External ports and interfaces
Routers can be purchased with different ports and interfaces.
Typical interfaces include:-
-Console port
-Auxiliary port
-Serial port

The CONSOLE port
-Starting point for configuration.
-Allows terminal connection (hyperTerminal in windows) via a PC and a cable.
-Typically basics like IP addresses will be assigned through the connection.
-Once basic configuration is in place further configuration can be done via telnet across the network.

The AUXILIARY port
-Allows a modem to be connected to the router.
-This can then be used to dial into the router for remote troubleshooting or configuration.
-Its main use is for remote configuration since troubleshooting and minor changes can be done via a telnet connection.

The SERIAL port
-Used for the connection of a leased line. IE kilo Stream or Mega Stream and carries user data.
-Represents the WAN side of the router
-Once enabled in the router’s software and given an IP address it can also be used for TELNET access into the router.
-This allows configuration and troubleshooting to be done remotely across the WAN link.

The AUI port
-Attachment unit interface
-Used for connection to the LAN
-Transceivers need to be purchased separately depending on the transmission used in your network.
-This allows the router to be transferable between different media without the need for different hardware configurations.
Internal components

Since the router is a complex device it has internal processing and file storage capabilities.
Typical components inside include:-
-ROM
-Flash
-RAM
-NVRAM

ROM – typically holds:-
-The bootstrap startup program
-Power On Self Test (POST)
-Can hold the routers Operating System on certain models.
-Generally installed on the motherboard in sockets to allow easy upgrade and replacement.

Flash
-Basically a form of Re-programmable ROM.
-It’s a non-volatile form of storage
-Not all models have flash memory
-If present will hold the IOS (routers operating system)
-Allows easy upgrade. No need to remove chips from the motherboard.

NVRAM
-Non-volatile RAM
-Stores the router’s startup configuration file
-This file is a bit like AUTOEXEC.BAT in that it contains all the instructions within the router’s configuration
-IE IP addresses for ports
-Interface descriptions
-Routes to remote networks

RAM
-Volatile memory
-Holds the running IOS
-Holds the route tables when the router is running
-Provides caching and packet buffering to increase performance

The boot procedure
When the power is turned on the router will begin its boot procedure. This progresses as follows:-
-The POST will run. This checks CPU, Memory and interface circuitry.
-The bootstrap program is initialised from ROM. This searches for the IOS.
-IOS is loaded from Flash or ROM

The bootstrap then searches for a startup configuration file
The startup configuration file is loaded from NVRAM.
If no startup configuration file is found the setup dialogue is displayed.
The internet work operating system.
The router is configured via a textual user interface known as the INTERNET WORK OPERATING SYSTEM (IOS)
This can be accessed via the console port or auxiliary port with a pc running terminal emulation software. EG HyperTerminal.
If the router already has a basic configuration we can telnet in via a network interface.

The Exec.
The IOS contains something known as the EXEC.
This is a bit like command.com in DOS.
It takes the command you enter and interprets it into an operation.
Unlike command.com the exec has different modes.
There are three exec modes:-
-User mode
-Very low risk level of access. Eg. check router status and view basic system info
-Privileged mode
-From here you can get detailed status info, use utilities like ping and access configuration mode
-Configuration mode
-Sometimes called global configuration mode.
-This mode of the exec is the highest risk.
-It allows manual configuration of the router.
-Allows access to the various interface configuration modes.

Command prompts.
You can tell which mode you are in by looking at the router prompt:-
-User mode = Router>
-Privileged mode = Router#
-Configuration mode = Router(config)#
? can be used from any mode to get help.

Configuration files
Each router has two configurations:-
-Startup configuration
-Loaded at router startup
-Stored in NVRAM and is loaded into DRAM at boot up
-Running configuration
-The configuration that is currently running
Any changes we enter in the router affect the running configuration.
This means if we want them to be permanent they need to be saved to the startup configuration.
This is useful since it allows us to make temporary changes to the router’s configuration.

Erase startup-config
-Erases the routers configuration. Basically returns it to the state it left the factory

Copy running-config startup-config
-Saves the currently running configuration to the startup-config file

The following commands are used to manage the configuration files:-
-Show startup-config
-Displays the startup configuration file
-Show running-config
-Displays the currently running configuration

Setting passwords
To set passwords you need to be in global configuration mode.
There are a number of passwords that can be used to protect your router
They are case sensitive
-Secret
-Used to restrict access to the privileged mode
-Virtual terminal
-Protects telnet access. Has to be configured for telnet access to work.
-Auxiliary
-Used to protect access from the router’s auxiliary (modem) interface.
-Console
-Used to protect direct cable access via the console port