Network Hubs

I will start with a hub then move on to talk about a switch highlighting the similarities and differences of the two devices.

• A hub operates at the Physical Layer (Layer 1), of the OSI reference model. It is mostly used in a star topology where all nodes are connected to a hub through a single cable.
• In a hub all packets transmissions that come into a physical port are rebroadcast to all connections. That means that if one device sends a network packet, all the other devices will receive it.
• All devices connected to a hub are said to be in the same collision domain.
• There are several types of hubs, namely: passive hubs, active hubs, switching hubs and intelligent hubs.

Network Switches

Switches on the other hand:

• Operate in the Data Link Layer (layer 2) of the OSI reference model.
• Like hubs, devices connected by a switch reside in the same collision domain.
• There are two types of switches, namely: Cut-through switch and a Store-and-Forward switch.

Comparing a network switch with a hub

• One of the differences between a switch and hub is that, a switch can filter and forward packets between LAN segments. It can therefore make a direct connection between the transmitting node and the receiving node unlike a hub which rebroadcasts signals out to all nodes connected to it beside the one in which it received the packet from.
• The main advantage of a switch over a hub is that it does not waste bandwidth by sending packets to nodes that are not intended to receive it. Each pair of nodes(sender and receiver) has a full bandwidth of the network dedicated to it when communication through a switch. For example, for a standard Ethernet LAN, when using a hub, say with 20 computers, all the 20 computers have to share the same 10 Mbps of bandwidth. By replacing a hub with a switch, each pair of nodes has its own dedicated 10 Mbps stream. This improves the overall performance of the network without any further modifications.
  
• A hub is mostly suitable for small networks. For much larger networks a switch will perform better than a hub. Switches are usually a little more expensive than hubs.

So deciding which one of the two to use will depend on several factors such as the size of the network, type of network and of course, how deep your pockets are. By compromising on the bandwidth utilization you can use a hub which is a little bit cheaper or if you want a much better network performance and optimal bandwidth usage a network hub is the way to go; moreso it is much more intelligent than a hub.

I hope this article clarifies the dilemma people usually have when deciding on the differences between a hub and a switch to be able to answer N+ exam questions like,

• Which of the following operates at the physical layer? A. Hub, B.Switches, C. Wireless Access Points D. Routers
• At which layer of the OSI reference model does a hub [or switch] operate?

1. This tutorial is for information and educational purposes only
2. Use your own wireless network to perform the steps in this guide
3. I hold no responsibility for how you use the information in this page

NB. The screenshots thumbnails are linked to larger more legible images.

First Steps - Downloading Backtrack and aircrack

• Download BackTrack live CD from the closest mirror at http://www.remote-exploit.org/backtrack_download.html

• Download aircrack from http://download.aircrack-ng.org/aircrack-ng-0.9.3.tar.gz and keep it handy e.g. save it in a USB stick.(you’ll need this later)

• Burn image to CD using Nero or any capable application.

• Reboot your system from the CD and wait as the system loads

NB. YOU’LL NEED TO DO A BIT OF TYPING TO EXECUTE PRETTY MUCH ALL OF THE COMMAND, BUT YOU ONLY NEED TO TYPE THE TEXT I’VE TYPED IN SQUARE BRACKETS AND REMEMBER TO TYPE ENTER AT THE END OF EVERY COMMAND

Launching Backtrack and loading X-server

4. Once the system has loaded, Login as “root” using the password “toor” without quotes by typing the following

bt-login: [root] <Enter>
Password: [toor] <Enter>

Once logged in, you’llbe taken to a command prompt, you’ll need to launch a graphical user interface(GUI), you do this by issuing the following command

bt ~ #[startx] <Enter> (see p1.jpg)

p1.jpg

then wait for the GUI to load(see p2.jpg)

p2.jpg

1. Using CDP to gather information
2. Building a Hosts table in a router

A Cisco Discovery Protocol (CDP) is a protocol which was designed by Cisco guys to promote “spying” on neighbouring devices by network administrators. Well, of course, not the Alex Litvinenko spying but spying as in being used to collect information about attached and remote devices, this could be hardware information, protocol information etc.

Cisco uses the commands such as:

• “show cdp neighbors” : to display information about other devices connected to your device. Using the configuration in Basic Router and Switch Configuration Lab, suppose that you are in Router 2 (PxR2) and you want to check all the devices connected to that router: The much simplier way will be to use the command

PxR2#show cdp neighbors
which outputs the information :
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater

Device ID	Local Intrfce	Holdtime	Capability	Platform	Port ID
PxR1	Ser 0/0/0	167	R	2610	Ser 0/0
PxS1	Fas 0/0	134	S I	WS-C2950-	Fas 0/1

A switch, PxS1 and one router, PxR1 are the only devices connected to router 2, PxR2.


• Obviously the information is not of much value, so that’s why Cisco came up with another command “show cdp neighbor detail” which provides a much more detailed information about connected devices such as their IP Addresses, Cisco version, holdtime etc are revealed.
  

PxR2#show cdp neighbor detail
————————-
Device ID: PxR1
Entry address(es):
IP address: 172.16.17.1
Platform: cisco 2610, Capabilities: Router
Interface: Serial0/0/0, Port ID (outgoing port): Serial0/0
Holdtime : 124 sec

Version :
Cisco Internetwork Operating System Software
IOS ™ C2600 Software (C2600-JK9O3S-M), Version 12.2(24), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2004 by cisco Systems, Inc.
Compiled Wed 28-Apr-04 15:30 by kellmill

advertisement version: 2

————————-
Device ID: PxS1
Entry address(es):
IP address: 192.168.27.1
Platform: cisco WS-C2950-24, Capabilities: Switch IGMP
Interface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/1
Holdtime : 151 sec

Version :
Cisco Internetwork Operating System Software
IOS ™ C2950 Software (C2950-I6Q4L2-M), Version 12.1(11)EA1, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2002 by cisco Systems, Inc.
Compiled Wed 28-Aug-02 10:25 by antonino

advertisement version: 2
Protocol Hello: OUI=0×00000C, Protocol ID=0×0112; payload len=27, value=00000000FFFFFFFF010221FF000000000000000
9E40FF0000
VTP Management Domain: ‘null’
Duplex: full

• The “show cdp ?” was rather useful when I needed to get any Cisco Discovery Protocol command. For instance

PxR2#show cdp ?
entry - Information for specific neighbor entry
interface - CDP interface status and configuration
neighbors - CDP neighbor entries
traffic - CDP statistics
| - Output modifiers

The next article will be about the second part of this laboratory session, that is, Building a Hosts table. In addition to that, I will talk about how I collected information of neighbouring devices (using Cisco Discovery Protocol + Hosts table) as well as obtaining information of neighbouring devices of neighbours and neighbouring devices of neighbour’s neighbouring devices; We could be here all day, I guess you see where I’m going with this.

Thanks for your comments

Basically, the aim of the lab was to configure the three routers by:

• Setting IP addresses
• Giving them Hostnames
• Configuring each routers interface
• Setting passwords
• Banner message of the day e.t.c

It was just a basic lab except I had a slight difficulty when setting password using the “service password-encryption” command but I finally figured it out by :

1. First setting a secret password for the router, using command
   secret password [your password]
2. Set the passwords for a line console, line aux and line vty, using command :
   password [your password]
3. And then finally to do the actual encryption of all the passwords I had to use “service password-encryption” command. I checked the passwords using “show running-config command”.
4. Afterwards I used “no service password-encryption” command. The passwords were still encrypted. So I was convinced that “no service password-encryption” command does not un-encrypt passwords, instead it leaves the passwords as there are (in this particular case encrypted).

All in all the lab was good experience. I am looking forward to my next lab which will be on the 6th September. Based on the notes so far, i have a feeling that it will be much more interesting than the other one. Apparently it is going to be about CDP (Cisco Discovery Protocol) lab.

Till next time

Getting the right domain name was a bit of a challenge seeing that the best domains seem to be taken but I settled for miss-networks.com because of my passion for all things networking ( IT and social). I have since registered with the Scheidegger/Skills Train for a CCNP certification but starting with N+ certification to try and ease my way into Cisco-land.

I think I’m now at a stage where I’m ready to finally take the N+ exam; I’ve procrastinated long enough. After I completed my MEng. in Computer Systems Engineering, I took a break from the books for a while trying to adjust myself to the work environment. As for work, well, work is just work; nothing much to write home about.

So I intend to use this blog as a log of my “journey” through acquiring the Cisco certification. As for how long that will take me is anybody’s guess.