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Chapter 9 Troubleshoot an Enterprisenetwork
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Situation
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Enterprise network requirements
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Most enterprises rely on theirnetworks to provide consistentand reliable access to shared
resources Without a reliable network,many organizations loseaccess to customer databasesand accounting records that
employees need to performtheir daily activities.
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Enterprise network requirements
Large enterprises generally span many different time zones andhave employees, customers, and suppliers accessing theirnetwork around the clock. For these organizations, anydowntime is extremely costly.
Cause of network downtime Weather and natural disasters
Security breaches
Man-made disasters
Power surges
Virus attacks Equipment failure
Misconfiguration of devices
Lack of resources
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Enterprise network requirements
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The three-layer hierarchical network design modelseparates the functionality of the various networking
devices and links. This separation ensures efficient
network performance.
Enterprise should have service level agreements(SLAs) with key suppliers.
An SLA clearly documents network expectations in
terms of level of service.
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Enterprise network requirements
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Enterprise network requirements
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To minimize this type ofdowntime requireswarrantees on all criticalpieces of equipment.
Warrantees provide for rapidreplacement of missioncritical components.
Business continuity plansprovide a detailed plan of
action in case of unexpectedman-made or naturaldisasters such as powerfailures or earthquakes.
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Monitoring and proactive maintenance
Several groups of tools are available for monitoringnetwork performance levels and gathering data. Thesetools include:
Network utilities
Packet sniffing toolsSNMP monitoring tools
Network monitoring tools, techniques, and programs relyon the availability of a complete set of accurate and currentnetwork documentation. This documentation includes:
Physical and logical topology diagrams
Configuration files of all network devices
A baseline performance level
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Monitoring and proactive maintenance
Use Ping, Tracert, packet sniffing.
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Monitoring and proactive maintenance
Packet sniffingtools monitor thetypes of traffic onvarious parts of thenetwork.
These tools mayalso be able toremedy thesituation before
network congestionbecomes critical.
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(SNMP) allows monitoring of individual devices on the network.SNMP-compliant devices use agents to monitor a number ofpredefined parameters for specific conditions.
These agents collect information and store it in a databaseknown as the management information base (MIB).
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9.1.2
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Troubleshooting and the Failure Domain
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The security of the network and the resources that it housesmust always be the highest priority.
security concerns in the business continuity plan. The planincludes:
oDocumentation of potential problems
oDescription of the appropriatecourse of action in the event ofproblems
oDetails of the security policy of thecompany
oDetails of the security risks of theactions
Refer 9.1.3
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Troubleshooting and the Failure Domain
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Activity 9.1.3
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Troubleshooting process
When a problem occurs on an enterprise network,troubleshooting that problem quickly and efficiently is veryimportant to avoid extended periods of downtime. Manydifferent structured and unstructured problem-solvingtechniques are available to the network technician. These
include:
Top-down
Bottom-up
Divide-and-conquer
Trial-and-error
Substitution
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Troubleshooting process
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Troubleshooting process
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Troubleshooting process
When a situation requires a more structured approach,most network personnel use a layered process based onthe OSI or TCP/IP models.
When approaching a problem situation, follow the genericproblem-solving model, regardless of the type oftroubleshooting technique used.
Define the problem
Gather facts
Deduce possibilities and alternatives
Design plan of action
Implement solution
Analyze results
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Troubleshooting process
OSI model
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OSI Model
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OSI Model
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OSI Model
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Activity 9.1.4.3
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Troubleshooting basic switching
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Switches are currently the mostcommonly used Access Layer
networking device.
The most common problems
with switches occur at thePhysical Layer
Ensure that the power LED is illuminated.
Ensure that the correct type of cable connects the end device to the
switch.
Reseat the cables at both the workstation and the switch end.
Check the configuration to ensure that the port is in a no shutdown
state.
Checking steps:
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Troubleshooting basic switching
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Commands: qshow running-configqshow port-security interface interface_id
qshow mac-address-table
qclear mac-address-table dynamic
qshow interface interface_id
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Troubleshooting basic switching
Switching loops are another potential source ofconnectivity issues.
STP prevents bridging loops and broadcast storms Indicators that a loop is present in a network include:
Loss of connectivity to, from, and through affected networkregions
High CPU utilization on routers connected to affectedsegments
High link utilization up to 100%
High switch backplane utilization as compared to thebaseline utilization
Syslog messages indicating packet looping, constantaddress relearning, or MAC address flapping messages
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Troubleshooting basic switching
A loop develops when the switch does not receive BPDUsor is unable to process them. This problem could be dueto:
Misconfigurations
Defective transceivers
Hardware and cabling issues
Overloaded processors
STP does not always identify the best root bridge or rootports.
Troubleshoote STPqshow spanning-tree
qshow spanning-tree interface interface_id
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Activity 9.2.1.5
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Troubleshooting VLAN Configuration Issues
If the Physical Layer is functioning correctly and communicationis still not occurring between end devices, check the VLANconfiguration.qshow vlan id vlan_number t
qShow vlan
qShow vlan brief
qShow vlan id 101Refer 9.2.2.1
If inter-VLAN routing is required, verify the followingconfigurations:
One port from each VLAN connects into a router interface orsubinterface.
Both the switch port and the router interface are configuredwith trunking.
Both the switch and router interface are configured with thesame encapsulation.
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Troubleshooting VLAN Configuration Issues
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Verify the interface configuration
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Access Port: switch considers the connected networkdevice as non-switch (unable to understand BPDU).
Trunk port: switch consider the connected network deviceas switch/bridge (able to speak and read BPDU).
Native VLAN is assigned to an 802.1Q trunk port. An802.1Q trunk port supports traffic coming from manyVLANs (tagged traffic) as well as traffic that does not comefrom a VLAN (untagged traffic)
Management VLANs is any VLAN you configure to access
the management capabilities of a switch.The native VLAN and management VLAN are VLAN1 by default.
Refer 9.2.2.3
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Troubleshooting VLAN Configuration Issues
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Activity 9.2.2.4
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Troubleshooting VTP
VTP simplifies the distribution of VLAN information tomultiple switches in a domain. Switches that participate inVTP operate in one of three modes: server, client, ortransparent. Only the server adds, deletes, and modifiesVLAN information.
When troubleshooting VTP on a network, ensure that: All participating devices have the same VTP domain name.
Two VTP servers exist in every domain, in case one fails.
All servers have the same information.
The revision numbers are the same on all devices. All devices use the same VTP version.
qshow vtp status
qvtp version
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Troubleshooting VTP
qvtp password passwordqshow vtp password
Refer 9.2.3.2
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Activity 9.2.3.3
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RIP Issues
RIP is a fairly basic and simple protocol to configure Compatibility issues exist between RIPv1 and RIPv2. If the
RIP routes are not being advertised, check for the followingproblems:
Layer 1 or Layer 2 connectivity issues
Requirements for VLSM subnetting but using RIPv1
RIPv1 and RIPv2 routing configurations mismatched
Network statements missing or incorrect
Interface IP addressing incorrect
Outgoing interface is down
Advertised network interface is down
Passive interface misconfigurations
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RIP Issues
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Activity 9.3.1.3
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EIGRP Issues
Possible reasons why EIGRP may not be working are: Layer 1 or Layer 2 connectivity issues exist. An interface has incorrect addressing or subnet mask.
AS numbers on EIGRP routers are mismatched.
The wrong network or incorrect wildcard mask is specified in the
routing process. The link may be congested or down.
The outgoing interface is down.
The interface for an advertised network is down.
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EIGRP Issues
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Activity 9.3.2.3
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OSPF Issues
OSPF Troubleshooting Issues
Neighbors must be part of the same OSPF area.
Interfaces for neighbors must have compatible IP addresses andsubnet masks.
Routers in an area should have the same OSPF hello interval anddead interval.
The routers must advertise the correct networks for interfaces toparticipate in the OSPF process.
The appropriate wildcard masks must be used to advertise thecorrect IP address ranges.
Authentication must be correctly configured on routers for
communication to occur.q show ip ospf
q show ip ospf neighbor
q show ip ospf interface
q debug ip ospf events
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OSPF Issues
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OSPF Issues
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9.3.3.2
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Route Redistribution Issues
Configuring a static default routeqip route 0.0.0.0 0.0.0.0 S0/0/0qdefault-information originate
qredistribute static
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RIP
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Route Redistribution Issues
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EIGRP
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Route Redistribution Issues
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OSPF
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Troubleshooting WAN connectivity
The IP address configured on an interface and the statusof the port and line protocol is viewable with the command:
qshow ip interface brief
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Troubleshooting WAN connectivity
the encapsulation in use on a serial line, use thecommand:qshow interfaces
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PPP?
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Troubleshooting WAN connectivity
The PPP process involves both the LCP and NCP phases.LCP establishes the link and verifies that it is of sufficientquality to bring up the Layer 3 protocols. NCP allows Layer3 traffic to move across the link.
When troubleshooting PPP connectivity, verify that: LCP phase is complete
Authentication has passed, if configured
NCP phase is complete
qshow interfaceqdebug ppp negotiation
qdebug ppp packet
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Troubleshooting WAN connectivity
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Commands are available that assist in troubleshooting PPP. Toshow the status of the LCP and NCP phase
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Troubleshooting WAN connectivity
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Display PPP packets transmitted during the startup phasewhere PPP options are negotiated
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Troubleshooting WAN connectivity
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Display real-time PPP packet flow, use:
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Activity 9.4.1.5
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Troubleshooting WAN Authentication
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Troubleshooting WAN Authentication
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Troubleshooting WAN Authentication
Authentication can be either one-way or two-way. Forenhanced security, use two-way or mutual authentication.Two-way authentication requires that each end deviceauthenticate to the other.
The most common problem with authentication is eitherforgetting to configure an account for the remote router ormisconfiguring the username and password.qppp pap sent-username username password password
Debugging the authentication process provides a quick
method of determining what is wrong.q debug ppp authentication
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Troubleshooting WAN Authentication
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Troubleshooting WAN Authentication
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Activity 9.4.2.3
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Determining if an ACL is the Issue
ACLs add a level of complexity to troubleshooting networkissues. When networks or hosts become unreachable and ACLs
are in use, it is critical to determine if the ACL is theproblem.
Is an ACL applied to the problem router or interface?
Has it been applied recently?
Did the issue exist before the ACL was applied?
Is the ACL performing as expected?
Is the problem with all hosts connected to the interface or only
specific hosts?
Is the problem with all protocols being forwarded or only specificprotocols?
Are the networks appearing in the routing table as expected?
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Determining if an ACL is the Issue
Logging shows the effect that ACLs are having on variouspackets.
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Determining if an ACL is the Issue
To display all ACLs configured on the router, whetherapplied to an interface or not:q show access-lists
To clear the number of matches for each ACL statement:q clear access-list counters
To display the source and destination IP address for eachpacket received or sent by any interface on the router:q debug ip packet
If the packets are simply passing through and the ACL
does not block a packet from this IP address, no debugmessage is generated.
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Determining if an ACL is the Issue
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9.5.1.3
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Activity 9.5.1.4
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ACL configuration and Placement Issues
If it appears that the ACL is the problem, there are severalissues to check.
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Refer 9.5.2.1
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ACL configuration and Placement Issues
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Activity 9.5.2.3
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Summary
When troubleshooting a network, determine the scope ofthe problem and isolate issue to a specific failure domain The most common problems with switches occur at the
Physical Layer
Use debug commands to isolate problems, not to monitornormal network operation
The most common physical layer WIN problems are notspecifying a clock rate on the link or ussing the wrong typeof cables.
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