1. Router Types: Cisco has wide
variety of router types starting from a low end fixed configuration
models to that of high-end carrier class routers.
The fixed configuration routers include 850, 870,
and 1800(fixed) routers.
The modular class routers include 1800(Modular),
2800, and 3800.
Cisco Access Routers include Cisco SOHO, SB 100,
820/830, 1700, 2600, and 3700 Series;
Cisco Aggregation Routers include Cisco 7200/7301,
7304, and 7600 Series
2. Hardware Components:
2.1. Chassis:
Below figure shows the chassis rear view of Cisco 2504
that has CPU motherboard with an onboard LAN (Ethernet/TokenRing),
multiple WAN ports, and a single power supply.
Fig : Typical 2504 chassis
2.2 Ports:
2.2.1 Ethernet port using AUI Connector:
AUI connector used for Ethernet ports. These ports are designed
for connecting to an external transceiver that converts to a
specific media type such as twisted pair, coax, or fiber. The
transceiver may be connected to the AUI port either directly
or through an AUI Cable.
2.2.2 Console and Auxiliary ports:
The console and auxiliary ports are asynchronous serial ports.
They provide an out-of-band method for managing the device by
providing an access to Command Line Interface (CLI) with out
affecting the traffic that is flowing through the device. Rollover
cable is used for connecting these ports with the external devices
like PC (or terminal) and modem. Console port is connected to
the COM port of the PC (or terminal) and Auxiliary port is connected
to the modem. The console port is configured as data communications
equipment (DCE), and the auxiliary port is configured as data
terminal equipment (DTE).
2.2.3 Ports Serial 0 and Serial 1:
These are used for connecting to synchronous serial interfaces.
DB-60 is a connector used for connecting these synchronous serial
interfaces to Cisco routers.
2.2.4 BRI port:
This is used for connecting to an asynchronous BRI interface.
RJ-45 is the connector used for ISDN BRI connection.
3. Router Architecture
3.a. Block Diagram:
3.b. The important architectural components of the router
are given below:
3.b.1 Processor:
The CPU used here is Motorola 68030 CISC. The following are
the characteristics of the CPU
32 bit bus, 20 MHz clock (25 MHz on some platforms).
256 Bytes internal Data Cache, 256 Bytes internal Instruction
Cache, both direct mapped.
3.b.2 System Control Logic
System Control Logic is used to help the main processor with
device control, interrupt handling, counting and timing, data
transfer, minimal First In, First Out (FIFO) buffering, and
communication with network interfaces and Dynamic RAM (DRAM).
3.b.3 Buses
CPU use Buses to access various components of the system.
In addition, Buses are used transferring instructions and data
to or from specified memory addresses.
CPU Bus:
It is used for high speed operations with direct Processor
access.
System Bus:
This allows communication with Ethernet or Token Ring controllers,
WAN port interfaces, and so on.
3.b.4. Dual UART
Dual Universal Asynchronous Receiver-Transmitter (UART) provides
the necessary user interface. It has one RS232 port, Data Communications
Equipment (DCE) (for Console port) RJ45, and Data Terminal Equipment
(DTE) (for Auxiliary port) RJ45.
3.b.5 This section describes memory details of a typical
Cisco 25xx Series routers.
3.b.5.a. DRAM
The DRAM has Main Processor Memory and Shared Input/Output
(I/O) memory.
Main Processor Memory is used for routing tables, fast
switching cache, running configuration, and so on. It can
take unused shared I/O memory, if needed.
Shared I/O memory is used for temporary storage of packets
in system buffers.
3.b.5.b Flash
Flash is a permanent storage for the Cisco IOS software image,
backup configurations, and any other files. Cisco IOS software
is run from the flash memory, these image files are defined
as re-locatable, because the Cisco IOS software image can be
executed from different locations in the Flash.
3.b.5.c NVRAM
NVRAM is a Not-Volatile RAM used as permanent storage of
the startup configuration. This memory is writeable. NVRAM size
is 32 KB.
3.b.5.d BOOT ROM.
This refers to erasable programmable read-only memory (EPROM)
and is used to permanently store the startup diagnostic code
(ROM Monitor), and RxBoot. Boot ROM size is 2 MB.
4.Hardware Features: The important
features offered by Cisco™ routers include the following:
high availability,
comprehensive security,
ease of management, and
advanced quality of service (QoS) for audio, video,
broadcast and other real-time applications.
