CCNA: Fast Pass phần 7 pdf

3.2 Perform LAN and VLAN troubleshooting 3.3 Troubleshoot routing protocols 3.4 Troubleshoot IP addressing and host configuration 3.5 Troubleshoot a device as part of a working network

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Configuring Static Routes

To forward traffic across the ISDN link, you configure static routes in each of the routers You certainly can configure dynamic routing protocols to run on your ISDN link, but then the link never drops So the better choice would be static routes Keep the following in mind when you are creating static routes:

 All participating routers must have static routes defining all routes of known networks

 Default routing can be used if the network is a stub network

Here’s an example of static routing with ISDN:

RouterA(config)#ip route 172.16.50.0 255.255.255.0

172.16.60.2

RouterA(config)#ip route 172.16.60.2 255.255.255.255 bri0

What this does is tell the router how to get to network 172.16.50.0 through 172.16.60.2 The second line tells the router how to get to 172.16.60.2

Specifying Interesting Traffic

After setting the route tables in each router, you need to configure the router to determine what brings up the ISDN line An administrator using the dialer-list global configuration com-mand defines interesting packets

The command to turn on all IP traffic is shown in this output:

804A(config)#dialer-list 1 protocol ip permit

804A(config)#int bri0

804A(config-if)#dialer-group 1

The dialer-group command sets the access list on the BRI interface Extended access lists can be used with the dialer-list command to define interesting traffic to just certain appli-cations I’ll cover that in a minute

If you use the dialer-list command, you must enter the dialer-group mand on an interface before this will work!

com-Configuring the Dialer Information

There are five steps to configuring the dialer information:

1. Choose the interface

2. Set the IP address

3. Configure the encapsulation type

4. Link interesting traffic to the interface

5. Configure the number or numbers to dial

2.11 Implement Simple WAN Protocols 219

Here’s how to configure those five steps:

Instead of the dialer string command, you can use a dialer map It provides more security

804A(config-if)#dialer map ip 172.16.60.2 name 804B

8350661

You can use the dialer map command with the dialer-group command and its associated access list to initiate dialing The dialer map command uses the IP address of the next hop router, the hostname of the remote router for authentication, and then the number to dial to get there

The five basic Dialer Map steps that you must be aware of are:

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

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Exam Essentials 221

Optional Commands

There are two other commands that you should configure on your BRI interface: the dialer load-threshold command and the dialer idle-timeout command

The dialer load-threshold command tells the BRI interface when to bring up the second

B channel The option is from 1 to 255, where 255 tells the BRI to bring up the second B channel only when the first channel is 100 percent loaded The second option for that command is in, out, or either This calculates the actual load on the interface either on outbound traffic, inbound traffic, or combined The default is outbound

The dialer idle-timeout command specifies the number of seconds before a call is connected after the last interesting traffic is sent The default is 120 seconds

dis-RouterA(config-if)#dialer load-threshold 125 either

RouterA(config-if)#dialer idle-timeout 180

The dialer load-threshold 125 tells the BRI interface to bring up the second B channel

if either the inbound or outbound traffic load is 50 percent The dialer idle-timeout 180 changes the default disconnect time from 120 to 180 seconds

DDR with Access Lists

You can use access lists to be more specific about what is, or is not interesting traffic In the ceding example you just set the dialer list to allow any IP traffic to bring up the line That’s great

pre-if you’re testing, but it can defeat the purpose of why you use a DDR line in the first place You can use extended access lists to set the restriction, for instance, to only e-mail or Telnet.Here’s how you define the dialer list to use an access list:

804A(config)#dialer-list 1 list 110

804A(config)#access-list 110 permit tcp any any eq smtp

804A(config)#access-list 110 permit tcp any any eq telnet

804A(config)#int bri0

804A(config-if)#dialer-group 1

I configured the dialer-list command to look at an access list This doesn’t have to be IP—it can be used with any protocol Create your list, then apply it to the BRI interface with the dialer-group command

Exam Essentials

Know the commands for PPP encapsulation You can use the encapsulation ppp command

on a serial interface to change from HDLC to PPP encapsulation Also, you can further configure authentication using the ppp authentication command

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Know the commands for Frame Relay encapsulation You can use the encapsulation

frame-relay command on a serial interface to change the encapsulation to Frame Relay Additionally, you will need to configure DLCI information, LMI and encapsulation types if they will differ from the default, and potentially subinterfaces if you will have multiple PVCs per physical interface

Understand the five basis dialer map steps for configuring DDR The five steps are as follows:

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6. If you configure the following access list:

access-list 110 deny 10.1.1.128 0.0.0.63 eq smtpaccess-list 110 deny any any eq 23

int ethernet 0

ip access-group 110 out

What will the result of this access list be?

A. E-mail and Telnet will be allowed out E0

B. E-mail and Telnet will be allowed in E0

C. Everything but e-mail and Telnet will be allowed out E0

D. No IP traffic will be allowed out E0

7. Which of the following series of commands will restrict Telnet access to the router?

A Lab_A(config)#access-list 10 permit 172.16.1.1 Lab_A(config)#line con 0

Lab_A(config-line)#ip access-group 10 in

B Lab_A(config)#access-list 10 permit 172.16.1.1 Lab_A(config)#line vty 0 4

Lab_A(config-line)#access-class 10 out

C Lab_A(config)#access-list 10 permit 172.16.1.1 Lab_A(config)#line vty 0 4

Lab_A(config-line)#access-class 10 in

D Lab_A(config)#access-list 10 permit 172.16.1.1 Lab_A(config)#line vty 0 4

Review Questions 225

9. Which of the following are the five basic steps to set up a dialer map command string?

A. Dial-string, dialer, map, protocol, next-hop

B. Dialer, dial-string, map, protocol, next hop

C. Dialer, map, protocol, next hop, dial string

D. Dialer, map, next-hop, protocol, dial-string

10. Which of the following are valid PPP authentication methods? (Choose two options.)

A. LCP

B. PAP

C. CHAP

D. MD5

Answers to Review Questions

1. D The command router eigrp followed by the autonomous system number is used to ment EIGRP Process numbers are not used by EIGRP All of the other command options have rad-ically incorrect command syntax

imple-2. B The show startup-config command will display the configuration that will be loaded the next time the router is booted

3. C The show controllers serial 0 command will show you whether either a DTE or DCE cable is connected to the interface

4. E To copy the IOS to a backup host, which is stored in flash memory by default, use the copy flash tftp command

5. C To copy a configuration of a router stored on a TFTP host to a router’s NVRAM, use the copy tftp startup-config command

6. D If you add an access list to an interface and you do not have at least one permit statement, then you will effectively shut down the interface because of the implicit deny any at the end of every list

7. C Telnet access to the router is restricted by using either a standard or extended IP access list to the VTY lines on the router The command access-class is used to apply the access list to the VTY lines

8. C If you just type from interface configuration mode, encapsulation frame-relay, the encapsulation type will be Cisco

9. C The five basic Dialer Map steps that you must be aware of are:

3.2 Perform LAN and VLAN troubleshooting

3.3 Troubleshoot routing protocols

3.4 Troubleshoot IP addressing and host configuration

3.5 Troubleshoot a device as part of a working network

3.6 Troubleshoot an access list

3.7 Perform simple WAN troubleshooting

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When networks first came into being, computers could typically communicate only with computers from the same manufacturer For example, companies ran either a complete DECnet solution

or an IBM solution—not both together In the late 1970s, the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) reference model to break this barrier

3.1 Utilize the OSI Model as a Guide for Systematic Network Troubleshooting

When networks first came into being, computers could typically communicate only with puters from the same manufacturer For example, companies ran either a complete DECnet solution or an IBM solution—not both together In the late 1970s, the International Organi-zation for Standardization (ISO) created the Open Systems Interconnection (OSI) reference model to break this barrier

com-The OSI model was meant to help vendors create interoperable network devices and ware in the form of protocols so that different vendor networks could work with each other Like world peace, it’ll probably never happen completely, but it’s still a great goal

soft-The OSI model is the primary architectural model for networks It describes how data and network information are communicated from an application on one computer, through the network media, to

an application on another computer The OSI reference model breaks this approach into layers

In this section, we will look at the OSI layered approach and it’s practical implications on real-world networks—the five steps of data encapsulation Understanding the five steps of data encapsulation is crucial in real-world troubleshooting

The Layered Approach

A reference model is a conceptual blueprint of how communications should take place It addresses all the processes required for effective communication and divides these processes into logical groupings called layers When a communication system is designed in this manner, it’s known as layered architecture

Think of it like this: you and some friends want to start a company One of the first things you’d do is sit down and think through what tasks must be done, who will do them, what order 4309c03.fm Page 228 Thursday, October 23, 2003 4:48 PM

3.1 Utilize the OSI Model as a Guide for Systematic Network Troubleshooting 229

they will be done in, and how they relate to each other Ultimately, you might group these tasks into departments Let’s say you decide to have an order-taking department, an inventory depart-ment, and a shipping department Each of your departments has its own unique tasks, keeping its staff members busy and requiring them to focus on only their own duties

In this scenario, I’m using departments as a metaphor for the layers in a communication system For things to run smoothly, the staff of each department has to trust and rely heavily upon the others to do their jobs and competently handle their unique responsibilities In your planning sessions, you should probably take notes, recording the entire process to facilitate later discussions about standards of operation that will serve as your business blueprint, or reference model.Okay, once your business is launched, your department heads, armed with the part of the blue-print that relates to their department, will need to develop practical methods to implement their assigned tasks These practical methods, or protocols, will need to be compiled into a standard operating procedures manual and followed closely Each of the various procedures in your manual will have been included for different reasons and have varying degrees of importance and implemen-tation If you form a partnership or acquire another company, it will be imperative for its business protocols—its business blueprint—to match yours (or at least be compatible with it)

The seven layers of the OSI model are discussed in detail in Chapter 4,

“Technology.”

Similarly, software developers can use a reference model to understand computer cation processes and see what types of functions need to be accomplished on any one layer If they are developing a protocol for a certain layer, all they need to concern themselves with is the specific layer’s functions, not those of any other layer Another layer and protocol will handle the other functions The technical term for this idea is binding The communication processes that are related to each other are bound, or grouped together, at a particular layer

communi-Advantages of Reference Models

The OSI is hierarchical, and the benefits and advantages of this type of model can apply to any layered model The primary purpose of all models, especially the OSI model, is to allow different vendors’ networks to interoperate

Advantages of using the OSI layered model include, but are not limited to, the following:

 Allows multiple-vendor development through standardization of network components

 Allows various types of network hardware and software to communicate

 Prevents changes in one layer from affecting other layers, so it does not hamper developmentThe implications of layered models have significant impact on troubleshooting scenarios Since changes in one layer do not affect other layers, if you can successfully identify which layer of a layered model is causing the problem, you can safely rule out the components of other layers (assuming you only have one problem, which is not always the case!) For example, if you know 4309c03.fm Page 229 Thursday, October 23, 2003 4:48 PM

230 Chapter 3  Troubleshooting

you have a TCP tuning problem (Layer 4), you can rule out Ethernet configuration (Layer 2) and IP addressing (Layer 3) This allows you to focus on the real problem and not waste time addressing layers that are already working and are not contributing to the problem Layered models contribute directly to problem isolation in troubleshooting scenarios

Next I will explain how the layered OSI model applies in network communications This process is called data encapsulation

Data Encapsulation

When a host transmits data across a network to another device, the data goes through sulation: it is wrapped with protocol information at each layer of the OSI model Each layer communicates only with its peer layer on the receiving device

encap-To communicate and exchange information, each layer uses Protocol Data Units (PDUs) These hold the control information attached to the data at each layer of the model They are usually attached to the header in front of the data field, but they can also be in the trailer, or end, of it.Each PDU is attached to the data by encapsulating it at each layer of the OSI model, and each has a specific name depending on the information provided in each header This PDU informa-tion is only read by the peer layer on the receiving device After it’s read, it’s stripped off, and the data is then handed to the next layer up

Figure 3.1 shows the PDUs and how they attach control information to each layer This ure demonstrates how the upper-layer user data is converted for transmission on the network The data stream is then handed down to the Transport layer, which sets up a virtual circuit to the receiving device by sending over a synch packet The data stream is then broken up into smaller pieces, and a Transport layer header (a PDU) is created and attached to the header of the data field; now the piece of data is called a segment Each segment is sequenced so the data stream can be put back together on the receiving side exactly as it was transmitted

fig-F I G U R E 3 1 Data encapsulation

Application Presentation Session Transport Network Data Link Physical

Segment

PDU

Packet Frame Bits

Upper layer data TCP header

Data

IP header

Data LLC header

Data MAC header

0101110101001000010

Upper layer data

FCS FCS

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3.1 Utilize the OSI Model as a Guide for Systematic Network Troubleshooting 231

Each segment is then handed to the Network layer for network addressing and routing through the internetwork Logical addressing (for example, Internet Protocol [IP]) is used to get each seg-ment to the correct network The Network layer protocol adds a control header to the segment handed down from the Transport layer, and what you have at this point is called a packet or dat- agram Remember that the Transport and Network layers work together to rebuild a data stream

on a receiving host, but it’s not part of their work to place their PDUs on a local network segment—which is the only way to get the information to a router or host

It’s the Data Link layer that’s responsible for taking packets from the Network layer and placing them on the network medium (cable or wireless) The Data Link layer encapsulates each packet in a frame, and the frame’s header carries the hardware address of the source and des-tination hosts If the destination device is on a remote network, then the frame is sent to a router

to be routed through an internetwork Once it gets to the destination network, a new frame is used to get the packet to the destination host

To put this frame on the network, you must first put it into a digital signal Since a frame is really a logical group of 1s and 0s, the Physical layer is responsible for encoding these digits into

a digital signal, which is read by devices on the same local network The receiving devices chronize on the digital signal and extract (decode) the ones and zeros from the digital signal At this point, the devices build the frames, run a cyclic redundancy check (CRC), and then check their answer against the answer in the frame’s frame check sequence (FCS) field If it matches, the packet is pulled from the frame, and what’s left of the frame is discarded This process is called de-encapsulation The packet is handed to the Network layer, where the address is checked

syn-If the address matches, the segment is pulled from the packet, and what’s left of the packet is carded The segment is processed at the Transport layer, which rebuilds the data stream and acknowledges to the transmitting station that it received each piece It then happily hands the data stream to the upper-layer application

dis-To summarize, at a transmitting device, the data encapsulation method works like this:

1. User information is converted to data for transmission on the network

2. Data is converted to segments and a reliable connection is set up between the transmitting and receiving hosts

3. Segments are converted to packets or datagrams, and a logical address is placed in the header so that each packet can be routed through an internetwork

4. Packets or datagrams are converted to frames for transmission on the local network ware (Ethernet) addresses are used to uniquely identify hosts on a local network segment

Hard-5. Frames are converted to bits, and a digital encoding and clocking scheme is used

The receiving device will follow these steps in reverse order to de-encapsulate the user information

In real-world troubleshooting scenarios, understanding these steps of data encapsulation is probably the most important skill that derives from the OSI layered model In many problem sit-uations, you end up looking at packet traces or sniffer traces as they are often called Sniffer is actually one of many tools called protocol analyzers that can capture frames from a wire and dis-play them These tools are able to analyze the headers on the PDUs, and display the frame, packet, segment, and often data headers However, they don’t always explain the headers, and thus a clear understanding of data encapsulation is required to understand the information they present.4309c03.fm Page 231 Thursday, October 23, 2003 4:48 PM

232 Chapter 3  Troubleshooting

Exam Essentials

Remember that the OSI model is a layered approach. Functions are divided into layers, and the layers are bound together This allows layers to operate transparently to each other; that is, changes in one layer should not impact other layers

Know the steps of data encapsulation. User information is encapsulated to data, data to segments, segments to packets or datagrams, packets or datagrams to frames, and frames

On a Catalyst 1900, you can verify VLAN configuration with the show vlan command (sh vlan for short):

3.2 Perform LAN and VLAN Troubleshooting 233

On a Catalyst 2950, you must examine the contents of the VLAN database To see the VLAN database, use the show vlan command or the show vlan brief command:

Switch#sh vlan brief

VLAN Name Status Ports

- -

-1 default active Fa0/ -1, Fa0/2, Fa0/3, Fa0/4

Fa0/7, Fa0/8, Fa0/9, Fa0/10

Maximum VLANs supported locally : 64

Number of existing VLANs : 7

VTP Operating Mode : Server

VTP Domain Name : routersim

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

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234 Chapter 3  Troubleshooting

MD5 digest : 0x4C 0x60 0xA6 0x5D 0xD7 0x41 0x8C 0x37

Configuration last modified by 172.16.10.1 at 3-1-94 06:40:09

Local updater ID is 172.16.10.1 on interface Vl1 (lowest numbered VLAN interface

found)

Exam Essentials

Know the commands to find which VLANs are configured on a switch Use the showvlan

command on the Catalyst 1900 to see the configured VLANs, or use the same command on the

Catalyst 2950 to examine the contents of the VLAN database

Know the implications of VTP Changes to VLAN configurations can propagate between

switches; your VTP mode must be transparent if you do not want your switches to participate

in VTP

3.3 Troubleshoot Routing Protocols

Troubleshooting routing protocols and issues is, in many ways, one of the most fundamental

skills you are expected to develop as a CCNA After all, if routers don’t route, what else really

matters? In this section, we will look at a few general commands to troubleshoot routing

protocols, and then take a closer look at each of the four routing protocols covered by the

CCNA exam

It’s important to verify your configurations once you’ve completed them, or at least once you think you’ve completed them The same commands are used to troubleshoot routing pro-

tocols that are used to verify them These commands tell you if the router is, well, routing The

following list includes the commands you can use to verify the routed and routing protocols

configured on your Cisco routers

The show ip route command This command is one of the most frequently used commands;

it displays the current contents of the routing table

Lab_A#sh ip route[output cut]

Gateway of last resort is not set

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3.3 Troubleshoot Routing Protocols 235

The show protocols command This command is useful because it displays all the routed

protocols and the interfaces upon which the protocol is enabled

Lab_B#sh protocol

Global values:

Internet Protocol routing is enabled

FastEthernet0 is up, line protocol is up

The show ip protocol command The show ip protocol command shows you the routing

protocols that are configured on your router If you look at the following output, you can see that both Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) are running on the router, but that only IGRP appears in the routing table because of its lower administrative distance (AD)

The show ip protocols command also displays the timers used in the routing protocol Now take

a look in the next section of output; you can see that RIP is sending updates every 30 seconds—the default Further down, you’ll notice that RIP is routing for all directly connected networks, and the two neighbors it found are 192.168.40.2 and 192.168.20.1

Lab_B#sh ip protocols

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 6 seconds

Invalid after 180 seconds, hold down 180, flushed after

240

Outgoing update filter list for all interfaces is

Incoming update filter list for all interfaces is

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Routing for Networks:

192.168.10.0 192.168.20.0 192.168.30.0 Routing Information Sources:

Gateway Distance Last Update 192.168.40.2 120 00:00:21 192.168.20.1 120 00:00:23 Distance: (default is 120)

Routing Protocol is "igrp 10"

Sending updates every 90 seconds, next due in 42 seconds Invalid after 270 seconds, hold down 280, flushed after 630

Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Default networks flagged in outgoing updates Default networks accepted from incoming updates IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 IGRP maximum hopcount 100

IGRP maximum metric variance 1 Redistributing: eigrp 10, igrp 10 Routing for Networks:

192.168.10.0 192.168.20.0 192.168.30.0 Routing Information Sources:

Gateway Distance Last Update 192.168.40.2 100 00:00:47 192.168.20.1 100 00:01:18 Distance: (default is 100)

The information included in the show ip protocols command includes the autonomous system (AS), routing timers, networks being advertised, gateways, and AD (100)

You can use these commands on all IP routing protocols; basically, regardless of which routing protocol you are running, these can and should be used What’s next? Now I’ll talk about some specific commands you can use with individual routing protocols to further troubleshoot their operation These commands, including their associated debug commands, are specific to the routing protocols mentioned I’ll begin with Routing Information Protocol (RIP) and then look

at Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF)