09 - Ethernet Switch Troubleshooting

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09 - Ethernet Switch Troubleshooting

By Muhammad Asghar Khan

Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom


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Agenda

Network Verification & TroubleshootingVerifying the Network TopologyExamining Interface Status

L1 & L2 Problems on Not Working InterfacesInterface Status CodesInterface Speed & Duplex Issues

L1 Problems on Working Interfaces

CRC CounterEthernet Collision vs Late Collision Interface Counters

Analyzing L2 Forwarding Path with MAC AddressTable

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Network Verification & Troubleshooting

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Verification refers to the process of examiningnetwork to confirm that it is working as designed

Troubleshooting refers to examining the network todetermine what is causing a problem so that it canbe fixed

For effective troubleshooting, you need toremember the theory of how networks should

work, how to interpret the show command, knowthe use of testing tools such as ping & traceroute

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Network Verification & Troubleshooting


Most of the troubleshooting processes involves a threesteps :1. Analyzing Normal Operation

Predict the details of what should happen if the network isworking correctly, based on documentation, configuration &show , debug command output

2. Problem IsolationDetermine how far along the expected path theframe/packet goes before it can not be forwarded any

further, again base on documentation, configuration andshow & debug command out put3. Root Cause Analysis

Identify the underlying causes of the problems identified inthe preceding step

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Verifying the Network Topology


The Cisco Discovery Protocol (CDP) discovers basicinformation about neighboring routers & switches

Cisco switches & routers send CDP messages out of each of their interfaces

Devices that support CDP learn information aboutothers by listening to these messages

CDP can be used to confirm or fix thedocumentation shown in a network diagram &discover the devices and interfaces used in anetwork

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Media that supports multicasts at data link layer,CDP uses multicast frames, while media that do notsupports multicasts CDP sends a copy of the CDPupdate to the known data link addresses

CDP can be used to discover:Device IdentifierAddress ListLocal InterfacePort IdentifierCapabilities List/Type of DevicePlatform/Model

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Cisco recommends that CDP be disabled on allinterfaces that do not have a specific need for it

The interfaces that need to use CDP are interfacesconnected to routers, switches and IP Phones

CDP for an interface can be enabled by using cdpenable interface sub command and disabled withno cdp enable interface sub command

Alternately, the no cdp run global commanddisables CDP for the entire switch, with cdp runglobal command re-enabling CDP globally

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Figure shows the small network

Display detailed information aboutneighboring devices using cdp, fromthis you can draw a basic diagramas shown above to confirm thedetails in the figure are correct

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Displays detailed information abouta neighbor(s)

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Displays detailed information one

neighbor at a time

Displays global Cisco DiscoveryProtocol information

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Displays information about theinterfaces on which Cisco DiscoveryProtocol is enabled

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Examining Interface Status


Layer 1 & 2 Problems on Not Working InterfacesInterface Status Codes

An interface status codes are used to determinewhether the interface is working or not

Cisco switches use two different sets of interface statuscodes

Set of two codes (Words) i.e line status & protocol statusThe show interfaces command shows the two code status

Line status refers to L1 working &

protocol status refers to L2 working,in this case both are down

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Single Code (Word)The show interface status command list the single codestatus

Status column shows the single code

status, a- before any state means thatthis value was found through auto-negotiation process

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Table lists the code combination & some causes thatcould have caused a particular interface status

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Interface Speed & Duplex IssuesSwitch interfaces can find their speed & duplex settingseither through the process of IEEE 802.3X standardauto-negotiation or by configuring to use specific speedor duplex using the speed {10|100|1000} andduplex {half|full} commandsThe defaults for speed & duplex are as follows:

If the speed is not known, use 10 Mbps, half duplex

If the speed is some how known to be 10 or 100 Mbps, defaultto use half duplexIf the speed is some how known to be 1000 Mbps, default touse full duplex

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If the duplex settings do not match on the ends of anEthernet segment, the switch interface will still be inconnect (up/up) state; In this case the interface works, butwith poor performance

Layer 1 Problems on Working InterfacesSwitch keep various interface countersThese interface counters can help to identify problemsthat can occur even though the interface is in a connectstateWhenever the an error occurs receiving device discardsthe frame & counts it as some kind of input errorCyclic Redundancy Check (CRC) Counter

Errors experienced during transmission due to damagedcable or electromagnetic interference (EMI) are referred asCRC errors

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CRC an older term referring to FCS

Ethernet Collision vs Late Collision Interface CountersEthernet collision occur as a normal part of the half-duplex logicIf LAN design follows cabling guidelines, all collisionsshould occur by the end of the 64 th byte of any frame

CRCcounter

Collisioncounter

Late Collisioncounter

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When a switch has already sent 64 bytes of a frame,and the switch receives a frame on that same interface,the switch senses a collision; this is called late collisionIn this case; switch increments the late collision counterand send a jam signal, wait random time, and try again

Incrementing late collision counter typically means;cabling length exceeds the Ethernet cabling standards,on one end of the interface half duplex while on otherside full duplex is used

Three common LAN problems can use these counters:Excessive Interface on the CableDuplex mismatchJabber

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Excessive Interface on the CableIt causes the problem of interference on the cableIt can cause various input error counters to keep growinglarger, especially the CRC counter

Duplex mismatch & JabberJabber refers to cases in which NIC ignores Ethernet rules &sends frame after frame without breakBoth duplex mismatch & jabber can be partially identified bythe collisions & late collision countersDuplex mismatch problems can be further isolated by usingthe show interface commandIsolating jabber problems requires the use of specialized LANcabling troubleshooting tools

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To know how switch will forward an Ethernetframe, you need to examine the MAC address tableon a switchThe show mac-address-table EXEC

command displays the contents of a switchs MACaddress tableThe forwarding logic can be summarized as:1. Determine VLAN in which frame should be

forwarded2. Look for frames destination MAC address in the

MAC address table; if destination MAC is:

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a) Unicast, forward the frame out the only interface listed inthe matched address table entry

b) Not Unicast, flood the frame out all other access ports(except the incoming port) in that same VLAN

c) Broadcast or Multicast, flood the frame out all otheraccess ports (except the incoming port) in that same VLAN

When MAC table allows a frame to be delivered, butthe frame still does not arrive, the problem is likely tobe related to some kind of filtering

LAN switches can be configured with Access ControlLists (ACL) which filter frames, also routers canconfigure & use ACLs

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ACLs are not covered in CCNA examsAlso port security configured as protect orrestrict violation action, the switch discards theoffending traffic, but it leaves the port in connect(up/up) state

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09 - Ethernet Switch Troubleshooting

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