Cisco Internetwork Operating System (IOS)

Cisco IOS is a collection of network operating systems used on Cisco Network Devices#, usually stored at the flash of the device or TFTP server. It can be used from input and output devices. It also provides the mean for memory management, process management, file systems management, security management, hardware management, network interface configuration, and enabling routing# and switching# functions.

ROM of the device will be used to partially boot up IOS by waking up the bootstrap program. Otherwise, it could be also used to perform power-on self test (POST) diagnostics. The flash of the device is used to store, locate and load IOS. Unless it doesn’t function, then Trivial File Transfer Protocol (TFTP) will be the second choice where the IOS will be transferred. The configuration file could be found in NVRAM, console and/or TFTP server. We can enter setup mode via them too.

We could access an IOS device’s command line interface (CLI) using either its console (by connecting to its console port with RJ-45/mini-USB to Serial/USB rollover cable), Telnet or Secure Shell (SSH)#, or auxiliary port (uses telephone line).

The emulation of IOS for different devices could be seen in #Cisco Packet Tracer.

Modes of Operations

There are several modes of operations in IOS which we can access via the console with built-in commands. They are User EXEC, Privileged EXEC, Global Configuration, Interface Configuration, Sub-Interface Configuration, Routing Engine Configuration, Line Configuration, Controller Configuration, Map-List Configuration, Map-Class Configuration, Router Configuration, IPX-Router Configuration, Route-Map Configuration, Setup, and ROM Monitor.

The following sections describe the commands used in these modes of operation. Additionally, we can add a ? to the end of an incomplete command in order to get hints on using the command or finding a command.

User EXEC

User EXEC is the default mode for an IOS device.

ping
show
enable (enter Privileged EXEC, can shorten as en)
traceroute

Privileged EXEC

Privileged EXEC could be entered via the command enable in User EXEC. It includes all User EXEC and debugging commands excepts the following:

reload
show, with subcommands running-config (show the system details) and ip route (shows Routing Table#)
copy, commonly copy running-config startup-config to save the configurations
erase, with subcommands startup-config (remove configurations)
reload (reload the system)
configure terminal (enter Global Configuration, can shorten as conf t)
disable (exit Privileged EXEC)

Global Configuration

Global Configuration will show up when the command configure is executed in Privileged EXEC. From here, we can configure the settings for the Network Device#.

hostname (set host name of the device)
enable secret (set password for Privileged EXEC)
ip route (Static Routing#)
interface, with subcommands ethernet, serial, dsl etc. (enter Interface Configuration)
router, with subcommands rip, ospf, eigrp etc. (enter Routing Engine Configuration)
line, with subcommands vty, console etc. (enter Line Configuration)
end (to end the configuration, available too in other Configuration modes)
service password-encryption (shows all the password configured in encrypted form)
banner motd (set banner, typically used to warn entering users)
no ip domain-lookup (disable Domain Name Server (DNS) lookup).
ipv6 unicast-routing (enable IPv6# unicast forwarding)

Interface Configuration

Interface Configuration could be entered via the command interface executed in Global Configuration. It is where we could configure the network interface of the device.

Note: We could define a range of interfaces with the same configuration by running the command interface range {interfaces}, e.g., interface range f0/1-12, f0/18-24.

ip address (set static IPv4 Address#)
ipv6 address (set static IPv6 Address#)
encapsulation
shutdown (disable the interface)
no shutdown (enable the interface and keep-alive mechanism)
bandwidth (adjusting the bandwidth of the interface in Kb/s, useful in Routing Protocol that use bandwidth as its metric)

Routing Engine Configuration

Routing Engine Configuration could be entered via the command router executed in Global Configuration. It is where we could configure the routing information.

network
version
auto summary
passive interface (prevent a router from sending updates through an interface)

Line Configuration

Line Configuration could be entered via the command line executed in Global Configuration. It is where we could configure the line access of the device including modem commands.

password (set password for the line)
login (activates the password immediately)

Note: Console port should be secured from unauthorised access by physically plugging in a cable to the device.

Links to this page

TRS2251 Chapter 1: Introduction to OSI Model and Internetworking

Cisco Internetwork Operating System (IOS)
Static Routing

In #Cisco Internetwork Operating System (IOS), this could be done using the command ip route and supply it with the destination network address, its subnet mask and the next-hop router’s IP address or the outgoing interface (called exit interface). Since using next-hop router’s IP address for static route could result in Recursive Route Lookup#, it is advised to use exit interface when ever possible. However, in the case of configuring on an Ethernet# link, the best practice is to configure with both the next-hop router’s IP address and the exit interface to avoid Address Resolution Protocol (ARP)# shenanigans.
Routing Information Protocol (RIP)

It could be enabled in #Cisco Internetwork Operating System (IOS) using the command router rip. To redistribute or advertising the routes in the routing table, we could run the command redistribute static.
Router

The physical structure of Router mainly consists of CPU, RAM, ROM, Flash, NVRAM and interfaces or ports. RAM not only stores the Routing Table#, but also holds Address Resolution Protocol (ARP) cache and fast-switching cache, performs packet buffering, and provides temporary memory for the running configuration file. NVRAM provides permanent storage for startup configuration file. Various versions of #Cisco Internetwork Operating System (IOS) are stored in Flash. ROM is where the power-on self test (POST) diagnostic software and bootstrap program (boot up partial IOS) located.
Roll-Over Cable

Roll-Over Cable is a #Twisted Pair Cable where the pin mapping is completely reverse, meaning the pin will be mapped to its total opposite order of the pin at the end of the cable. It is usually used to connect PC to Router, especially in Cisco Internetwork Operating System (IOS).
RIP version 2 (RIPv2)

RIPv2 can support discontiguous network by turning off auto summarisation (it is enabled by default). In Cisco Internetwork Operating System (IOS)# this can be done by no auto-summary in the Router Configuration mode.
Open Shortest Path First (OSPF)

OSPF is a #Layer 3 #classless #Link-State Routing Protocol developed by Internet Engineering Task Force (IETF)# where its metric is an arbitrary value called cost (defined by RFC 2328). In Cisco, the cost calculated using bandwidth. It is secure, scalable and provides fast Convergence#. It supports both IPv4 and IPv6. The implementation of OSPF varies from vendor to vendor.
Default Route

To define a Default Route in Cisco Internetwork Operating System (IOS)#, use the command ip route and assign two quad zeros# (0.0.0.0) to it with an interface’s IP address or name.
Cisco Switch Configuration

To configure #Virtual LAN (VLAN) with Switch#, there is a need to set up an IP Address, subnet mask, and default gateway (router to direct switch management traffic for remote access such as Telnet and Secure Shell (SSH)) in a switch. The configuration could be done in IOS# with the command vlan {number} in Interface Configuration Mode follows by the IP address assigned to the VLAN. We can attach a name to a VLAN by simply running the command name {vlan-name} right after vlan {number} command in Global Configuration Mode. Define the VLAN membership mode for the assigned interface (port) as access (see Dynamic Trunking Protocol (DTP)# for other possible modes) with switchport mode {mode} and assign VLAN to it by switchport access vlan {number} in the Interface Configuration Mode.
Cisco Packet Tracer

Cisco Packet Tracer is a networking simulation tool developed by Cisco. It allows user to build up virtual topologies and emulate designed computer networks. This includes the emulation of Cisco Internetwork Operating System (IOS)#.
Cisco OSPF Router Configuration

To enable #Open Shortest Path First (OSPF) to a #Cisco router, run the command router ospf {process-id} to enter Router Configuration Mode with the process ID ranging from 1 to 65,535. A Router# can run multiple OSPF interfaces (differ by the process ID) at the same time.
Cisco Discovery Protocol (CDP)

CDP is a #Data Link Layer Cisco proprietary tool used to gather information about other directly connected Cisco network devices. However, it is recommended to disable CDP by no cdp run for the entire device or no cdp enable for one particular interface.
Administrative Distance (AD)

In #Cisco Internetwork Operating System (IOS), it could be shown using the command show ip route and inspect the routing table. It is typically in the format of:

#operating-system #networking #)