CCNA study notes on Cisco IOS

Cisco IOS

1. Cisco router boot configuration commands:

2. To enable the Cisco IOS to forward packets destined for obscure subnets of directly connected networks onto the best route, you use "ip classless" command.

3. Internal memory components of a Cisco router:

4. The Cisco router can be configured from many locations.

5. Router modes of operation:

6. There are three ways a router learns how to forward a packet:

1. Static Routes - Configured by the administrator manually. The administrator must also update the table manually every time a change to the network takes place. Static routes are commonly used when routing from a network to a stub (a network with a single route) network

The command is

ip route network mask address/interface [distance]

ex: ip route

Here, is the destination network or subnet is the subnet mask is the default gateway

2. Default Routes - The default route (gateway of last resort) is used when a route is not known or is infeasible. The command is

ip route 165.44.56.

The default gateway is set to

3. Dynamic Routes - In dynamic routing, the routing tables are automatically updated. Dynamic routing uses broadcasts and multicasts to communicate with other routers.

The commands to enable rip are:

router rip

network <major network number>

7. The following are some important commands that can be used to edit and review command history buffer on a Cisco router. It will be useful to practice these commands.

8. There are five different types of passwords:

9. The Catalyst IOS software is very similar to a router IOS. IOS image files are stored in the Flash memory on a switch.

Show running-config [interface / | vlan | module ]: The command displays the contents of the configuration file.Show tech-support: The command is primarily used to send switch information to Cisco TAC support personnel.

Verify flash: - This command is used to verify whether the Flash contents are intact, and not corrupted. The checksum of the flashfile specified is verified for correctness.

10. By default, Cisco routers support 5 simultaneous telnet sessions. This number can be configured using IOS commands.

11. Routers can make alternate route decisions based on ICMP messages, if appropriate. Routers send an ICMP message if the destination is unreachable.

ICMP (Internet Message Control Protocol) messages are used for basic error reporting between host to host, or host to gateway. It is not used for error reporting between Gateways. ICMP messages are encapsulated using the IP protocol. For example, the command “ping” uses ICMP protocol. In the OSI Reference model, ICMPs are generally considered part of the IP layer.

i. CDP

ii. Router Commands

1. The following are some frequently used COPY commands:

2. SHOW command is extensively used for seeing the status and configuration information of the router.

Some of the frequently used commands are:

3. Show IP protocol: This command will show information on RIP timers including routing update timer (30sec default), hold-down timer (default 180sec). It also displays the number of seconds due for next update (this is fraction of update timer). This command also gives the network number for which IP RIP is enabled, Gateway, and the default metric.

4. Show IP route: This command will display the IP routing table entries. In addition, it displays the Gateway of last resort (if one is assigned). It also displays the codes used for various types of routes. Some of the important codes are:

Show IP interface: This command shows you interface-wise information such as IP address assigned to each interface, whether the interface is up, MTU etc.

Debug IP RIP: Debug IP RIP will turn the RIP debugging ON. This will display a continuous list of routing updates as they are sent and received. This leads to lot of overhead, which is the reason that you use "undebug ip rip" to turn-off debugging as soon as you finish with debugging.

5. The banner is displayed whenever anyone logs in to your Cisco router. The syntax is

"banner motd # " . MOTD stands for "Message Of The Day".

# symbol signifies the start of the banner message to the router. You will be prompted for the message to be displayed. You need to enter "#" symbol at the end of the message, signifying that the message has ended.

6. Copy running-config startup-config : allows the running configuration file to be saved onto the startup configuration file on the switch. Make sure that you use this command whenever you have made any configuration changes to the switch. Otherwise, your configuration command are not permanently saved in the switch memory, and lost soon after power cycling the switch.

The command:

Copy startup-config running-config : allows startup configuration file to be copied into the current running configuration file.

Copy running-config tftp: Copies the running configuration of a switch to a TFTP server. You will be prompted for the server address and destination filename.

Copy tftp: startup-config : This command is useful to restore the startup-config file incase the original is lost or corrupt. The command loads the startup-config file from a remote tftp server.

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