routers communicate with each other by WAN
connections. Routers are the backbone devices of large
intranets and of the Internet. They operate at Layer 3 of the
OSI model, making decisions based on network addresses. The two
main functions of a router are the selection of best path for
and the switching of frames to the proper interface. Routers
accomplish this by building routing tables and exchanging
network information with other routers. An administrator can
maintain routing tables by configuring static routes, but
generally routing tables are maintained dynamically through the
use of a routing protocol that exchanges network topology
(path) information with other routers. If, for example,
computer (x) needs to communicate with computer (y) on one side
of the world, and with computer (z) in another distant
location, a routing feature for information flow is required as
well as redundant paths for reliability. Many network design
decisions and technologies can be traced to this desire for
computers x, y, and z to be able to communicate. A correctly
configured internetwork provides the following:
- Consistent end-to-end addressing
- Addresses that
represent network topologies
- Best path selection
- Dynamic or static routing
- Switching
Web Links How Routers Work
http://www22.verizon.com/about/community/
learningcenter/articles/dslarticle1/
0%2C16157%2C1080z1%2C00.html
Content 1.1 WANs
1.1.4 Router role in a WAN A WAN is said
to operate at the physical layer and at the data link layer.
This does not mean that the other five layers of the OSI model
are not found in a WAN. It simply means that the
characteristics that separate a WAN from a LAN are typically
found at the physical layer and the data link layer. In other
words, the standards and protocols used in WANs at Layer 1 and
Layer 2 are different from those used in LANs at the same
layers. The WAN physical layer describes the interface between
the data terminal equipment (DTE) and the data
circuit-terminating equipment (DCE). Generally, the DCE is the
service provider and the DTE is the attached device. In this
model, the services offered to the DTE are made available
through a modem or a CSU/DSU. The principal function of a
router is routing. Routing occurs at the network layer, Layer
3, but if a WAN operates at Layers 1 and 2, is a router a LAN
device or a WAN device? The answer is both, as is so often the
case in the field of networking. A router may be exclusively a
LAN device, it may be exclusively a WAN device, or it may sit
at the boundary between a LAN and a WAN and be a LAN and WAN
device at the same time. One of the roles of a router in a WAN
is to route packets at Layer 3, but this is also a role of a
router in a LAN. Therefore routing is not strictly a WAN role
of a router. When a router uses the physical and data link
layer standards and protocols that are associated with WANs, it
is operating as a WAN device. The primary WAN roles of a router
are therefore not routing, but providing connections to and
between the various WAN physical and data-link standards. For
example, a router may have an ISDN interface using PPP
encapsulation and a serial interface terminating a T1 line
using Frame Relay encapsulation. The router must be able to
move a stream of bits from one type of service, such as ISDN,
to another, such as a T1, and change the data link
encapsulation from PPP to Frame Relay. Many of the details of
WAN Layer 1 and Layer 2 protocols will be covered later in the
course, but some of the key WAN protocols and standards are
listed here for reference. WAN physical layer standards and
protocols: - EIA/TIA-232
- EIA/TIA-449
- V.24
- V.35
- X.21
- G.703
- EIA-530
- ISDN
- T1, T3, E1, and E3
- xDSL
- SONET (OC-3, OC-12, OC-48, OC-192)
WAN data link layer standards and protocols:
- High-level data link control (HDLC)
- Frame Relay
- Point-to-Point Protocol (PPP)
- Synchronous
Data Link Control (SDLC)
- Serial Line Internet
Protocol (SLIP)
- X.25
- ATM
- LAPB
- LAPD
- LAPF
Web Links
Choosing a WAN Router http://www.naspa.com/PDF/97/
T9704009.pdf
Content 1.1 WANs
1.1.5 Academy approach to hands-on labs In the
academy lab, all the networks will be connected with serial or
Ethernet cables and the students can see and physically touch
all the equipment. Unlike the academy lab setup, the serial
cables in the real world are not connected back to back. In a
real world situation, one router could be in New York, while
another router could be in Sydney, Australia. An administrator
located in Sydney would have to connect to the router in New
York through the WAN cloud in order to troubleshoot the New
York router. In the academy lab, devices that make up the WAN
cloud are simulated by the connection between the back-to-back
DTE-DCE cables. The connection from one router interface s0/0
to another router interface s0/1 simulates the whole circuit
cloud. Interactive Media Activity Drag and Drop:
Academy Lab Equipment Setup When the student completes this
activity, the student should know the correct order in which to
connect all devices and cables to create the CCNA lab equipment
setup. Web Links Introduction to WAN Technologies
http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/
introwan.htm
Content 1.2 Routers
1.2.1 Router internal components While the
exact architecture of the router varies between router models,
this section will introduce the major internal components.
Figures and show the internal components of some of the Cisco
router models. The common components are covered in the
paragraphs below. CPU – The Central Processing Unit
(CPU) executes instructions in the operating system. Among
these functions are system initialization, routing functions,
and network interface control. The CPU is a microprocessor.
Large routers may have multiple CPUs. RAM –
Random-access memory (RAM) is used for routing table
information, fast switching cache, running configuration, and
packet queues. In most routers the RAM provides run time space
for executable Cisco IOS software and its subsystems. RAM is
usually logically divided into main processor memory and shared
input/output (I/O) memory. Shared I/O memory is shared among
interfaces for temporary storage of packets. The contents of
RAM are lost when power is removed. RAM is generally dynamic
random-access memory (DRAM) and can be upgraded by adding
additional Dual In-Line Memory Modules (DIMMs). Flash –
Flash memory is used for storage of a full Cisco IOS software
image. The router normally acquires the default IOS from flash.
These images can be upgraded by loading a new image into flash.
The IOS may be in uncompressed or compressed form. In most
routers an executable copy of the IOS is transferred to RAM
during the boot process. In other routers the IOS may be run
directly from flash. Adding or replacing the flash Single
In-Line Memory Modules (SIMMs) or PCMCIA cards can upgrade the
amount of flash. NVRAM – Nonvolatile random-access
memory (NVRAM) is used to store the startup configuration. In
some devices, NVRAM is implemented using separate
electronically erasable programmable read-only memory (EEPROMs)
in some devices. In other devices it is implemented in the same
flash device from which the boot code is loaded. In either case
these devices retain contents when power is removed.
Buses – Most routers contain a system bus and a CPU bus.
The system bus is used for communication between the CPU and
the interfaces and/or expansion slots. This bus transfers the
packets to and from the interfaces. The CPU bus is used by the
CPU for accessing components from router storage. This bus
transfers instructions and data to or from specified memory
addresses. ROM – Read-only memory (ROM) is used for
permanently storing startup diagnostic code (ROM Monitor). The