Networking

Talking Shop: Protocol pointers to help you pass the 640-607 CCNA exam

Drills down on networking concepts surrounding network protocols that you must know to pass this difficult exam

If you want to obtain the Cisco Certified Network Associate (CCNA) certification, you must pass Cisco Systems certification exam 640-607. The exam is broken into eight categories:
  • Bridging/switching
  • OSI reference model and layered communication
  • Network protocols
  • Routing
  • WAN protocols
  • Network management
  • LAN design
  • Cisco basics, IOS, and network basics

In this Daily Drill Down, Ill show you the networking concepts surrounding network protocols that you must know to pass the exam.

TCP/IP
In order to pass the exam, there are different Classes of IP addresses you must know. IP addresses whose first octet start with 1-126 are considered Class A addresses. The range 128-191 is for Class B addresses and the range 192-223 is for Class C addresses. The address range of 127 is reserved for loopback. There is also a Class D, with a range of 224-239, which is used for multicasting. An example of a Class A address would look like this: 64.10.10.10. A Class B address example is 131.10.10.10, and a Class C address would be 199.10.10.10.

The following IP ranges are considered private and can be used on your intranet. They do not get passed onto the Internet:
  • 10.0.0.0
  • 172.16.0.0
  • 192.168.0.0

The term window determines how much data the receiver will accept before sending an acknowledgement back to the sender indicating success. Since windowing requires an acknowledgement, it is very reliable, but adds overhead. It is connection-oriented and uses TCP as the transport layer protocol. It is best used for applications that are intolerant of delays and packet resequencing, such as voice and video. UDP is the other transport layer protocol you need to understand. It is connectionless, which means it does not guarantee delivery. It is faster than TCP and uses dynamic path selection instead of static path selection.

Network addresses are made up of two parts: network ID and host ID. An IP address consists of a 32-bit dotted decimal. IPX addresses consist of an 80-bit hexadecimal number. With IPX, the first 8 hex digits represent the network ID and the remaining 12 hex digits represent the host ID, which is also the MAC address.

Subnetting
Subnetting is the process of dividing the total available IP addresses for a network into subnetworks, or subnets. On the exam, you will have many questions about subnetting, so you will have to understand it extremely well. You will need to be able to calculate them quickly in order to save time. There are many methods for figuring subnets and hosts. One of the simplest and quickest ways is to memorize the table below. You can also use the following formulas.

The formula for figuring the number of subnets is:
(The number of bits borrowed)^2 - 2

The formula for figuring the number of hosts in a Class B address is:
(16 the number of bits borrowed)^2 - 2

The formula for figuring the number of hosts in a Class C address is:
(8 the number of bits borrowed)^2 2

The table below illustrates these formulas. You can memorize Table A below, which makes answering questions faster, but just adds to the amount of information that you must know.
Table A
Class B        
Number of bits
Subnet mask
Number of networks
Number of hosts
Range
2
255.255.192.0
2
16,382
64
3
255.255.224.0
6
8,190
32
4
255.255.240.0
14
4,094
16
5
255.255.248.0
30
2,046
8
6
255.255.252.0
62
1,022
4
7
255.255.254.0
126
510
2
8
255.255.255.0
254
254
1
9
255.255.255.128
510
126
128
10
255.255.255.192
1,022
62
64
11
255.255.255.224
2,046
30
32
12
255.255.255.240
4,094
14
16
13
255.255.255.248
8,190
6
8
14
255.255.255.252
16,382
2
4
Class C        
Number of bits
Subnet mask
Number of networks
Number of hosts
Range
2
255.255.255.192
2
62
64
3
255.255.255.224
6
30
32
4
255.255.255.240
14
14
16
5
255.255.255.248
30
6
8
6
255.255.255.252
62
2
4

Once you have figured out the subnet mask, number of hosts, or number of subnets, you will have to know how to apply this information to answer a question. For instance, lets look at the question below.

Which is a valid host IP address given the subnet mask 255.255.255.240?
A. 198.10.10.16
B. 198.10.10.31
C. 198.10.10.46
D. 198.10.10.64

The answer to this question is C. Lets take a look at how I got that answer. Whenever you subnet, you lose two IP addresses per subnet because each subnet must have a network address and a broadcast address. Looking at the table above, you can see that the subnet mask 255.255.255.240 has a range of 16 with 14 hosts per subnet. The subnets will be broken out as follows:

Subnet 1
Network address 198.10.10.0
Valid host address 198.10.10.1 - 198.10.10.14
Broadcast address 198.10.10.15

Subnet 2
Network address 198.10.10.16
Valid host address 198.10.10.16 - 198.10.10.30
Broadcast address 198.10.10.31

Subnet 3
Network address 198.10.10.32
Valid host address 198.10.10.33 - 198.10.10.46
Broadcast address 198.10.10.47

Subnet 4
Network address 198.10.10.48
Valid host address 198.10.10.49 - 198.10.10.62
Broadcast address 198.10.10.63

Subnet 5
Network address 198.10.10.64
Valid host address 198.10.10.65 - 198.10.10.78
Broadcast address 198.10.10.79
And so on.

A valid IP address for a host cannot be a network or broadcast address. In the example, all of the addresses except for 198.10.10.46 falls on a network or broadcast address.

You may also see IP addresses and subnet masks represented in the format 198.10.10.46/28. The 28 tells you that there are 28 bits used for the subnet mask, which leaves 4 bits for the hosts.

Routing protocols
Address resolution protocol (ARP) is sent as a broadcast to all stations on the subnet. TCP/IP uses ARP to find the MAC address from a given IP address. TCP/IP uses reverse address resolution protocol (RARP) to find the IP address from a given MAC address. This can be a little confusing, so to help you remember, keep in mind that ARP and IP begin with vowels and RARP and MAC begin with consonants.

A default route is used when the next hop has not been explicitly listed in the routing table. A default route will appear as 0.0.0.0 0.0.0.0, followed by an IP address in the routing table.

Three utilities that can verify IP addresses are telnet, ping, or trace. Telnet is the most complete test utility because it verifies the application-layer software between source and destination stations. Ping uses the ICMP protocol to verify the hardware connection at the logical address of the network layer. Trace generates messages from each router used along the path and helps locate failures from the source to the destination.

Distance vector protocols and routing
With distance vector routing, each router shares a copy of its routing table with its neighbor. This means that all routing information is learned second hand and that no one router has a complete view of the entire network. Distance vector routing is prone to routing loops and counting to infinity. To overcome these problems, you can implement the following solutions:
  • Defining a maximum number of hops: Usually it is 16 hops with RIP and 256 hops with IGRP.
  • Split horizon: This prevents a router from sending information about a route back out the same interface in which it came.
  • Route poisoning: Information that is passed out on an interface is marked unreachable by setting the hop count to 16 hops for RIP.
  • Hold down timers: This causes the router to ignore network update information for some period of time. Timers are reset when the timer expires, when infinity is finally defined as some maximum number, or when another update is received indicating that the original route to the network has been restored.

Link state routing protocols allow the router to have a complete understanding of the network. Open shortest path first (OSPF) is an example of a link state routing protocol. It requires more power, memory, and bandwidth, and it is harder to set up. Whereas distance vector updates topology by sending its entire routing table to its neighbor every 30 seconds, link state can be triggered by topology changes resulting in faster convergence times.

RIP is a distance vector routing protocol that uses hop count as its metric. The maximum hop count for RIP is 15. If it takes more than 15 hops to get to a destination, then it is deemed unreachable. To enable RIP, type router rip at the Router(config)# prompt. After starting RIP, you must configure each directly connected network by typing the network number at the Router(config-router)# prompt. A key point to remember here is that this is for directly connected networks, not subnetworks. For example, if you wanted to connect a network of 10.2.10.0, you would do the following:
Router(config)# router rip
Router(config-router)# network 10.0.0.0

RIP is capable of load sharing over multiple paths, and it broadcasts updates every 30 seconds.

IGRP is also a distance vector routing protocol and was designed by Cisco. The maximum hop count for IGRP is 255, so it is a better candidate for use on a larger network. It uses bandwidth, delay, load, reliability, and maximum transmission unit (MTU) as its metric. To enable IGRP, type router igrp 12 at the Router (config)# prompt. In the example, 12 is the autonomous system number. Directly connected networks must be entered as with RIP. IGRP broadcasts routing updates every 90 seconds.

WAN protocols
Point-to-point protocol (PPP) provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. It supports multiple protocols over a single serial link. It uses link control protocol (LCP) to establish and configure the link and network control protocols (NCP) to identify upper layer protocols. It replaced SLIP, which was an older WAN protocol. PPP does not handle address management and is fully compatible between vendors.

ISDN
Integrated services digital network (ISDN) is a digital service designed to run over existing telephone networks. It can carry voice and data simultaneously, is faster than a typical modem, and has a faster call setup. ISDN basic rate interface (BRI) consists of two B channels of 64 Kbps and one D channel of 16 Kbps. The ISDN protocols you need to be familiar with are:
  • E: Deals with existing telephone networks
  • I: Deals with concepts, terminology, and services
  • Q: Deals with switching and signaling

Devices that connect to an ISDN network are known as terminals and have the following types:
  • TE1: Terminal equipment type 1 understands ISDN standards, like a BRI Interface on a router.
  • TE2: Terminal equipment type 2 predates ISDN standards. To use a TE2, you must have a terminal adapter (TA).

ISDN has the following reference points:
  • R: Defines the reference point between non-ISDN equipment and a TA
  • S: Defines the reference point between user terminals and an NT2
  • T: Defines the reference point between NT1 and NT2 devices
  • U: Defines the reference point between NT1 devices and line termination equipment for North America only

Frame Relay
Frame relay is a fast packet-switching WAN protocol that operates at the physical and data link layers and works between DTE and DCE devices. It is connection-oriented and requests retransmission of any missing frames. Frame relay uses permanent virtual circuits (PVCs).

The router commands that deal with frame relay that you should be familiar with are listed below:
  • show frame-relay ip: Shows frame relay IP statistics
  • show frame-relay lmi: Shows LMI statistics. Local management interface (LMI) provides keepalives in a frame relay environment
  • show frame-relay map: Shows the map table
  • show frame-relay pvc: Shows PVC statistics and DLCI information

A data link connection identifier (DLCI) is a number between 16 and 1007 that uniquely identifies a logical connection to the local frame relay switch. The DLCI is locally significant and can change on each hop through the frame relay cloud.

Access lists
Access lists control access to a routers interface by comparing each packet with each line of the access list in sequential order. When a match is found, the packet is either permitted or denied and no more comparison is done to the remainder of the access list. There is an implicit deny at the end of each access list. The range of access list numbers that you must know includes:
  • 1-99: IP standard access lists
  • 100-199: IP extended access lists
  • 800-899: IPX standard access lists
  • 900-999: IPX extended access lists
  • 1000-1099: IPX SAP access

A standard IP access list examines the source address of the packet and matches it against the access list. For example, entering the command access-list 1 permit 162.114.0.0 0.0.255.255 would only allow traffic from network 162.114.0.0 to enter the interface.

An extended IP access list has more options available, such as protocol and port number. For example, entering Access-list 100 permit tcp any 205.10.10.0 0 0.0.0.255 eq www would only allow WWW traffic into network 205.10.10.0.

As a rule of thumb, you place a standard access list close to the destination and you place an extended access list close to the source.

You would issue the show ip interface command to see which IP access lists are set for an interface. The show access-lists command displays all access lists running on the router. The show ip access-lists command displays all IP access lists running on the router. To apply an access list to an interface, use the access-group command followed by the number of the access list.

Cisco discovery protocol
Cisco discovery protocol (CDP) is enabled by default and allows you to access configuration information on other routers and switches with a single command. To disable CDP, type:
Router(config-if)# no cdp enable
To enable CDP again, you type,
Router(config)# cdp run

This is a test; this is only a test
In order to properly administer your Cisco routers, you must have a good knowledge of network protocols. The CCNA exam thoroughly tests your knowledge of these protocols. Once you know what youll face on the exam, youll be able to better guide your study regimen.
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