CompTIA Network+ Certification Study Guide part 62 pdf

In a loop, when the bridges and switches don’t know the location of a destination computer, they send the data frame across the bridge or switch.. The frames coming from the other bridge

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ChApTEr 12 : Network Troubleshooting Methodology

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should be local (occurring on the same side of the bridge or switch), and no more than 20 percent should cross the bridge or switch For best performance, ensure that those computers that communicate with one another most often are on the same side of the bridge or switch Frequently accessed file or print servers should be placed on the same side of the bridge or switch as those clients that use them most often Before implementing a bridging or switching solution, you should carefully analyze the normal flow of network traffic and try to group nodes so that most communication, and especially transfer of large amounts of data, takes place without the need to cross the bridge

Identifying the Cause of an Infrastructure Problem

One issue that can sometimes occur with bridges and switches is called

looping This can occur when there is more than one active bridge or switch

on a network In a loop, when the bridges and switches don’t know the location of a destination computer, they send the data frame across the bridge

or switch This results in multiple copies of the same data frame on the network, causing unnecessary congestion – but it gets even worse than that

As each device detects the frame sent by the other bridge or switch, it passes the frame back across to the other side The frames coming from the other bridge cause each bridge or switch to make incorrect entries in its routing table for the destination computer, and this in turn intermittently prevents the destination computer from receiving data The problem is intermittent because the bridges keep resetting the entries in the routing table based on where the data frames are coming from This can go on forever in an endless loop, hence the term looping See Figure 12.5 for an example of how this can happen

In the scenario shown in Figure 12.5, if Computer B sends a message

to Computer A, both bridges would detect the data frame Neither bridge knows where Computer A is located, so both bridges would transmit the frame to the other segment They would put an entry in the routing tables identifying Computer B as being off the left-side port Two copies of the data frame have now been transmitted onto the right-side bridge port Now each bridge will also detect the copy of the data frame sent by the other bridge on the right-side port They see the source address and think this is Computer

B sending Computer A another frame They will now pass the frame back

to the left-side port Assuming Computer B is now on the right-side port, they change the table to reflect that status This can go on forever, with both bridges detecting each other’s transmitted frames and passing them across, then changing Computer B’s status in the table from the right- to the left-side port over and over again When the routing table is incorrectly

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set, Computer B will not be able to receive any data When the table changes

again and Computer B is identified as being on the correct bridge port, it will

be able to receive data, but only until the tables are changed once more The

problem here is that a bridge looks at the source and destination addresses,

but cannot identify duplicate frames

This does not mean that you can’t have two bridges on a network In fact,

redundancy is a good idea, in case one bridge “dies” So how do you prevent

the looping behavior? The answer is the Spanning Tree Protocol If your

bridge supports and is configured to use this protocol, it will be able to

com-municate with other bridges on the network The two bridges will then work

FIGurE 12.5 Looping Can Occur on a Bridged or Switched Network.

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cooperatively, with one functioning in active mode and the other on standby unless or until it detects a failure of the first bridge At that point, the second bridge will take over passing data frames With only a single pathway avail-able at any given time, there is no possibility of a loop

TrouBlEShooTING ThE NETworK lAyEr

The Network layer of the OSI model is Layer 3, and it houses the addressing protocol of the TCP/IP protocol suite, IP The protocols TCP and UDP both operate at the transport layer, which is Layer 4, and they both rely on IP to address data packets from the sending to the receiving computer

As you’ve worked through this guide, you’ve learned that routing takes

place at the network layer, and routing is all about recognizing network and host IP addresses and mapping out the most efficient way to get from one address to another

For instance, let’s assume that you were to take on the job of navigator

on a cross-country automobile trip Just as TCP and IP, working together, have different responsibilities, you and the driver could divide the duties

so that the journey goes more smoothly It’s the driver’s job to get the car

to the destination safely and in one piece, and the navigator’s job to know where the next stop is along the route Comparatively, it is TCP’s job to move the data to its destination safely, whereas it is the function of IP to know the appropriate next hop along a route to an end destination If you were to perform the function of identifying the next stop along the way to the end destination, you would be performing a function similar to that of the IP protocol at the network layer

It is also the job of the navigator to consider factors such as the size of each thoroughfare, known areas of congestion, and anything else that might make one route more desirable than another This is similar to the tasks given to the network layer within the OSI model Protocols that exist at the network later are responsible for finding a path through a network to

the destination computer They are also responsible for translating logical addresses In the case of TCP/IP, the IP addresses assigned to a machine must be translated into physical addresses The physical, or MAC address is

burned into a chip on the NIC by its manufacturer

IP routes messages based on the network number of the destination

address Every computer has a table of network numbers, known as a routing table If there is an entry in the routing table for the destination network ID, the computer sends it to a gateway address, which represents the first router

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in the path to the destination A default gateway address is included in the

routing table to send packets to when a specific route to the destination

network ID isn’t found in the routing table The default gateway must be on

the same network as the source computer Each gateway or router that the

message must go through on the way to its destination is called a hop You

might say that a journey of a thousand hops begins with a single step: the

gateway address listed in the routing table for a particular network number

The network layer can use either static or dynamic routing to find a path

from a source to a destination computer It’s easy to map out a static route

to a friend’s house that is four blocks away – the path that you take will

likely never change, and it’s a simple one to remember However, if you’re

trying to get to the home of a relative who lives in the backwoods in another

state, you may need more than a good map You may need to call ahead and

get directions from someone who has traveled there recently As networks

become larger and more complex, it becomes more difficult to manually

maintain routing tables When this happens, you will want to use a dynamic

routing protocol Dynamic routing protocols automatically update routes on

all routers on the network The most common dynamic routing protocols

available are the Router Information Protocol (RIP) and the Open Shortest

Path First (OSPF) protocol

The TCP/IP suite includes several protocols that operate at the

net-work layer of the OSI model, including one of the two major components

of the suite: IP IP handles addressing and routing at the network Level,

relying on logical IP addresses It can use packet switching methods to

route different packets, which are all part of the same message, via

differ-ent pathways It can use dynamic routing protocols to determine the most

efficient routes on a per-packet basis IP is a connectionless protocol; it

depends on TCP at the transport layer above it to provide a connection,

if necessary However, it is able to use number sequencing to break down

and reassemble messages, and uses a checksum to perform error checking

on the IP header

Two additional TCP/IP protocols that operate at the network layer are the

ARP and the Reverse Address Resolution Protocol (RARP) ARP’s job is to

translate logical IP addresses to physical MAC addresses ARP discovers this

information by way of broadcasts, and keeps a table of IP-to-MAC entries

Test day Tip

Remember that routers, whether they are dedicated hardware devices or routing software

running on a Windows server, all operate at the network layer.

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hEAd oF ThE ClASS…

packet Switching and Circuit Switching –

deciphering the Terminology

Many people easily confuse the terms packet

switch-ing and circuit switchswitch-ing Even experienced network

administrators, if they haven’t had much exposure to

the conceptual and hardware sides of WAN technology,

find them a little mysterious They sound like the same

thing, but they’re not.

Circuit switching technology is something we use

all the time, whether we’re aware of it or not The public

telephone system (which is formally called PSTN

or Public Switched Telephone Network) is the most

familiar example of switched-circuit communication

An end-to-end communication link is established when

you place a telephone call, and that same physical path

from one end (your telephone) to the other (Aunt Mary’s

telephone in Boise, Idaho, for example) is maintained

for the duration of that call The path is reserved until

you break the connection by hanging up.

If you call Aunt Mary again next week, the pathway

(also called the circuit) used may be completely different

That’s where the switching comes in, and that explains

why sometimes when you talk to Aunt Mary, the con-nection is clear, while other times there’s so much noise and static on the line that you have to ask her to repeat herself when she tells you whose quilt won first prize at this year’s county fair.

Packet switching is different in that there is no

dedicated pathway or circuit established It is known as

a connectionless technology for that reason If you send

data from your computer to your company’s national headquarters in New York over a packet-switched network, each individual packet, or chunk of data, can take a different physical route to get there Most traffic sent across the Internet uses packet switching.

Another type of digital packet switching network

called X.25 can also support virtual circuits, in which

a dedicated logical connection is established between

two parties for a certain duration A permanent virtual circuit, or PVC, creates an ongoing, dedicated logical

connection between two locations, even though the physical network connection can be shared by more than one logical connection.

This table is referred to as the ARP cache RARP is a similar protocol that does

the opposite of ARP; instead of starting with an IP address and finding the matching MAC address, it uses the MAC address to find the IP address The ICMP also exists at the network layer and is known as a maintenance protocol It is invaluable in TCP/IP troubleshooting and it allows two computers on an IP network to share IP status and error information

ICMP is used by the ping and tracert utilities that have been discussed in Chapter 11, as well as the traceroute utility for UNIX and Linux computers

Computers and routers using IP can report errors and exchange control and status information via ICMP

Finally, the last protocol that exists at the network layer is Internet Group Management Protocol (IGMP) IGMP allows computers on a network such

as the Internet to participate in IP multicasting A multicast address allows

an application to send a message to a large number of recipients without requiring the source computer to know the addresses of all the recipients

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Network routers use IGMP to translate the multicast address into host

addresses This works because each computer involved in a multicast group

will use IGMP to report its multicast group memberships to the necessary

routers to receive the appropriate multicast messages For example, if you

sign up for a real-time Webcast from your laptop computer, your laptop will

use IGMP to register with the multicast router that will be transmitting

the Webcast When the Webcast starts, the Webcast application will send

its information to the multicast router, which will use IGMP to send the

Webcast to any computers who signed up for it (including yours)

layer 3 Troubleshooting

You’ll spend quite a bit of time troubleshooting at the network layer, because

this is the layer that really governs whether two computers can communicate

with one another A failure at the network layer can create connectivity issues

where a single client or an entire subnet cannot communicate with another

portion of a network, either because of a physical device failure or because of

some type of misconfiguration

It is helpful to have documentation of the physical and logical network

design available so that you can understand how traffic should be flowing on

CoNFIGurING ANd IMplEMENTING …

The 6to4 protocol

The Internet Engineering Task force (IETF) has created

a new protocol called 6to4, the purpose of which is

to encapsulate IPv6 packets inside IPv4 packets This

will allow networks that migrate to IPv6 early to be

able to send their data across the Internet, even if the

Internet Service Providers (ISPs) don’t yet support the

new version of IP.

Many ISPs are now using network address

transla-tion (NAT) to allow for the translatransla-tion of multiple

pri-vate IP addresses that don’t have to be registered, to a

smaller number of public assigned addresses For this

reason, many ISPs have not been in a hurry to

imple-ment IPv6 support Reconfiguring all of their

equip-ment to use IPv6 addresses would be a big project,

requiring a great deal of time and effort The recent

popularity of NAT devices and software

implementa-tions of NAT (along with inexpensive proxy software) has

taken the edge off of the urgency of upgrading, at least for some companies NAT is built into Windows 2000 Server products, and a simple, lighter version of NAT called Internet Connection Sharing (ICS) is included in the Windows 2000, Windows 98SE, Windows XP, and Windows Server 2003 operating systems Using one

of these operating systems, all of the computers on a network can access the Internet using just one public registered IP address.

The new 6to4 Protocol will solve the compatibil-ity problem for those corporate networks that do wish

to adopt IPv6 sooner rather than later and may make migration more attractive for others as well The 6to4 Protocol is installed on a router that serves as a gate-way from the IPv6 network to the Internet It works by automatically assigning a prefix to each IPv6 address, which identifies it as a 6to4 address The 6to4 protocol then establishes a tunnel over IPv6 network.

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your network, especially when you’re troubleshooting IP connectivity issues

on a large network This allows you to compare the desired path to the actual path that the data may be taking You’ll recall that network routes are measured in hops, where each hop represents a single router between the source and destination computer Because of this, you have the potential for failure at every hop along the way, and so you may need to test connectivity

at every single hop

When troubleshooting network layer issues, there are a few common situations that tend to be the source of most connectivity issues If a source computer is trying to send information to a destination computer, and the

source computer’s default gateway does not have a route to the destination,

the packet will never reach the destination computer because the gateway doesn’t know where to send it This can happen, not only at the default gateway, but at every router along the path There might also be a physical connectivity issue between the source and destination computers, where either a router or a network link that’s required to transmit the information has failed or gone offline The best tools to check connectivity at the network

layer are ping, tracert, traceroute (fr UNIX/Linux), and pathping (for Windows

2000/2003), which we discussed in Chapter 11

You should also check for configuration issues on each router along the path to ensure that nothing has changed or been configured incorrectly

A common misconfiguration, especially when new routers or network links

are added to a network, is a routing loop, which occurs in this fashion:

A source computer on Network A is trying to reach a destination

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computer on Network C The source computer sends the packet to its default gateway; let’s call it Router A

Router A checks its routing table to figure out how to reach

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Network C Router A sees that the next hop to Network C is Router B So Router A sends the packet to Router B

Router B receives the packet that’s intended for Network C Router

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B checks its routing table, and sees that the next hop to Network

C … is Router A So it sends the packet back to Router A.

I bet you can guess what happens next: Router A receives the

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packet that’s intended for Network C It checks its routing table, and sends the packet to Router B, which then proceeds to

send it back to Router A, and the process repeats itself until the

time-to-live (TTL) of the packet has been exceeded and the packet

is dropped

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The good news is that a routing loop is easy to detect using the tracert or

traceroute command, because you’ll see the path that the packet is taking

bounce back and forth between the same two routers over and over again as

follows:

9 29 ms 29 ms 28 ms p10-0.sjc01.atlas.cogentco.com [154.54.2.1]

10 30 ms 29 ms 40 ms p4-0.sfo01.atlas.cogentco.com [66.28.4.93]

If the connectivity failure is limited to a single workstation, use the

ipconfig command to verify that the IP address, subnet mask, and default

gateway have been configured correctly You can also use the route command

to verify that the default gateway and other routing table entries are correct

for an individual PC

Test day Tip

If all physical links and routers are functioning between a source and destination

computer, but traffic is still not getting through, verify that a packet filter hasn’t been

configured on a router or firewall between the two computers that is preventing traffic

from getting through.

TrouBlEShooTING ThE TrANSporT lAyEr

The transport layer’s main purpose in life is ensuring reliability The transport

layer must verify that any data sent by one computer arrives at its intended

destination in good condition It also needs a way to differentiate between

different communications that may be addressed to different applications

that are being served up by the same IP address This is accomplished

through the use of port numbers.

Thanks to the multitasking capabilities of Windows and other modern

operating systems, you can use more than one network application

simultaneously For example, you can use your Web browser to access

your company’s homepage at the same time your e-mail software is

down-loading your e-mail You already know that TCP/IP uses an IP address

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to identify your computer on the network, and gets the messages to the correct system, but how does it separate the response to your browser’s request from your incoming mail when both messages arrive at the same

IP address? This is where ports come in The two parts of an IP address that represent the network address and the host address are somewhat like a street name and an individual street number In this analogy, the port number would identify the specific apartment or suite within the building

TCP and UDP, the transport layer protocols, assign port numbers to each application so the data intended for the Web browser in Apartment A doesn’t get sent to the e-mail program living in Apartment B The transport layer uses two types of connection services: connection-oriented and connectionless Which type of connection is most appropriate for sending

a particular message depends on whether you are more concerned with reliability or speed

In TCP/IP communications, data is sent over the network as a sequence

of datagrams A datagram is a collection of data sent as a single message

Each datagram is sent across the network individually

A connection-oriented protocol such as TCP offers better error control, but can’t send information as quickly because it needs to confirm that each datagram has reached its destination successfully A connectionless protocol such as UDP, on the other hand, suffers in the reliability department but has better speed because it doesn’t need to confirm the delivery of any datagrams that it sends

The easiest way to differentiate between connection-oriented and connectionless communications is by comparing it to the different ways you can send a letter from your local post office If you need to send an important report to the manager of your company’s branch office in El Paso, you could put it in an envelope, affix the required amount of postage, and drop it in the corner mailbox This would be the easiest, quickest way to take care of the task, but you would have no idea whether or when the report reached its destination, like connectionless services

On the other hand, you could go to the post office and fill out a card to send the report via registered, certified mail, with a return receipt requested This would cost more and it would take more time and effort on your part, but it would be a more reliable form of communication Just like a connection-oriented protocol, you would get back an acknowledgment when the package was delivered, showing that it was indeed received by the person

to whom it was addressed

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Connection-oriented services establish a connection before sending the

data This would be as if, before you sent your certified mail, you first got

on the telephone with the El Paso manager and let him know the report

was coming so he could be on the lookout for its arrival If you’re really

detail-minded (or paranoid), you could even ask that he call you back when

it gets there, and let you know that all the pages are there in sequence and

it wasn’t damaged along the way You’ve taken pains to make sure that your

communication is as reliable as possible, but at a cost in terms of time to

both you and the intended recipient

Exam warning

As a provider of connection-oriented services, TCP first establishes a virtual connection

between the sending and receiving computers This is done through the use of

acknowledgments and response messages.

understanding TCp

TCP works on the transport layer of the TCP/IP model, providing connection-

based communication with other IP hosts When an application passes

data to the transport layer, it is often too much data to transmit in one

packet, so TCP segments the data on the sending side and reassembles it at

the receiving end according to sequence information that is packaged with

the packet TCP sends acknowledgments to confirm successful delivery

and analyzes each packet according to checksum information to ensure

data integrity

TCP uses a system of ports to manage communication Applications

bind to a specific TCP port, and any inbound traffic delivered to that port

will be picked up by the application This enables multiple applications

on one host to use TCP at the same time and also standardizes the way

a client can connect to a given service on a server For instance, Telnet’s

standard TCP port is 23, so Telnet clients try to establish connections on

port 23 by default Port assignments are flexible; that is, you can change

the port a client or server uses for a specific application if needed Although

Web servers typically use port 80 for Hypertext Transfer Protocol (HTTP)

communication, the Web server application can be bound to a different

port You should be aware of the default TCP ports that are used by major

applications when you’re troubleshooting network issues at the transport

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