A complete manual for troubleshooting network problems. You will never be afraid of problems again after reading this.

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1. Network complexity

General network includes routing, dial-up, switching, video, WAN (ISDN, frame relay, ATM, ...), LAN, VLAN, ...

2. Fault Handling Model

1. Define the Problem

Describe the symptoms and potential causes of the fault in detail and precisely

2. Gather Facts

R>Information">RIP Failure

RIP is a distance vector routing protocol, and the metric is the number of hops. The maximum number of hops in RIP is 15. If the number of hops to the destination exceeds 15, it is unreachable.

RIP V1 is a classful routing protocol, and RIP V2 is a non-classful routing protocol that supports CIDR, routing summarization, and VLSM, and uses multicast (224.0.0.9) to send routing updates.

RIP related show commands:

• Show ip route rip; only display the RIP routing table
• Show ip route; Display all IP routing tables
• Show ip interface; Display IP interface configuration
• Show running-config
• Debug ip rip events;

Common RIP failures: Inconsistent RIP versions, RIP updates using UDP broadcast

3. Troubleshooting IGRP

IGRP is a Cisco-specific routing protocol and a distance vector protocol.

The IGRP metric can be based on five factors: bandwidth, delay, load, reliability, and MTU. By default, only bandwidth and delay are used.

IGRP-related show commands:

• Show ip route igrp; Display IGRP routing table
• Debug ip igrp events;
• Debug ip igrp transactions;

Common IGRP failures: access lists, incorrect configuration, line down to neighboring routers

4. Troubleshooting EIGRP

EIGRP is a hybrid protocol of link state protocol and distance vector protocol, and is a CISCO-specific routing protocol. EIGRP uses the multicast address 224.0.0.10 to send routing updates and uses the DUAL algorithm to calculate routes.

EIGRP metrics can be based on bandwidth, delay, load, reliability, and MTU. By default, only bandwidth and delay are used.

EIGRP uses three databases: routing database, topology database, and neighbor router database.

EIGRP related show commands:

• Show running-config
• Show ip route
• Show ip route eigrp ; only show EIGRP routes
• Show ip eigrp interface; Display the peer information of this interface
• Show ip eigrp neighbors; Display all EIGRP neighbors and their information
• Show ip eigrp topology ; Display the contents of the EIGRP topology table
• Show ip eigrp traffic ; Displays a summary of EIGRP routing statistics
• Show ip eigrp events; Display the most recent EIGRP protocol event records
• EIGRP related debug commands:
• Debug ip eigrp as number
• Debug ip eigrp neighbor
• Debug ip eigrp notifications
• Debug ip eigrp summary
• Debug ip eigrp

Common EIGRP failures: loss of neighbor relationships, default gateway, etc., routing of old versions of IOS, and stuck in active.

When troubleshooting EIGRP problems, first use show ip eigrp neighbors to view all neighboring routers, then use show ip route gigrp to view the router's routing table, and then use show ip eigrp topology to view the router's topology table. You can also use show ip eigrp traffic to check whether routing updates are being sent.

5. Troubleshooting OSPF Failures

OSPF is a link-state protocol that maintains three databases: adjacency database, topology database, and routing table.

OSPF related show commands:

• Show running-config
• Show ip route
• Show ip route ospf ; only show OSPF routes
• Show ip ospf process-id ; Display information related to a specific process ID
• Show ip ospf; Display OSPF related information
• Show ip ospf border-routers; Display border routers
• Show ip ospf database; Display the summary database of OSPF
• Show ip ospf interface; Display OSPF information on the specified interface
• Show ip ospf neighbor; Display OSPF neighbor information
• Show ip ospf request-list; Display link state request list
• Show ip ospf summary-address; Display the redistribution information of the summary route
• Show ip ospf virtual-links; Display virtual link information
• Show ip interface; Display the IP settings of the interface
• OSPF related debug commands:
• Debug ip ospf adj ;
• Debug ip ospf events
• Debug ip ospf flood
• debug ip ospf lsa-generation
• Debug ip ospf packet
• Debug ip ospf retransmission
• Debug ip ospf spf
• Debug ip ospf tree

Common OSPF failures: Each OSPF area does not exceed 100 routers, and the entire network does not exceed 700 routers; the wildcard mask is improperly configured;

6. Handling BGP Failures

The key configuration of BGP (including IBGP and EBGP) is the neighbor relationship. BGP uses TCP to establish neighbor relationships.

BGP related show commands:

• Show ip bgp; Display the routes learned by BGP
• Show ip bgp network; Display BGP information for a specific network
• Show ip neighbors; Display BGP neighbor information
• Show ip bgp peer-group; Display BGP peer group information
• Show ip bgp summary; Displays a summary of all BGP connections
• Show ip route bgp; Display BGP routing table
• BGP related debug commands:
• Debug ip bgp 192.1.1.1 updates
• Debug ip bgp dampening
• Debug ip bgp events
• Debug ip bgp keepalives
• Debug ip bgp updates

7. Re-issuing routing protocols

TCP/IP symptoms and causes

Causes of symptoms

The local host cannot communicate with the remote host:

1) DNS is not working properly

2) There is no route to the remote host

3) Missing default gateway

4) Management Deny (ACL)

An application is not working properly:

1) Management Deny (ACL)

2) The network is not properly configured to handle the application

Startup failure:

1) The BootP server does not have an entity with a MAC address

2) Missing IP helper-address

3) ACL

4) Modify NIC or MAC address

5) Duplicate IP addresses

6) Abnormal IP configuration

Cannot ping remote host:

1) ACL

2) There is no route to the remote host

3) No default gateway is set

4) The remote host is down

Missing routes:

1) The routing protocol is not configured correctly

2) Release List

3) Passive interface

4) No neighbor advertising routes

5) Routing protocol versions are inconsistent

6) Neighbor relationship is not established

Neighbor relationship is not established:

1) Incorrect routing protocol configuration

2) Incorrect IP configuration

3) No network or neighbor statement is configured

4) Hello interval is inconsistent

5) Inconsistent area ID

High CPU utilization:

1) Unstable routing updates

2) Debug is not turned off

3) Process is too heavy

Routing triggers active mode:

1) Inconsistent intervals

2) Hardware issues

3) Unstable link

TCP/IP Symptoms and Action Plan

DNS is not working properly:

1) Configure the DNS host configuration and DNS server. You can use nslookup to verify the DNS server's operation.

No route to remote host:

1) Use ipconfig /all to check the default gateway

2) Use show ip route to check whether the corresponding route is available.

3) If there is no such route, use show ip route to check whether there is a default gateway.

4) If there is a gateway, check the next hop to the target; if there is no gateway, correct the problem

ACL There are separate issues related to ACL:

The ACL must be analyzed or rewritten and applied.

The network is not configured to handle the application:

View Router Configuration

Booting failed:

1) Check the DHCP or BootP server and see if there is a MAC address of the faulty machine.

2) Use debug ip udp to verify the packets received from the host

3) Verify that helper-address is configured correctly

4) Check whether the ACL prohibits the packet

Missing routes:

1) Use show ip route on the first router to view the learned routes

2) Verify adjacent routers

3) Have correct routing network and neighbor statements

4) For OSPF, verify the wildcard mask

5) Check the distribute list applied to the interface

6) Verify the neighbor's IP configuration

7) If the route is redistributed, verify the metric

8) Verify that the route is redistributed normally

No neighbor relationship is established:

1) Use show ip protocol neighbors to list the neighbor relationships that have been established

2) Check the protocol configuration that does not form a neighbor relationship

3) Check the network statement in the routing configuration

4) Use show ip protocol/interface to view specific interface information, such as Hello interval

Chapter 2 ▶ Troubleshooting Serial Line and Frame Relay Connections

1. Handling Serial Line Failures

1. HDLC encapsulation

High-level Data Link Control (HDLC) is an encapsulation method used for serial links. HDLC is the default encapsulation method for serial interfaces on Cisco routers. The first step in troubleshooting a serial link is to check that both ends of the link use the same encapsulation type.

Common commands:

Show interface serial 1: View interface information

Clear counters serial number: reset the interface counter to 0

Under normal circumstances, the interface and line are up.

Cable failure, carrier failure, and hardware failure can all cause the interface to go down. You can verify the cable connection, replace the hardware (including the cable), and check the carrier signaling to locate the problem.

Interface up, line down: CSU/DSU failure, router interface problem, CSU/DSU or carrier time inconsistency, no keepalive signaling received from the remote router, carrier problem. Verify the configuration of the local interface and the remote interface.

Reasons for interface restart:

• Packets queued for several seconds were not sent;
• Hardware problems (router interfaces, cables, CSU/DSU);
• Clock signaling is inconsistent
• Loop interface
• Interface shutdown
• The line protocol is down and the interface restarts periodically

show controllers serial 0 ; Displays the interface status, whether there is a cable connected, and the clock rate

show buffers; view the system buffer pool and interface buffer settings

debug serial interface; Display HDLC or Frame Relay communication information

2. CSU/DSU loop test

There are four types of loop tests:

Test the local loop on the local CSU/DSU;

Test the local loop at the remote CSU/DSU;

Test the remote loop from the local NIU to the remote CSU/DSU;

Test the remote loop from the remote NIU to the local CSU/DSU;

On a serial link encapsulated with PPP, PPP uses the negotiated Magic Number to detect the loopback network.

3. Summary of serial line

1) Symptoms and Problems:

Interface is administratively down; line protocol is down:

1) The interface is shut down from the command line

2) Duplicate IP addresses are not allowed. Two interfaces with the same IP address will be down.

Interface is down; line protocol is down:

1) Unqualified cables

2) No signaling from local providers

3) Hardware failure (interface or CSU/DSU, cable)

4) Clock

Interface is up; line protocol is down:

1) Unconfigured interface: local or remote

2) Local provider issues

3) Keepalive sequence number does not increase

4) Hardware failure (local or remote interface, CSU/DSU)

5) Line noise

6) Clock inconsistency

7) Layer 2 (such as LMI)

Interface is up; line protocol is up (looped) The link is looping somewhere Incrementing carrier transition counter

1) Unstable signal from local providers

2) Cable failure

3) Hardware failure

Incrementing interface resets

1) Cable failure, resulting in loss of CD signal

2) Hardware failure

3) Line congestion

Input drops, errors, CRC, and framing errors

1) The line rate exceeds the interface capability

2) Local provider issues

3) Line noise

4) Cable failure

5) Unqualified cables

6) Hardware failure

Output drops: The interface transmission capacity exceeds the line rate

2) Issues and actions

Local Provider Issues:

1) Check the CD signal and other signals of the CSU/DSU to see if the link is sending and receiving information.

2) If there is no CD signal or other problems, contact your local provider to troubleshoot.

Defective or faulty cables:

1) Use cables that meet the device requirements

2) Use breakout box check

3) Replace the faulty cable

Unconfigured interfaces:

1) Use show running-config to verify the interface configuration

2) Confirm that both ends of the link use the same encapsulation type

Keepalive issues:

1) Verify that keepalive is sent

2) Configure keepalive sending and debug keepalive

3) The verification number is increasing

4) If the sequence number does not increase, run a loop test

5) If the serial number of the CSU/DSU loop does not increase, it is a hardware failure.

Hardware failure:

1) Replace the hardware

The interface is in loopback mode:

1) Check the interface configuration

2) If there is a loop in the interface configuration, remove

3) If the interface configuration is cleared, clear the CSU/DSU loop mode

4) If the CSU/DSU is not in loop mode, it may be that the provider has set the loop mode.

The interface is administratively down:

1) Check if there are duplicate IP addresses

2) Enter the interface configuration mode and execute no shutdown

Line rate is greater than the interface capability:

1) Use hold-queue to reduce the incoming queue size

2) Increase the output queue size

The interface rate is greater than the line rate:

1) Reduce broadcast traffic

2) Add output queue

3) Use a queue algorithm if necessary

2. Handling Frame Relay Failures

DLCI is used to identify a virtual link in frame relay. DLCI is only local signaling and is mapped to a Layer 3 IP address.

Steps to handle frame relay:

1) Check the physical layer, cable or interface problems;

2) Check the interface encapsulation;

3) Check the LMI type;

4) Verify the mapping of DLCI to IP;

5) Check the PVC of Frame Delay;

6) Check the LMI of Frame Delay;

7) Check Frame Delay mapping;

8) Verification loop test;

1. Frame Relay show command

show interface

show frame-relay lmi ; Display LMI related information (LMI type, update, status)

show frame-relay pvc; output PVC information, LMI status of each DLCI, ...)

show frame-relay map ; provides DLCI number information and encapsulation of all FR interfaces

2. Frame Relay debug commands

debug frame-relay lmi ; Display LMI exchange information

debug frame-relay events; Displays details of protocols and applications using DLCIs

3. Frame Relay

1) Symptoms and Problems

Frame Realy link is down:

1) Cable failure

2) Hardware failure

3) Local service provider issues

4) LMI type is inconsistent

5) Keepalive is not sent

6) Inconsistent packaging type

7) DLCI is inconsistent

Cannot ping remote host from Frame Delay network

1) DLCI specifies the wrong interface

2) Inconsistent packaging type

3) ACL issues

4) Interface configuration error

Cable failure:

1) Check the cables and test the connectors

2) Replace the cable

Hardware failure:

1) Perform a loop test to isolate the hardware

2) Connect the cable to another port on the router with the same configuration. If it works, you need to replace the hardware.

Local Service Provider Questions:

1) If the loop test makes the LMI status up, but cannot connect to the remote site, contact the local carrier

2) Involves carrier issues, such as FR configuration errors, such as inconsistent DLCI or inconsistent encapsulation.

LMI type inconsistency:

1) Verify that the LMI type of the router is consistent with that of each device on the PVC

2) If you are using a public provider network and cannot access LMI, contact your provider

Keepalive issues:

1) Use show interface to check whether keepalive is disabled, or verify that keepalive is configured normally.

2) If the keepalive setting is incorrect, enter configuration mode and specify the keepalive interval on the interface

Package Type:

1) Verify that the encapsulation methods of the routers at both ends are the same. If there is a non-Cisco router, IETF must be used. Use the show frame-relay command to display the encapsulation information.

2) Use encapsulation frame-relay ietf to change the encapsulation mode, and use frame-relay map to set the encapsulation of a certain PVC.

DLCI is inconsistent:

1) Use show running-config and show frame-relay pvc to display the DLCI number assigned to an interface.

2) If the DLCI number is configured correctly, contact the supplier to verify whether the FR switch has the same DLCI number.

ACL issues:

1) Use show ip interface to display the ACL applied to the interface

2) Analyze the ACL and delete or modify it if necessary

Chapter 3▶Troubleshooting ISDN

1. Basic Principles of ISDN

2. Common ISDN Failures

ISDN problems fall into three categories: improperly configured routers, physical cabling and ISDN protocols, and improperly configured switches.

1. Improperly configured router

Improper configuration due to various reasons: typographical error, wrong information from the service provider, incorrect configuration of the router

1) SPID (Service Profile Identifiers): If the SPID and LDN are configured incorrectly, there will be ISDN connection problems. SPID is only used in North America and should only be set when required by the service provider.

2) CHAP: CHAP authentication is used on interfaces that use PPP encapsulation. The CHAP configurations on both routers must be identical. In PPP, usernames and passwords are case-sensitive.

3) Dialer Map Entity: Dialer map associates high-level addresses to related phone numbers. Each protocol requires a dialer map statement.

4) Access List: ACL can be used for ISDN connections to prevent certain types of traffic from triggering a connection.

5) PPP:

2. Physical layer connection

1) BRI: Provides digital services over existing telephone lines.

2) ISDN BRI channel: 2B+D (2*64+16+48=192kbps); the physical frame of ISDN BRI is 48 bits, and the link sends 4000 frames per second.

3) Local loop: The link between the customer and the CO, connecting the ISDN equipment to the ISDN switch.

4) Physical layer: reference points (R, S, T, U); equipment (LT/ET, NT1, NT2, TE1, TE2, TA)

3. Improperly configured telephone switches

In new ISDN installations, the possibility of misconfiguration of the service provider's ISDN switch must be considered.

1. Layer 2 troubleshooting:

ISDN Layer 2 troubleshooting objectives: q.921 protocol and PPP.

1) q.921: Layer 2 of ISDN is defined in q.921. Q.921 signaling is transmitted on the D channel using the LAPD protocol.

The most commonly used command for handling q.921 faults is debug isdn q921. ​​Problems are often related to TEI (terminal endpoint identifier), SAPI (service access point identifier) ​​and SABME (set asynchronous balanced mode extended).

TEI=127 indicates broadcast; TEI=64-126 are reserved for dynamic allocation.

SAPI=0 indicates the current layer 3 signaling; 63 indicates the management SAPI used for TEI value allocation; 64 is call control.

2) PPP: PPP uses LCP to set up and maintain links; NCP configures and maintains network layer protocols.

2. Layer 3 troubleshooting:

ISDN layer 3 is also called q.931. Use the debug isdn q931 command to view call setup, connect, release, cancel, status, disconnect, and user information.

The ISDN layer 3 connection is between the local router (TE) and the remote ISDN switch (ET).

ISDN call establishment process:

1) SETUP: Send information between the local TE and the remote ET

2) CALL_PROC: call processing signaling

3) ALERT:

4) CONNECT

5) CONNECT_ACK

3. Switch type:

When configuring ISDN, you must use the isdn switch-type command to specify the switch for the local loop.

4. ISDN troubleshooting commands

1. Ping: In DDR, the ping command triggers a call, and the router has completed the ping before the second B channel is up.

2. clear interface bri n: resets different counters on the interface and terminates the connection on the interface.

3. show interface bri n: Displays information about ISDN BRI D channel

4. show interface bri n 1 2: displays the B channel information of ISDN BRI.

5. show controller bri: Displays interface hardware controller information and U interface for Cisco's TAC to troubleshoot.

6. show isdn status: displays the ISDN interface status and detailed information of each layer.

7. show dialer: Displays information about DDR connections, including dialing, successful connections, IDLE time, and number of calls.

8. show ppp multilink:

5. Debug ISDN

1. debug bri: Provides information about BRI B channels, including bandwidth information

2. debug isdn q921: Get information about the D channel of the interface. The D information is used to transmit signaling between the switch and the local ISDN equipment.

3. debug dialer: the reason for the call connection and the status of the connection.

4. debug isdn q931: monitors events occurring at layer 3.

Cause ID shows the reason why the call was rejected;

The information sent and returned by CallRef ID is used to analyze specific sessions of different calls between routers and switches.

5. debug ppp negotiation: provides real-time information about establishing PPP sessions, and can view CHAP and PAP authentication

6. debug ppp packet: reports real-time PPP packet flow, including packet type and B channel used

Chapter 4 ▶ Switched Ethernet Troubleshooting

1. Switch, Bridge, Hub

Broadcast domain: controlled by Router

Conflict domain: controlled by Switch or Bridge

Switch vs Hub:

TypeSwitch Hub

Unicasts are sent to all ports only.

Broadcasts Send to all ports in the same VLAN to all ports

Aggregate bandwidth is equal to the bandwidth of each port × the number of ports equals the media rate

Full/half-duplex Full-duplex connection is possible, only half-duplex

Support for mixed media: Token Ring, Ethernet, FDDI... Relying on switch, it can be transmitted between different frame types and physical media. Only the same media is supported.

Support for mixed media depends on the bridge configuration

Processing frame hardware (ASIC) software or

Number of ports: 4 to over 100, usually less than 16

Frame type conversion depends on bridge configuration

2. Catalyst troubleshooting tools

1. Catalyst command line interface:

The command line interface has native mode and hybrid mode. Native mode configures Layer 3 and Layer 2 together; hybrid mode configures Layer 3 and Layer 2 under different CLIs, usually set-based CLIs.

2. CLI in hybrid mode:

1) show system: A high-level summary of the switch, including power status, uptime, and management settings

2) show port: Displays the information of a specified port or all ports on a module (VLAN, rate, duplex, status, type, etc.)

3) show log: reports important events, including restart, trap, power failure, etc. of all modules.

4) show logging buffer: Equivalent to the router's show log command, reports port up or down, STP, etc., depending on the logging level.

5) show interface: Reports the IP configuration on the management module and the VLAN information on the SC0 interface. (sl0, sc0)

6) show cdp: Displays information about adjacent CISCO devices

7) show config: Equivalent to the show running-config command, it displays all settings on all modules of the switch except the MSFC, and only displays non-default settings. Show config all displays all settings.

8) show test: only displays the status of the switch management module, including interface card, power supply, memory, etc.

9) show mac: Displays a large number of counts, including frame traffic per port, total number of outgoing and incoming frames, discards, ...

10)show vtp domain:

11)show cam: Displays the MAC address associated with the port

12) Duplicate MAC Address

13)show spantree: Displays the SPT process status of each VLAN

14)show version: Displays hardware and software version numbers, including memory, system UP time statistics, etc.

3. RMON (Remote Monitoring)

RMON is based on RMONProbe and collects data information from the circuit (physical medium). Routers and switches do not support all levels of RMON information. More monitoring can be achieved using SPAN (Switched Port Analyzer, also called Port Mirroring).

4. Indicator light:

The supervisor engine includes a load LED that indicates the current load of the switch. During the boot process, the LED will flash; under normal circumstances, the LED is always green; the orange LED indicates a problem; the red LED indicates a fault.

3. Use STP to control the loop

The STP algorithm is defined in 802.1D and is used to control duplicate paths and avoid network loops when there are multiple switches.

When Cisco uses Port Fast and Uplink Fast, network loops must be prevented.

VLAN

VLAN has port-based static VLAN and MAC-based dynamic VLAN

1. ISL: A Cisco-specific protocol used to connect two devices to support multiple VLANs.

ISL can only be used on products that support ISL:

ISL must be point-to-point

ISL is only used for 100Mb full-duplex

ISL requires the router's IOS and memory upgrades;

ISL can support Token Ring;

ISL adds 30Bit to the original frame;

ISL includes a CRC at the end of the frame.

2. 802.1Q: Used to connect non-Cisco trunks to Cisco devices.

3. VTP: VTP uses multicast to inform all other switches in the VTP domain about the information about the VLANs in the domain.

VTP Server:

VTP Client

5. Cable Problems

Physical layer standards:

Cable 10Mb 100Mb

Category 3 cable distance 100m not available

Category 5 distance 100m 100m

Multimode fiber distance 2000m 2000m

Single mode fiber distance up to 100km Up to 100km

1. Cable problem:

1) Multimeters and Cable Testers

Multimeters and volt-ohm meters are used to verify cable continuity and can only be used to test copper wire or other electrical signal-based cables, not optical fibers.

Cable Testers can test both electrical cables and optical cables, providing users with more information about the tested cables, such as: connectivity, open circuit, short circuit, long distance, noise, MAC information, line load, etc.

2) Time Domain Reflectometers (TDRs) and Optical Time Domain Reflectometers (OTDRs)

TDR is a more complex cable tester that can be used to locate electrical faults and detect abnormalities such as breaks, shorts, curls, etc.

2. Cross lines

A crossover cable is used to directly connect two hosts or two network devices.

Ethernet uses four cores 1, 2, 3, and 6 (white-orange, orange, white-green, and green), while T1 circuits use four cores 1, 2, 3, and 5 of RJ-45.

6. Switch connection troubleshooting

Common faults that occur on switches include speed and duplex settings.

1. SPAN (Switched Port Analyzer):

Also called Port Mirroring (port monitor), the switch copies all packets sent to the workstation interface to another interface that is not assigned a VLAN.

Set span enable ; Configure SPAN

Use SPAN to monitor received, sent, or all packets.

2. Multilayer Switch Feature Card (MSFC) and Catalyst Routing:

The MSFC is a Cisco router on a daughterboard that is installed on the management module and provides inter-VLAN routing.

Accessing the MSFC in the CLI: session

3. VLAN between router and switch:

Routers provide communication between VLANs.

1) Broadcast management:

The router does not forward broadcasts, and the switch controls the broadcast to be forwarded only to the ports that are members of the VLAN of the source port.

2) Policy control: Switches have no policies, while routers provide security and policy control for connecting VLANs

3) VLAN switching: forwarding a packet to different interfaces in the same VLAN through a router

4) VLAN transmission: transmission between two VLANs using different VLAN protocols or transmission from a VLAN protocol to a non-VLAN Layer 2 protocol.

5) Routing: Communication between different VLANs or non-VLAN networks

6) VLAN troubleshooting on routers:

• show vlans
• show arp
• show interface
• show cdp neighbor
• debug vlan packet
• debug spantree

7) show vlans: Executed on a router, it displays the details of the router's VLAN configuration, including: VLAN name, interface, IP address, VLAN encapsulation protocol, and interface protocol.

8) debug vlan packet: Determine the VLAN of the data sent to the router on the trunk.

4. VLAN design and troubleshooting

Things to note when designing VLANs:

1) The network diameter should be less than 8 switches;

2) VLANs must be numbered within certain limits;

7. Hybrid/Native Mode Command Conversion

Mixed Mode Native Mode Explanation

Clear vlan No vlan Removes a VLAN from the configuration.

Set cam agingtime Mac-address-table aging-time Set the timeout value for retaining MAC addresses

Set port dulex Duplex Configures the duplex mode on a specific port.

Set port name Description Set the port name

Set port speed speed Set the port speed

Set span monitor session Set span port

Set spantree Spanning-tree Set STP information

Set vlan Switchport access vlan Assign a port to a given VLAN

Show cam dynamic Show mac-address-table dynamic Display MAC to port relationship

Show port Show interface Display port information

Show span Show monitor Display the SPAN port

Show test Show diagnostic Show startup test results

Show version Show version Display the switch IOS version information

Show vlan Show vlan Display VLAN information

Show vtp domain Show vtp status Display VTP information

Chapter 5 ▶ Isolating and Correcting Physical and Data Link Layer Failures

1. Identify symptoms of physical layer problems

Physical layer components include: interfaces/ports, modules, cables, repeaters, network cards, converters, etc.

Physical layer problems will result in complete or intermittent loss of data on the link, application failure, and low data transmission rate.

The LEDs of ports and specific components of the device are steady when operating normally, and will be off, blinking, or in other colors when a fault occurs.

2. Identify the symptoms of data link layer problems

Data link layer problems include: abnormal frame types (incompatible encapsulation), duplicate MAC addresses, switching, and other improper behavior of layer devices.

Layer 2 and Layer 3 testing tools (CDP, PING) can help verify and troubleshoot data link layer problems.

3. Commands and applications for separating physical layer and data link layer issues:

1) ES command:

Ping host|ip-address ;

Arp –a ;

Netstat –rn ;

Ipconfig /all ;

Tracert ;

Winipcfg ;

Ifconfig –a ;

Traceroute ;

2) Cisco IOS Commands

Ping ;

Traceroute ;

Debug;

Show version;

Show ip interface brief;

Show interface e 1 ;

Show cdp neighbor detail;

Show controllers ;

Debug ppp|isdn|serial|asynch|frame-relay

Show arp ;

Debug arp|lapb|stun ;

4. Correct the commands and applications that occur at the physical layer and data link layer

arp –d ;

interface ;

no shutdown ;

encapsulation ;

clock rate ;

controller ;

duplex full|half|auto

speed 10|100|auto

1) Commands to correct T1|E1 issues

channel-group channel-no timeslots timeslot-list speed 56|64

clock source line|internal

framing sf|esf;framing crc4|no-crc4

linecode ami|b8zs;linecode ami|hdb3

pri-group timeslote range

Chapter 6 ▶ Isolating and Correcting Network Layer Problems

1. Symptoms of network layer problems

2. ES commands to separate network layer issues

1) General commands:

ping

arp –a

netstat

2) WINDOWS

Route print

Ipconfig /all

Tracert

Winipcfg

3) UNIX&MAC

Ifconfig –a

Traceroute

Route –n

3. Cisco IOS commands to isolate network layer issues

1) General:

ping

trace

debug

show running-config

2) ARP

Show ip arp

Debug arp

3) Routing Table

show ip route

debug ip routing

4) IP interface

Show ip interface brief

5) BGP

Show ip bgp

Show ip bgp summary

Show ip bgp neighbors

Debug ip bgp

6) IP traffic

Show ip traffic

Debug ip icmp

Debug ip packet

7) IP access list

Show ip access-list