CCNA: Cisco IOS An Introduction

Cisco IOS (short for Internetwork Operating System) is the software used on a majority of Cisco Systems routers and switches.  IOS consists of routing, switching, internetworking and telecommunications functions in a multitasking operating system. Cisco IOS has uses command line interface (CLI), and provides a fixed set of multiple-word commands. A Cisco IOS command line interface can be accessed through either a console connection, modem connection, or a telnet session. The set of commands available at any particular level is determined by the "mode" and the privilege level of the current user. Cisco IOS follows a command hierarchy, with each level offering different set of commands  All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. Through the CLI, the commands available to each privilege level can be defined.

Some of the widely used command hierarchy levels are given below:
User EXEC level: This is the level that a connected user is allowed initially. User EXEC allows access to a limited set of basic monitoring commands. A ">" sign denotes User EXEC mode.
Privileged EXEC level: Privileged EXEC level allows access to all router commands including router configuration and management commands. This level is usually password protected for security reasons. A "#"sign denotes privileged EXEC mode.
When a user is connected to a Cisco IOS, a User EXEC prompt appears. Now, the user can enter privileged EXEC mode by typing the password shown as below:
Router> enable
Password: [enable password]
Router# configure terminal
Router(config)#
Global configuration mode: "Global configuration mode" provides commands to change the system's configuration. This is typically represented by "(config)#" sign as shown in the above example.
Interface configuration mode: "Interface configuration mode" provides commands to change the configuration of a specific interface of the router.  An interface configuration mode is denoted by "(config-in)#".
A summary of Cisco IOS router command prompt is given below:

Prompt	Explanation
Router>	User EXEC mode
Router#	Privileged EXEC mode
Router(config)#	Global configuration mode. # sign indicates this is only accessible at privileged EXEC mode.
Router(config-if)#	Interface level configuration mode.
Router(config-router)#	Routing engine level within configuration mode.
Router(config-line)#	Line level (vty, tty, async) within configuration mode.

Context Sensitive Help

Cisco IOS CLI offers context sensitive help. At any time during an EXEC session, a user can type a question mark (?) to get help.
Two types of context sensitive help are available:

• Word help and
• Command syntax help.

Word help: Word help can be used to obtain a list of commands that begin with a given character string. To use word help, type in the characters in question followed immediately by the question mark (?).  The following is an example of word help:

Router# co?
configure connect copy

Command syntax help: Command syntax help can be used to obtain a list of commands, keyword, or argument options that are available starting with the keywords that the user had already entered. To use command syntax help, enter a question mark (?) after hitting a space.  The router will then display a list of available command options with <cr> standing for carriage return. The following is an example of command syntax help:

Router# configure ?
memory Configure from NV memory
network Configure from a TFTP network host
terminal Configure from the terminal
<cr>

Cisco IOS also allows abbreviated commands support. For example, consider the following:

Router#configure terminal
Router(config)#
Router#config term
Router(config)#

Both the above commands to the same job. The IOS correctly interprets the full command words. However, if there is any ambiguity, an error message is generated as below:

Router(config)#c
% Ambiguous command: "c"

Checkout a ccna router simulator available from certexams.com.
Example Question: 
What is the command used to add a banner to a Cisco router configuration?
A. add banner
B. banner motd #
C. motd banner #
D. add banner #

Correct answer: B
Explanation:
The banner is displayed whenever anyone logs in to your Cisco router. The syntax is
"banner motd # 
MOTD stands for "Message Of The Day".
# symbol signifies the start of the banner message to the router. You will be prompted for the
message to be displayed. You need to enter "#" symbol at the end of the message, signifying
that the msg has ended.
Alternatively, you can enter the banner in the same line as below:
"banner motd # your message here#

Readmore...

CCNA: Routing Fundamentals

When IP packets travel over the Internet, routing information is exchanged between the devices that control the flow of information over the Internet. These devices are known as routers, and they use the IP address as the basis for controlling the traffic. These devices need to talk the same language to function properly, though they belong to different administrative domains. For example, one router may be in Newyork(US), and the receiving router may be in London (UK). It is necessary that a routing protocol is followed for smooth flow of traffic. Given below are the widely used routing protocols for routing Internet traffic: RIP v1 RIP v2 OSPF IGRP EIGRP BGP Notations used: Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP).

One often get confused between a routing protocol and a routed protocol. A routing protocol such as RIP is used to route information packets over the Internet, where as a routed protocol such as IP (or IPX) is the payload (contains data) that get routed from source to the destination.
Routing protocols are primarily distinguished into three types:

• Distance Vector Protocols
• Link State Protocols
• Hybrid Protocols

RIP is an example of distance vector protocol. IS-IS is an example of Hybrid protocol, and OSPF is an example of Link State Protocol.
The table below provides the routing protocol used with different routed protocols:

Routing Protocol	Routed Protocol
RIP, OSPF,IS-IS, BGP,EIGRP	IP
RIP, NLSP, EIGRP	IPX
RTMP, EIGRP	AppleTalk

The list of routed, and routing protocols given in the above table is not complete, and given to serve as an example only.
Routing Metric: This is a fundamental measure that routing protocols use for determining appropriate route to deliver packets. Each routing protocol uses its own measure of metric, and a sample of routing metrics used by different routing protocols is given below:

Routing Protocol	Metric
RIPv2	Hop count
EIGRP	Bandwidth, Delay, Load, Reliability, and MTU
OSPF	Cost (Higher bandwidth indicates lower cost)
IS-IS	Cost

The best route in RIP is determined by counting the number of hops required to reach the destination. A lower hop count route is always preferred over a higher hop count route. One disadvantage of using hop count as metric is that if there is a route with one additional hop, but with significantly higher bandwidth, the route with smaller bandwidth is taken. This is illustrated in the figure below:

The RIP routed packets take the path through 56KBPS link since the destination can be reached in one hop. Though, the alternative provides a minimum bandwidth of 1MBPS (though using two links of 1MBPS, and 2MBPS each), it represents 2 hops and not preferred by the RIP protocol.

Link State vs. Distance Vector

Distance Vector routing protocols usually send their entire routing table to their nearest neighbors at regular intervals. A router that receives several such routing tables filter the routes and arrive at its own and retransmits it to its neighbouring routers. There will some period of time where different routers hold non-optimized routes initially. After some time, known as convergence time, a final routing table is arrived at by all the routers. A faster convergence time results in a stable network. 
RIP, as mentioned earlier uses hop count as the metric for computing a route to a given destination. Other Distance Vector routing protocols, such as IGRP, improve on this by using hop count, bandwidth, current load, cost, and reliability to determine the best path.
Link State routing protocols usually send only the routing changes to every other router within their area. Unlike Distance Vector, routers using Link State routing protocols maintain a picture of the entire network. A router can use this network wide information to determine the best route for traffic.
Example Question:
What is true about IP routing?
A. The frame changes at each hop
B. The source IP address changes at each hop
C. The destination IP address changes at each hop
D. The hardware interface addresses remain constant

Correct answer: A
Explanation:
IP Packets are transported from source network to the destination network by what is known as routing. Hop-by-hop routing model is used by the Internet for delivery of packets. At each hop, the destination IP address is examined, the best next hop is determined by the routing protocol (such as RIP, OSPF or BGP) and the packet is forwarded by one more hop through this route. The same process takes place at the next hop. During this process, the logical addresses remain same. In an IP network, the logical addresses are IP addresses. The hardware interface addresses, such as MAC address change with each hop.

Readmore...

CCNA: Subnet masking -II

What we discussed in the previous section is Classful subnet masking. A Subnetmask normally contains the host portion of the bits also. This is called Classless Inter Domain Routing (CIDR). This will enable more networks for a given class of network address. For example, allowing 3 host bits towards subnet portion in our previous IP address, will allow us to offer 2X2X2 or 8 additional subnetworks. Traditionally, all zeros, and all ones subnets are not used, and hence we are left with 6 subnets. 192.189.210.078: 1100 0000.1011 1101.1101 0010.0100 1110 Class C IP Address 255.255.255.224: 1111 1111.1111 1111.1111 1111.1110 0000 Class C subnet mask with 3 additional bits of host portion used for Subnetting. Broadcast address: 1100 0000.1011 1101.1101 0010.0101 1111 :192.189.210.95 The above is the broadcast address for a given subnet (192.189.210.078). Under Classful routing, the broadcast address would have been 192.189.210.255.

Note that by using Subnetting, we are able to increase the number of networks available within a given IP address. On the otherhand, we will be loosing the number of hosts available within a subnet to 2⁴  or 16 hosts per subnet. Again, all zeros, and all ones host addresses are traditionally reserved for other purposes.
CIDR (Classless InterDomain Routing) notation: Subnet mask is also represented as below:
192.189.210.078/27, where 27 is the number of bits in the network portion of the IP address.
Why use CIDR?
Normally, ISPs allocate the IP addresses for individuals or Corporates.  The reason being that it is almost impossible to allocate a classful IP address to every individual or a corporate. Using CIDR, the biggest ISPs are given large pool of IP address space. The ISP's customers such as individual or Corporates are then allocated networks from the big ISP's pool. This kind of arrangement will enable efficient management and utilization of the Internet.
Classful addresses can easily be written in CIDR notation
Class A =  A.B.C.D/8, Class B = A.B.C.D/16, and Class C = A.B.C.D/24
Where A,B,C,D are dotted decimal octets.
Example Question:
You have an IP of 156.233.42.56 with a subnet mask of 7 bits. How many hosts and subnets are possible?

A. 126 hosts and 510 subnets
B. 128 subnets and 512 hosts
C. 510 hosts and 126 subnets
D. 512 subnets and 128 hosts

Correct answer: C
Explanation:
Class B network has the form N.N.H.H, the default subnet mask is 16 bits long.
There is additional 7 bits to the default subnet mask. The total number of bits in subnet are 16+7 = 23.
This leaves us with 32-23 =9 bits for assigning to hosts.
7 bits of subnet mask corresponds to (2^7-2)=128-2 = 126 subnets.
9 bits belonging to host addresses correspond to (2^9-2)=512-2 = 510 hosts.

Readmore...