Configure Router Serial Interfaces, DTE (Data Terminal Equipment) and DCE (Data Communications Equipment)

When connecting a serial cable to the serial interface of the router, clocking is provided by an external device, such as a CSU/DSU device. A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a router to a digital circuit. The router is the DTE (Data Terminal Equipment) and the external device is the DCE (Data Communications Equipment), where the DCE provides the clocking. However, in some cases we might connect two routers back-to-back using the routers’ serial interfaces (Example: Inside the router labs). Each router is a DTE by default. The cable decides which end to be DCE or DTE and it is usually marked on the cable. The picture below shows back to back cable.

Smart Serial to Smart Serial back to back cable

If is not marked, we can use the Cisco IOS show command “show controller” command to determine the interface is DTE or DCE. Since clocking is required to enable the interface, one of the two routers should function as DCE and should provide clocking. This can be done by using the “clock rate” command, from the interface configuration mode.

To find the possible clock rate values, get the command help by using a question mark after the “clock rate” from the serial interface configuration mode, as shown below.

Router(config-if)# clock rate ?

The output for above command is shown below.

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface serial 1/0
R1(config-if)#clock rate ?
With the exception of the following standard values not subject to rounding,

          1200 2400 4800 9600 14400 19200 28800 38400
          56000 64000 128000 2015232

  accepted clockrates will be bestfitted (rounded) to the nearest value
  supportable by the hardware.

  <246-8064000>    DCE clock rate (bits per second)

These values are in bites per second. You can find the possible values by using help.

Router(config)# interface s0/1
Router(config-if)# clock rate 64000

All interfaces have a bandwidth value assigned to them. This is used by certain routing protocols, such as IGRP,OSPF, and EIGRP, when making routing decisions.

For LAN-based interfaces, the speed of the interface becomes the bandwidth value (In kilobits per second (Kbps)). However, on synchronous serial interfaces, the bandwidth defaults to 1,554Kbps and this is not related with the clock rate set. To change the bandwidth value for an interface, use the bandwidth Interface Subconfiguration mode command:

Router(config)# interface  s0
Router(config-if)#bandwidth 64


Posted By – RamCruiseWalker


Null Interface in a Cisco Router

Null interfaces are virtual interfaces and are always up. A virtual interface is not a physical interface like Fast Ethernet interface or Gigabit Ethernet interface.

Null interfaces never forward or receive traffic; packets routed to a null interface are dropped. Null interface is a logical interface absorbs packets without forwarding them to another interface or destination. Null interface is also known as bit bucket because the IP datagram reaching Null interface are dropped as soon as they are received.

The Null interface in a Cisco Router is a mechanism for preventing routing loops. Enhanced Interior Gateway Routing Protocol (EIGRP) creates a route to the Null0 interface when it summarizes a group of routes.

 Posted By – RamCruiseWalker


loopback interface in a Cisco Router

A loopback interface is a logical, virtual interface in a Cisco Router. A loopback interface is not a physical interface like Fast Ethernet interface or Gigabit Ethernet interface.

A loopback interface has many uses. Loopback interface’s IP Address determines a router’s OSPF Router ID. A loopback interface is always up and allows Border Gateway Protocol (BGP) neighborship between two routers to stay up even if one of the outbound physical interface connected between the routers is down.

Loopback interfaces are used as the termination points for Remote Source-Route Bridging (RSRB), and Data-Link Switching Plus (DLSW+). Loopback interfaces interfaces are always up and running and always available, even if other physical interfaces in the router are down.

A loop back interface is a software interface which can be used to emulate a physical interface. By default, router doesn’t have any loopback interfaces (loopback interfaces are not enabled by default), but they can easily be created.

Loopback interfaces are treated similar to physical interfaces in a router and we can assign IP addresses to them. The command syntax to create a loopback interface is shown below.

Router(Config)#int loopback <loopback_interface_number>
Router(Config-if)#ip address <ip_address> <subnet_mask>

To create a loopback interface, use the following command in a Cisco Router.

Router(Config)#int loopback 2
Router(Config-if)#ip address

Posted by – RamCruiseWalker


Trivial File Transfer Protocol (TFTP)

Trivial File Transfer Protocol (TFTP) is a file transfer protocol, which is the basic form of File Transfer Protocol (FTP).

Trivial File Transfer Protocol (TFTP) has a very simple design and it requires only a very small amount of memory.

Trivial File Transfer Protocol (TFTP) is mainly used for network booting of computers and network infrastructure devices such as routers and switches.

Trivial File Transfer Protocol (TFTP) is used in Cisco networking environment to back up Cisco IOS (Operating System) image file, configuration files, Network Booting and for an IOS upgrade.

Trivial File Transfer Protocol (TFTP) is a simple file transfer protocol and it is implemented on top of the User Datagram Protocol (UDP). The standard UDP port number for Trivial File Transfer Protocol (TFTP) is 69.

Many Trivial File Transfer Protocol (TFTP) server software products for windows Operating System are available for free download on the Internet. Some are listed below.

Solarwinds Trivial File Transfer Protocol (TFTP) server

tftpd32 Trivial File Transfer Protocol (TFTP) server

Open Trivial File Transfer Protocol (TFTP) server

Posted By – RamCruiseWalker


Collision Domain and Broadcast Domain

Collision Domain

                                                A term collision is described as an event that usually happens on an Ethernet network when we use a “Shared Media” to connect the devices in an Ethenrnet network. A “Shared Media” is a type of connecting media which is used to connect different network devices, where every device share the same media. Example: 1) Ethernet Hubs, 2) Bus Topology

In a “Shared Media” there are no separate channels for sending and recieving the data signals, but only one channel to send and recieve the data signals.

We call the media as shared media when the devices are connected together using Bus topology, or by using anEthernet Hub. Both are half-duplex, means that the devices can Send OR Recieve data signals at same time. Sending and recieving data signals at same time is not supported.

Collisions will happen in an Ethernet Network when two devices simultaneously try to send data on the Shared Media, since Shared Media is half-duplex and sending and recieving is not supported at same time. Please refer CSMA/CD to learn how Ethernet avoid Collision.

Collisions are a normal part of life in an Ethernet network when Ethernet operates in Half-duplex and under most circumstances should not be considered as a problem.

A Collision Domain is any network segment in which collisions can happen (usually in Ethernet networks). In other words, a Collision Domain consists of all the devices connected using a Shared Media (Bus Topolgy or usingEthernet Hubs) where a Collision can happen between any device at any time.

Collision Domain

For Example, if “Computer A” send a data signal to “Computer X” and “Computer B” send a data signal to “Computer Y”, at same instance, a Collision will happen.

As the number of devices in a collision domain increases, chances of collisions are also more. If there is more traffic in a collision domain, the chances of collisions are also more. More collisions will normally happen with a large number of network devices in a Collision domain.

Increased collisions will result in low quality network where hosts spending more and more time for packet retransmission and packet processing. Usually switches are used to segment (divide) a big Collision domain to many small collision domains. Each port of an Ethernet Switch is operating in a separate Collision domain.

In other words, Collision cannot happen between two devices which are connected to different ports of a Switch.

No need to worry much about collision and related network problems now because we are not using Network Hubs to connect our devices. Ethernet Network Hubs are replaced with Ethernet Network Switches long way back.

Broadcast Domain

                                             Broadcast is a type of communication, where the sending device send a single copy of data and that copy of data will be delivered to every device in the network segment. Brodcast is a required type of communication and we cannot avoid Broadcasts, because many protocols (Example: ARP and DHCP) and applications are dependent on Broadcast to function.

A Broadcast Domain consists of all the devices that will receive any broadcast packet originating from any device within the network segment.

          In above picture, “Computer A” is sending a broadcast and switch will forward it to every ports and all the switchs will get a copy of broadcast packet. Every switch will flood the broadcast packet to all the ports. Router also will get a copy of broadcast packet, but the Router will not forward the packet to the next network segment.

As the number of devices in the Broadcast Domain increases, number of Broadcasts also increases and the quality of the network will come down because of the following reasons.

1) Decrease in available Bandwidth: Large number of Broadcasts will reduce the available bandwidth of network links for normal traffic because the broadcast traffic is forwarded to all the ports in a switch.

2) Decrease in processing power of computers: Since the computers need to process all the broadcast packets it recieve, a portion of the computer CPU power is spent on processing the broadcast packets. Normally a Broadcast packet is relevent to a particular computer and for other computers that broadcast packet is irrelevant (For example, DHCPDISCOVER message is relevent only for a DHCP Server. For other computers DHCPDISCOVER is irrelevant and they will drop the packet after processing). This will reduce the processing power of computers in a Broadcast domain.

By design, Routers will not allow broadcasts from one of its connected network segment to cross the router and reach another network segment. The primary function of a Router is to segment (divide) a big broadcast domain in to multiple smaller broadcast domains.

Posted By – RamCruiseWalker


TCP/IP And OSI Model

         TCP/IP and the OSI Model


                      In the previous chapter we were introduced to computer networks and we got to know their importance in everyday lives. In this chapter, we will look at the layered models that are used in communication over networks.


Layer Models

To better understand the various protocols and how they work with each other, we use layered models. A layered model shows how the protocols work at each layer as well as how the layers interact with the layers either above them or below them. The layered models that are used in modern computer networks are two; the OSI model (Open Systems Interconnection) and the TCP/IP model (Transmission Control Protocol/ Internet Protocol)

 There are several benefits that to using the OSI and TCP/IP models in explaining how network protocols work and these include the following.

  • Each layer in a model defines different protocols, therefore by using a layered model, network engineers can be able to define and design protocols which conform to the specific layer.
  • Competitions between different vendors is increased, this is because the models define standards and therefore product superiority is not based on the use of certain protocols since all products support them.
  • The layered model is useful since it allows for independence between other layers. This means that if a change in technology or capabilities is made in one layer, it will not affect another layer either above it or below it.
  • Since the layered model is an open standard, it provides for conformity and interoperability between different networking devices.

OSI Model

The OSI model provides an extensive list of functions and services that can occur at each layer. It also describes the interaction of each layer with the layers directly above and below it.

TCP/IP Model

                                        This model was first introduced in the 1970’s. There are four categories as you can see from the output above. Network communications were first defined using this model and for successful communication to occur, the functions of each layer must be in place in a network.

     From the output below, you can see the function of each and every layer of this model. The application layer, is the component that interfaces with the user, when you are using a web browser, this is a component of the application layer

The transport layer defines the various ports and helps differentiate the different types of communication from a single user. You may be sending an email, browsing and listening to internet radio on one computer. It is the work of the transport layer to differentiate the different types of communications. The transport layer also helps in interoperability between different network devices such as a PDA and a computer.

 The internet layer is meant to provide the best path to remote networks, this differentiates the different devices on a network. If a message is to be sent from one computer to another on a remote network, it is the work of the internet layer to make sure the message gets to the intended recipient. You may compare the internet layer to an address you use when you want to send a letter.

  The network access layer acts as an interface between the hardware and software components in the network. The application, transport and internet layers are all implemented by software, however, the network access translates the messages from these layers to a form that can be transmitted over various media such as fiber optic cables, copper wire and wirelessly.

 The protocols that are defined in the TCP/IP model describe the various functions and processes at each layer. This means that the protocols at each layer have to have specific functions as described by the TCP/IP model.


1. At the application layer, we would create the email and this would be the data that would be communicated over the network.

2. The transport layer would then break this data into segments and add information in a process known as encapsulation.

3.The segments would then be passed down to the internet layer and encapsulated into packets, in this layer, logical addressing would be added. (more on logical addressing will be discussed later)

4.The packets would then be passed to the network access layer, the network access layer would then prepare the packets for transmission over the physical media such as fiber optic cable by converting the data to light signals.

5.When the data is received at the destination, the reverse process would happen, i.e., removal of protocol specific information – decapsulation as well as reassembly into the application data would be carried out.

6.The data would then be passed to the user. This process is illustrated below.

TCP/IP and OSI Model

  1. Data – the end user information, this may include, email content, website information among others. This is the information presented to the user.
  2. Segment – as mentioned earlier, this is the PDU at the transport layer.
  3. Packet – in the internetwork layer, the packets are the PDUs and they include the logical addressing for remote delivery.
  4. Frame – this is the form that data at the network access layer takes, there is also addressing at this layer which is physical addressing such as the MAC address.
  5. Bits – the form that is carried over the physical media form is Bits, these may be in many forms such as electrical signals, light signals and others.

PDUs and communication over a layered mode

The OSI model defines how messages are encoded, formatted, encapsulated, and segmented so that they can be transmitted over networks. As we mentioned earlier, the data is usually broken down into different PDUs and the layers in the OSI model define how each PDU is controlled so as to make communication successful.

Addresses are one of the ways that communication is made successful in the network. If we can use the post office analogy, you can imagine how difficult it would be if not impossible to send letters without a destination address or how difficult it would be if the recipient would not know who to reply to. The diagram below shows the various addresses that are used in communication over the network.


                               In this chapter, we have discussed how communication works over the layered model. We have looked at the TCP/IP and OSI reference models and how they define communication at each layer. We have also looked at the protocol data units and compared the two models. In the next chapter, we will look at the application layer.

                                                                             Post By – RamCruiseWalker

Different Types Of Router Memory, Introducion

Different Types Of Router Memory


Different Types Of  Router Memory



               ROM is read-only memory available on a router’s processor board. The initial bootstrap software that runs on a Cisco router is usually stored in ROM. ROM also maintains instructions for   Power-on Self Test (POST) diagnostics. For ROM Software upgrades, the plugable chips on the motherboard should be replaced.
Flash Memory

                                         Flash memory is an Electronically Erasable and Re-Programmable memory chip. The Flash memory contains the full Operating System Image (IOS- Internetwork Operating System). This allows you to upgrade the OS without removing chips. Flash memory retains content when router is powered down or restarted.


            RAM is very fast memory that loses its information when the router is shutdown or restarted. On a router, RAM is used to hold running Cisco IOS Operating System, IOS system tables and buffers RAM is also used to store routing tables, keep ARP cache, Performs packet buffering (shared RAM). RAM Provides temporary memory for the router configuration file of the router while the router is powered on.

RAM Stores running Cisco IOS Operating System, Active program and operating system instructions, the Running Configuration File, ARP (Address Resolution Protocol) cache, routing tables and buffered IP Packets.

NVRAM (Non-volatile Random Access Memory)

              NVRAM is used to store the Startup Configuration File. This is the configuration file that IOS reads when the router boots up. It is extremely fast memory and retains its content when the router is restarted.


 Post By – RamCruiseWalker


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