The Router
Router Components
(internal) Router Components
(external) Router's Startup
Procedure Configuration Register Cisco® CLI Command Modes User Exec Mode Privileged Exec Mode Setup Mode ROM Monitor Mode
A Router is a layer 3 network device that moves
data between different network segments and can look into a packet header to
determine the best path for the packet to travel. Routers can connect network
segments that use different protocols. They also allow all users in a network
to share a single connection to the Internet or a WAN. It is used to improve
network performance by:-
• segmenting the network and creating separate
collision & broadcast domains.
• reducing competition for bandwidth.
• Broadcasts are not forwarded to other network
segments.
• Increases security by using Access Lists.
Router Components (internal)
ROM ROM is used to store the router's bootstrap startup program, operating system software, and power-on diagnostic tests programs. In order to perform ROM upgrades you remove and replace pluggable chips on the motherboard.
Flash Memory
It holds operating system image(s). Flash memory
is erasable, reprogrammable ROM. You can perform Cisco® IOS software upgrades
without having to remove and replace chips. Flash content is retained when you
switch off or restart the router.
RAM
RAM is used to store
operational information such as routing tables, router's running configuration
file. RAM also provides caching and packet buffering capabilities. Its contents
are lost when you switch off or restart the router.
NVRAM
NVRAM (nonvolatile RAM), is used to store the
router's startup configuration file. It does not lose data when power is
switched off. So the contents of the startup configuration file are maintained
even when you switch off or restart the router.
Network Interfaces
The router's network interfaces are located on
the motherboard or on separate interface modules. You configure Ethernet or
Token Ring interfaces to allow connection to a LAN. The synchronous serial
interfaces are configured to allow connection to WANs. You can also configure
ISDN BRI interfaces to allow connection to an ISDN WAN..
A router can be
configured over any of its network interfaces. You can supply configuration
information to a router using:-
TFTP servers : Trivial File Transfer Protocol; A simplified
version of FTP that allows files to be transferred from one computer to another
over a network.
virtual terminals
network management stations
TFTP servers : Trivial File Transfer Protocol; A simplified
version of FTP that allows files to be transferred from one computer to another
over a network. virtual terminals network management stations
Each time you switch on the router, it goes
through power-on self-test diagnostics to verify basic operation of the CPU, memory
and network interfaces.
The system bootstrap software in ROM (boot image) executes and searches for valid router operating system software (Cisco® IOS image). IOS is acronym for Internetwork Operating System.
There are three places to find the Cisco® IOS image to load:
• Flash memory
• A TFTP server on the network
• ROM
The source of the Cisco® IOS image is determined from the boot field setting of the router's configuration register.
Configuration Registration: A 16-bit register used to control how the router boots up, where the IOS image, how to deal with the NVRAM configuration, setting the console baud rate and enabling or disabling the break function.
The default setting for the configuration register indicates that the router should attempt to load a Cisco® IOS image from flash memory.
If the router finds a valid IOS image, it searches for a valid configuration file. If your router does not find a valid system image, or if its configuration file is corrupted at startup, and the configuration register (bit 13) is set to enter ROM monitor mode, the system will bypass the NVRAM setting and enters ROM monitor mode. This also allow access to the router in the event a password is lost.
The system bootstrap software in ROM (boot image) executes and searches for valid router operating system software (Cisco® IOS image). IOS is acronym for Internetwork Operating System.
There are three places to find the Cisco® IOS image to load:
• Flash memory
• A TFTP server on the network
• ROM
The source of the Cisco® IOS image is determined from the boot field setting of the router's configuration register.
Configuration Registration: A 16-bit register used to control how the router boots up, where the IOS image, how to deal with the NVRAM configuration, setting the console baud rate and enabling or disabling the break function.
The default setting for the configuration register indicates that the router should attempt to load a Cisco® IOS image from flash memory.
If the router finds a valid IOS image, it searches for a valid configuration file. If your router does not find a valid system image, or if its configuration file is corrupted at startup, and the configuration register (bit 13) is set to enter ROM monitor mode, the system will bypass the NVRAM setting and enters ROM monitor mode. This also allow access to the router in the event a password is lost.
The configuration file,
saved in NVRAM, is loaded into main memory and executed one line at a time.
These configuration commands start routing processes, supply addresses for
interfaces, and set media characteristics.
If no configuration file exists in NVRAM, the operating system executes a question-driven initial configuration routine called the system configuration dialog.
This special mode is also called the Setup mode.
If no configuration file exists in NVRAM, the operating system executes a question-driven initial configuration routine called the system configuration dialog.
This special mode is also called the Setup mode.
Cisco® CLI Command Modes
The Cisco® IOS software provides you with access
to several different command modes. Each command mode provides a different
group of related commands.
The Cisco® Command Line Interface (CLI) is called EXEC. EXEC has two modes:-
• User mode
• Privileged mode
For security purposes the two EXEC modes serve as two levels of access to Cisco® IOS commands.
The Cisco® Command Line Interface (CLI) is called EXEC. EXEC has two modes:-
• User mode
• Privileged mode
For security purposes the two EXEC modes serve as two levels of access to Cisco® IOS commands.
EXEC user commands allow you to
• connect to remote devices
• make temporary changes to terminal settings
• perform basic tests
• list system information
If you want to access privileged mode you have to enter a password. The commands available in Privileged mode also include all those available in User mode. You can use Privileged EXECcommands to:-
• set operating parameters
• perform a detailed examination of the router's status
• test and debug router operation
• access global and other included configuration modes
• connect to remote devices
• make temporary changes to terminal settings
• perform basic tests
• list system information
If you want to access privileged mode you have to enter a password. The commands available in Privileged mode also include all those available in User mode. You can use Privileged EXECcommands to:-
• set operating parameters
• perform a detailed examination of the router's status
• test and debug router operation
• access global and other included configuration modes
From Privileged mode you can enter global
configuration mode. This gives you access to configuration commands that affect
the system as a whole, and to other configuration modes.
You can specify the source of the configuration commands as being from :-
• a terminal
• memory
• the network
You can access many other specific configuration modes from Global Configuration mode that allow complex configurations to be performed.
Setup Mode: If the router does not have a configuration file it will automatically enter Setup mode when you switch it on. Setup mode presents you with a prompted dialog, called the system configuration dialog, in which you establish an initial configuration.
You can specify the source of the configuration commands as being from :-
• a terminal
• memory
• the network
You can access many other specific configuration modes from Global Configuration mode that allow complex configurations to be performed.
Setup Mode: If the router does not have a configuration file it will automatically enter Setup mode when you switch it on. Setup mode presents you with a prompted dialog, called the system configuration dialog, in which you establish an initial configuration.
Rom Monitor Mode: If the router does not
find a valid operating system image, or if you interrupt the boot sequence, the
system may enter ROM monitor mode. From ROM monitor mode you can boot the
device or perform diagnostic tests.
in-bottM �
. ��~ @'} in-left:0in;line-height:14.25pt'>• The network layer sends packets from source
network to destination network.
• It provides consistent end-to-end packet delivery
services to its user, the transport layer.
In wide area networking a substantial geographic distance and many networks can separate two end systems that wish to communicate. Between the two end systems the data may have to be passed through a series of widely distributed intermediary nodes. These intermediary nodes are normally routers.
Routers are special stations on a network, capable of making complex routing decisions.
• The network layer is the domain of routing.
Routing protocols select optimal paths through the series of interconnected networks.
Network layer protocols then move information along these paths.
In wide area networking a substantial geographic distance and many networks can separate two end systems that wish to communicate. Between the two end systems the data may have to be passed through a series of widely distributed intermediary nodes. These intermediary nodes are normally routers.
Routers are special stations on a network, capable of making complex routing decisions.
• The network layer is the domain of routing.
Routing protocols select optimal paths through the series of interconnected networks.
Network layer protocols then move information along these paths.
• One of the functions of the network layer is
"path determination".
Path determination enables the router to evaluate all available paths to a destination and determine which to use. It can also establish the preferred way to handle a packet.
After the router determines which path to use it can proceed with switching the packet.
It takes the packet it has accepted on one interface and forwards it to another interface or port that reflects the best path to the packet's destination.
Devices:-
Path determination enables the router to evaluate all available paths to a destination and determine which to use. It can also establish the preferred way to handle a packet.
After the router determines which path to use it can proceed with switching the packet.
It takes the packet it has accepted on one interface and forwards it to another interface or port that reflects the best path to the packet's destination.
Devices:-
• IP, IPX, Routers, Routing Protocols
(RIP, IGRP, OSPF, BGP etc), ARP, RARP, ICMP.
Layer two of the OSI reference model is the
data-link layer. This layer is responsible for providing reliable transit of
data across a physical link. The data-link layer is concerned with
• physical addressing; Bridges, Transparent
Bridges, Layer 2 Switches
• network topology; CDP
• line discipline (how end systems will use the network link)
• error notification
• ordered delivery of frames
• flow control
• network topology; CDP
• line discipline (how end systems will use the network link)
• error notification
• ordered delivery of frames
• flow control
• Frame Relay, PPP, SDLC, X.25, 802.3, 802.3,
802.5/Token Ring, FDDI.
At the data-link layer, the bits that come up from the physical layer are formed into data frames, using any of a variety of data-link protocols. Frames consist of fields, containing bits.
The data-link layer is subdivided into two sub layers:
• the logical link control (LLC) sub layer
• the media access control (MAC) sub layer
The LLC sub layer provides support for
• the media access control (MAC) sub layer
The LLC sub layer provides support for
•
connections between applications running on a LAN
• flow control to the upper layer by means of ready/not ready codes
• sequence control bits.
The LLC sub layer rests on top of other media access protocols to provide interface flexibility.
Because the LLC sub layer operates independently of specific media access protocols, upper layer protocols, for example IP at the network layer, can operate autonomously without concern as to the specific type of LAN media. The LLC sub layer can depend on lower layers to provide access to the media. It provides Service Access Points (SAP's) and flow control. This layer puts 1's & 0's into a logical frame.
The MAC sub layer provides orderly access to the LAN medium. For multiple stations to share the same medium and still uniquely identify each other, the MAC sub layer defines a hardware, or data-link address called the "MAC address". The MAC address is unique for each LAN interface. On most LAN interface cards the MAC address is burned into ROM.
The ROM MAC address is sometimes known as the burned-in address (BIA).
The MAC address is a 48-bit address expressed as 12 hexadecimal digits written in three groups of four digits. The first six hexadecimal digits (the first 24 bits) represent a vendor code known as the organizationally unique identifier (OUI). To ensure vendor uniqueness, the IEEE administers OUIs. The last six hexadecimal digits are administered by the vendor and often represent the interface serial number.
• flow control to the upper layer by means of ready/not ready codes
• sequence control bits.
The LLC sub layer rests on top of other media access protocols to provide interface flexibility.
Because the LLC sub layer operates independently of specific media access protocols, upper layer protocols, for example IP at the network layer, can operate autonomously without concern as to the specific type of LAN media. The LLC sub layer can depend on lower layers to provide access to the media. It provides Service Access Points (SAP's) and flow control. This layer puts 1's & 0's into a logical frame.
The MAC sub layer provides orderly access to the LAN medium. For multiple stations to share the same medium and still uniquely identify each other, the MAC sub layer defines a hardware, or data-link address called the "MAC address". The MAC address is unique for each LAN interface. On most LAN interface cards the MAC address is burned into ROM.
The ROM MAC address is sometimes known as the burned-in address (BIA).
The MAC address is a 48-bit address expressed as 12 hexadecimal digits written in three groups of four digits. The first six hexadecimal digits (the first 24 bits) represent a vendor code known as the organizationally unique identifier (OUI). To ensure vendor uniqueness, the IEEE administers OUIs. The last six hexadecimal digits are administered by the vendor and often represent the interface serial number.
Before a frame is exchanged with a device on
the same LAN, the sending device needs to have a MAC address it can use as a
destination address.
The sending device may use an address resolution protocol (such as TCP/IP's address resolution protocol (ARP)) to discover the destination's MAC address. In other protocols the MAC address can be determined directly from the network address.
For example, assume that host Y and host Z are on the same LAN. Host Y broadcasts an ARP request onto the LAN looking for host Z. Because it is a broadcast message all devices on the LAN, including host Z, process the request. However, host Z is the only device to respond and it does so with its MAC address. Host Y receives host Z's reply and stores the MAC address in local memory. This is often called an "ARP cache". The next time host Y needs to communicate with host Z it recalls host Z's stored MAC address.
Process of Finding Hosts on the Different Network Segment- ARP + Router
The sending device may use an address resolution protocol (such as TCP/IP's address resolution protocol (ARP)) to discover the destination's MAC address. In other protocols the MAC address can be determined directly from the network address.
For example, assume that host Y and host Z are on the same LAN. Host Y broadcasts an ARP request onto the LAN looking for host Z. Because it is a broadcast message all devices on the LAN, including host Z, process the request. However, host Z is the only device to respond and it does so with its MAC address. Host Y receives host Z's reply and stores the MAC address in local memory. This is often called an "ARP cache". The next time host Y needs to communicate with host Z it recalls host Z's stored MAC address.
Process of Finding Hosts on the Different Network Segment- ARP + Router
Let's
look at how host Y communicates with host X on a different LAN, which it can
access via router A.
As before host Y broadcasts its ARP request. Router A, along with all the other devices on the LAN, processes the request. It knows that host X will not see the request because it is on another LAN, and that any packets destined for host X will have to be relayed. So instead, router A provides its own MAC address to host Y as a "proxy" reply to the ARP request. Host Y receives the router's response and saves the MAC address in its ARP cache memory. The next time host Y needs to communicate with host X, it recalls the stored MAC address of router A.
As before host Y broadcasts its ARP request. Router A, along with all the other devices on the LAN, processes the request. It knows that host X will not see the request because it is on another LAN, and that any packets destined for host X will have to be relayed. So instead, router A provides its own MAC address to host Y as a "proxy" reply to the ARP request. Host Y receives the router's response and saves the MAC address in its ARP cache memory. The next time host Y needs to communicate with host X, it recalls the stored MAC address of router A.
Layer one of the OSI model is the physical
layer. The physical layer is concerned with the interface to the transmission
medium. At the physical layer, data is transmitted onto the medium (e.g.
coaxial cable or optical fiber) as a stream of bits.
So, the physical layer is concerned, not with networking protocols, but with the transmission media on the network.
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. This layer puts 1's & 0's onto the wire.
Characteristics specified by the physical layer include
So, the physical layer is concerned, not with networking protocols, but with the transmission media on the network.
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. This layer puts 1's & 0's onto the wire.
Characteristics specified by the physical layer include
• voltage levels
• timing of voltage changes
• physical data rates
• maximum transmission distances
• physical connectors
• timing of voltage changes
• physical data rates
• maximum transmission distances
• physical connectors
Devices:-
•
Hubs, FDDI Hardware, Fast Ethernet, Token Ring Hardware.
Example of Layered Transmission
Let's look at the transport layer in TCP/IP as
an example.
The transport layer must use the services of the network layer in order to communicate to the peer TCP function on another system. Each lower layer in turn takes upper layer information as part of the PDUs it exchanges with its peer layer.
The transport layer must use the services of the network layer in order to communicate to the peer TCP function on another system. Each lower layer in turn takes upper layer information as part of the PDUs it exchanges with its peer layer.
Each lower layer adds whatever headers and trailers
it requires to perform its functions. This is called "data
encapsulation".
The transport layer's segments become part of the network layer's "packets" exchanged between IP peers. Network layer packets are also known as "datagrams".
The network layer adds to the start of the PDU, a header to the data that identifies the source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.
The Host-to-network layer takes the IP packet and adds a header to form a "frame". The header contains information required to complete the data-link functions. For example, the frame header contains a physical address which allows the network device to communicate over its interface to the next directly connected network device on the link.
Ultimately, these frames must be converted into electrical pulses as the data is finally transmitted by the physical layer protocol across the wire or other physical medium used by the network.
The transport layer's segments become part of the network layer's "packets" exchanged between IP peers. Network layer packets are also known as "datagrams".
The network layer adds to the start of the PDU, a header to the data that identifies the source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.
The Host-to-network layer takes the IP packet and adds a header to form a "frame". The header contains information required to complete the data-link functions. For example, the frame header contains a physical address which allows the network device to communicate over its interface to the next directly connected network device on the link.
Ultimately, these frames must be converted into electrical pulses as the data is finally transmitted by the physical layer protocol across the wire or other physical medium used by the network.
No comments:
Post a Comment