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| Characteristics of Data Link Layer Problems |
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| Data link layer problems overview |
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| No component above the data link layer is functional |
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| The network is functional, but operating at less than the baseline level |
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| Framing errors |
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| Encapsulation errors |
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| Layer 2 to Layer 3 address mapping errors |
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| Critical characteristics – no network layer connectivity |
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| Upper layer component operation |
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| Critical characteristics – console messages |
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| Troubleshooting Switched Ethernet Networks |
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| Troubleshooting the Spanning Tree Protocol |
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| Troubleshooting STP loops |
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| Preventing STP loops |
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| Troubleshooting incorrect STP root configuration |
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| Troubleshooting Ethernet broadcast traffic |
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| Troubleshooting Ethernet switch flooding |
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| Troubleshooting VTP misconfiguration |
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| Troubleshooting EtherChannel |
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| Troubleshooting Gateway Redundancy Protocols |
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| Gateway redundancy |
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| HSRP configuration and verification |
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| How to debug HSRP operations |
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| Troubleshooting ISDN |
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| T1 framing errors |
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| Troubleshooting ISDN primary rate switch types |
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| Troubleshooting ISDN BRI to telco communications |
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| Troubleshooting ISDN BRI dialer problems |
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| Debugging ISDN call setup failures |
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| Troubleshooting Frame Relay |
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| Steps for troubleshooting Frame Relay |
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| The show frame-relay lmi command |
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| The show frame-relay pvc command |
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| Frame Relay encapsulation type |
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| Troubleshooting Frame Relay using the line status |
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| Ping the local IP address on a multipoint Frame Relay |
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| Reconfiguring a subinterface |
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| PPP and Layer 2 Considerations for Routed and Routing Protocols |
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| Link Control Protocol |
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| Troubleshooting PPP authentication PAP |
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| Troubleshooting PPP authentication CHAP |
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| IP split horizon checking |
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| OSPF in an NBMA environment |
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| The keyword broadcast |
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| Excessive fragmentation |
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