show ip interface brief
Provides a summary of
all interfaces and their IP
Shows entire routing
table or a subset if other
parameters are entered
show ip arp [ip-address] [host-name] [mac-address] [type number]
Displays IP ARP cache
debug ip packet
Issues log messages for
each IP packet
decimal | hexadecimal}
Sets type of display for
subnet masks in show
tag] {host-name | system-address}
Sends and receives ICMP
echo messages to verify
trace [protocol] [destination]
Sends a series of UDP
packets with increasing
TTL values to verify the
current route to a host

Typical Router Configuration

  • A host name for the router
  • Reference to a DNS so that commands typed on the router can refer to host names
  • instead of IP addresses
  • Set a password on the console port
  • Set a password for those Telnetting to the router
  • Set the enable secret password to protect access to privileged mode
  • Create a banner stating an appropriate warning, depending on the security practices at that company

exec timeout minutes seconds: sets the inactivity timeout

logging synchronous line subcommand: tells the router not to interrupt the output of a show command with debug messages

terminal ip netmaskformat decimal exec command:: If you prefer to see the subnet masks instead of the prefix

Seeding the Routing Table with Connected IP Routes

The Cisco IOS routes IP packets by default (you do not need to type any

commands to tell the router to enable IP routing). Before the router will route packets in or out of an interface, the interface must have an IP address.

IOS adds connected routes to the routing table when:

  • The interface has been configured with a valid IP address.
  • The interface is in an up and up status according to the various interface-oriented show commands.

Up-and-up status: First state refers to Layer 1 status; If there is no cable plugged in, the first status word would be down instead of up. Second state is Layer 2 (HDLC on one end of the serial link, but other end setup using PPP).

- > Errors can also occur if keepalive messages don't succeed. Default is every ten seconds, and if down for 3x interval (30 seconds), reports down status.

- > Disable keepalives with no keepalive subcommand or change interval with keepalive interval

- > To administratively bring down an interface, shutdown (and no shutdown to bring back up)

Bandwidth, Clock Rate, and Serial Lines in the Lab

When creating WAN without CSU/DSU, use DTE/DCE connectors. One router must supply the clocking (the end with the DCE).

1. Determine which router has DCE

a. Show controllers 0/0

2. Set clock rate

a. Clock rate clock rate in bps

3. Configure bandwidth (optional)

a. Bandwidth bandwidth in kbps

IP Troubleshooting Features

Internet Control Message Protocol (ICMP): provides a wide variety of information about the health and operational status of a network (sit inside an IP packet, with no transport layer header at all)

Message Types

Message Purpose
Destination unreachable Tells source host there is a problem delivering the packet.
Time exceeded Time it take to deliver packet has expired; and the packet has been discarded.
Redirect Message tells sender to use the "better" route that is available.
Echo Used by ping to verify connectivity.

Echo Request/Echo Reply: Send a request, if receive a reply, connectivity is okay

- > Default is 5 echo requests, timeout is 2 seconds

Destination Unreachable Reasons

Unreachable Code When is it Used? Sent By?
Network unreachable No match in routing table for destination Router
Host unreachable Can locate router connected to subnet, host not responding. Router
Can't fragment Packet has Don't Fragment bit set, and router must fragment to send. Router
Protocol unreachable Packet delivered to host, but transport protocol not available Endpoint Host
Port unreachable Destination port not opened by application Endpoint Host

Ping Command Codes

Code Description
! ICMP Echo received
. Nothing received before ping timed out
U ICMP unreachable (destination) received
N ICMP unreachable (network) received
P ICMP unreachable (port) received
Q ICMP source quench received
M ICMP Can't Fragment message received
? Unknown packet received

IP Naming Commands

Statically Configure hosts: ip host Chris

DNS Server: ip domain-name

ip name-server [address1] [address]
ip domain-lookup
  • Multiple DNS often used for redundancy
  • ip domain-lookup enables IOS to ask name server
  • if FQDN not given, assumes ip domain-name


Function Command
Use a DNS ip domain-lookup
Configure IP address of name server ip name-server svr1 svr2…
Statically assign hosts ip host hostname ipaddress
Display current hostname information show hosts

Telnet and Suspend: telnet IOS command allows you to telnet from one device to another. Suspend connection, resume using connection number.

- > To suspend connection: ctrl+shift+6, followed by letter x
- > To resume, resume connection number or connection number
- > To display session, show sessions or where commands
- > To quit session, quit command

*Note: Pressing enter automatically connects to most recently suspended connection

Cisco Discovery Protocol (CDP): Discovers basic information about neighboring routers and switches, supports any interface that supports SNAP headers. CDP devices advertise their information and listen to other devices advertisements.

Obtains this information:

  • Device Identifier: Typically the host name
  • Address list: IP/MAC addresses
  • Port identifier: Another name for an interface
  • Capabilities list: What type of device it is (router/switch)
  • Platform: Model/OS level running in device

- > CDP is enabled by default, no cdp run disables it (cdp run re-enables it).

- > To disable particular interface, no cdp enable (cdp enable re-enables it).

1. show CDP neighbor command: shows each neighbor (1 line of output/neighbor)

2. Then, to display further information, enter: show cdp entry entryname to display a specific neighbors details.

3. Or, to show all neighbors and details: show cdp neighbor detail

With show cdp features, possible to create network map (lists port/interface connections)

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