Cisco has a number of different routers, amongst them are the popular 1600 series, 2500 series & 2600 series. There is more. The ranges start from the 600
series & go up to the 12000 series (now we’re talking about a lot of money). All the above equipment runs special software called the Cisco Internetwork
Operating System or IOS. This is the kernel of their routers & most switches. Cisco has created what they call Cisco Fusion, which is supposed to make all these
devices run the same operating system.
We are going to begin with the basic components which make up a Cisco router (and switches) & I’ll be
explaining what they’re used for, so grab that tea or coffee & let us get going !
The basic components of any Cisco router are
- Interfaces
- The Processor (CPU)
- Internetwork Operating System (IOS)
- RXBoot Image
- RAM
- NVRAM
- ROM
- Flash memory
- Configuration Register
Now I just hope you have not looked at the list & thought “Stuff this, it looks hard & complicated” because I
assure you, it is less painful than you might think ! In actual fact, once you read it a couple of times, you’ll find all of it easy to remember & understand. Interfaces These
allow us to use the router ! The interfaces are the various serial ports or ethernet ports which we use to connect the router to our LAN. There’re a number of
different interfaces but we’re going to hit the basic stuff only. Here are some of the names they have given some of the interfaces: E0 (first Ethernet interface), E1
(second Ethernet interface). S0 (first Serial interface), S1 (second Serial interface), BRI 0 (first B channel for Basic ISDN) & BRI 1 (second B channel for Basic
ISDN).
You have to connect a digital phone to an ISDN line & since this is an ISDN router, it has this option with the router. I should, how ever, explain that
you do not normally get routers with ISDN S/T & ISDN U interfaces together. Any ISDN line requires a Network Terminator (NT) installed at the customer’s premises
and you connect your equipment after this terminator. An ISDN S/T interface does not have the NT device built in, so you need an NT device in order to use the
router. On the other hand, an ISDN U interface has the NT device built in to the router.
Apart from the ISDN interfaces, we also have an Ethernet interface
that connects to a device in your LAN, usually a hub or a computer. If connecting to a Hub uplink port, then you set the small switch to “Hub”, but if connecting to a
PC, you really need to set it to “Node”. This switch will simply convert the cable from a straight through (hub) to a x- over (Node):
The Config or Console port is a
Female DB9 connector which you connect, using a special cable, to your computers serial port & it allows you to directly configure the router.The Processor (CPU)
All Cisco routers have a main processor that takes care of the main functions of the router. The CPU generates interrupts (IRQ) in order to communicate with the
other electronic components in the router. The routers utilize Motorola RISC processors. Usually the CPU utilisation on a normal router would not exceed 20
%.
The IOS
The IOS is the main operating system on which the router runs. There is more. The IOS is loaded upon the router’s bootup. It usually is around
2 to 5MB in size, but can be a lot larger depending on the router series. There is more. The IOS is currently on version 12, & they periodically releases minor versions every couple
of months e.g 12.1 , 12.3 etc. to fix small bugs & also add extra functionality.
The IOS gives the router its various capabilities & can also be updated or
downloaded from the router for backup purposes. On the 1600 series & above, you get the IOS on a PCMCIA Flash card. This Flash card then plugs into a slot
located at the back of the router & the router loads the IOS “image” (as they call it). Usually this image of the operating system is compressed so the router must
decompress the image in its memory in order to use it.
The IOS is one of the most critical parts of the router, without it the router is pretty much useless. Just
keep in mind that it’s not necessary to have a flash card (as described above with the 1600 series router) in order to load the IOS. You can actually configure most
routers to load the image off a network tftp server or from another router which might hold multiple IOS images for different routers, in which case it will have a large
capacity Flash card to store these images.
The RXBoot Image
The RXBoot image (also known as Bootloader) is nothing more than a
“cut-down” version of the IOS located in the router’s ROM (Read Only Memory). If you had no Flash card to load the IOS from, you can configure the router to load
the RXBoot image, which would give you the ability to perform minor maintenance operations & bring various interfaces up or down.
The
RAM
The RAM, or Random Access Memory, is where the router loads the IOS & the configuration file. It works exactly the same way as your
computer’s memory, where the operating system loads along with all the various programs. There is more. The amount of RAM your router needs is subject to the size of the IOS
image & configuration file you have. To give you an indication of the amounts of RAM we’re talking about, in most cases, smaller routers (up to the 1600 series)
are happy 
with 12 to 16 MB while the bigger routers with larger IOS images would need around 32 to 64 MB of memory. Routing tables are also stored in the system’s
RAM so if you have large & complex routing tables, you’ll obviously need more RAM ! When I attempted to upgrade the RAM on a 1600 router, I unscrewed the case
and opened it & was amazed to find a 72 pin SIMM slot where you needed to attach the extra RAM. For all those who do not know what a 72 pin SIMM is, it is basically
the type of RAM the older Pentium socket 7 CPUs took, back in ‘95. This type of memory was replaced by today’s standard 168 pin DIMMs or
SDRAM.The
NVRAM (Non-Volatile RAM)
The NVRAM is a special memory place where the router holds its configuration. When you
configure a router & then save the configuration, it’s stored in the NVRAM. This memory isn’t big at all when compared with the system’s RAM. On a 1600 series,
it is only 8 KB while on bigger routers, like the 2600 series, it’s 32 KB. Normally, when a router starts up, after it loads the IOS image it will look into the NVRAM &
load the configuration file in order to configure the router. The NVRAM isn’t erased when the router is reloaded or even switched off.
ROM (Read Only
Memory)
The ROM is used to start & maintain the router. It contains some code, like the Bootstrap & POST, which helps the router do some basic
tests & bootup when it is powered on or reloaded. You can not alter any of the code in this memory as it has been set from the factory & is Read
Only.
Flash Memory
The Flash memory is that card I spoke about in the IOS section. All it is, is an EEPROM (Electrical Eraseable
Programmable Read Only Memory) card. It fits into a special slot normally located at the back of the router & contains nothing more than the IOS image(s). You can
write to it or delete its contents from the router’s console. Usually it comes in sizes of 4MB for the smaller routers (1600 series) & goes up from there depending on
the router model.
Configuration Register
Keeping things simple, the Configuration Register determines if the router is going to boot the IOS
image from its Flash, tftp server or just load the RXBoot image. This register ® is a 16 Bit register, in other words has 16 zeros or ones. A sample of it in Hex would be
the following: 0×2102 & in binary is : 0010 0001 0000 0010.
Bob Bello, is the owner of http://www.CiscoKits.com At his site you’ll find over
200 various articles dedicated to Cisco CCNA & CCNP study help, free CCNA &
CCNP study exams, step by step labs along with CCNA & CCNP home lab kits.
