Serial line troubleshooting:

http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html
http://www.cisco.com/application/pdf/paws/14170/T1_alarms.pdf
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00801069ff.shtml
http://www.n-cg.net/ncgpdf/T1-Troubleshooting.pdf
http://www-china.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a0080104552.shtml

MPLS:
https://nws.niif.hu/ncd2001/docs/eloadas/98/index.html

Troubleshooting serial lines:
http://petespacket.com/2009/03/22/serial-lines-troubleshooting-serial-line-input-errors/
http://penta2.ufrgs.br/trouble/trserline.htm
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a758d.shtml

http://alpha.tmit.bme.hu/meresek/interf.htm

DSL és egyéb hibák üzenetei
http://www.cisco.com/en/US/docs/ios/12_3/sem1/system/messages/emgdscre.html

Cisco IOS System Messages
http://www.cisco.com/en/US/docs/ios/12_3/sem1/system/messages/123semv1.html

Synchronous Digital Hierarchy (SDH) Graphical Overview
http://www.cisco.com/en/US/tech/tk482/tk876/technologies_tech_note09186a008011927d.shtml

ATM Control Cells Illustrated
http://www.cisco.com/en/US/tech/tk39/tk49/technologies_tech_note09186a00801111e2.shtml

Framing Formats on ATM DS-3 and E3 Interfaces
http://www.cisco.com/en/US/tech/tk39/tk49/technologies_tech_note09186a0080093c89.shtml

ATM Network Slow Performance
http://www.cisco.com/en/US/tech/tk39/tk371/technologies_tech_note09186a00802878e7.shtml

Framing Formats on ATM DS-3 and E3 Interfaces
http://www.cisco.com/en/US/tech/tk39/tk49/technologies_tech_note09186a0080093c89.shtml

Troubleshooting Line Problems and Errors on DS-3 and E3 ATM Interfaces
http://www.cisco.com/en/US/tech/tk39/tk48/technologies_tech_note09186a0080111093.shtml

Troubleshooting Frame Relay Connections
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1918.html

Troubleshooting Serial Lines
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html

Comprehensive Guide to Configuring and Troubleshooting Frame Relay
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a008014f8a7.shtml

Determining DLCI Limits From LMI Status Updates
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a0080094183.shtml

Equivalent MIB Objects for Frame Relay show Commands
http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080483107.shtml

Frame Relay Glossary
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a00801e32e7.shtml

Frame Relay Traffic Shaping – Token Bucket Flowchart
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a0080093c07.shtml

Frame Relay Traffic Shaping Flowchart
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a0080094a3d.shtml

How to Detect Frame Relay PVC State Changes Using SNMP
http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a9f.shtml

How to Measure Frame Relay Circuit Utilization Using SNMP

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a9d.shtml
http://www.cisco.com/application/pdf/paws/13500/frcircuit.pdf
http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a9e.shtml

Show Commands for Frame Relay Traffic Shaping
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a0080093c06.shtml

Understanding Queueing on Frame Relay Router Interfaces
http://www.cisco.com/en/US/tech/tk543/tk544/technologies_tech_note09186a00800a4754.shtml

Troubleshooting ISDN Connections
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1917.html

Common Problems in Debugging RADIUS, PAP and CHAP
http://www.cisco.com/en/US/tech/tk59/technologies_tech_note09186a0080093f4b.shtml

Configure a Cisco Router with TACACS+ Authentication
http://www.cisco.com/en/US/tech/tk59/technologies_tech_note09186a00800946a3.shtml

Configuring and Troubleshooting PPP Password Authentication Protocol (PAP)
http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a0080093c6f.shtml

Connecting a SLIP/PPP Device to a Router’s AUX Port

http://www.cisco.com/en/US/tech/tk801/tk36/technologies_tech_note09186a0080093c76.shtml
http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a0080093c49.shtml
http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a0080093c49.shtml

Troubleshooting Serial Lines
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html

Blue and Yellow T1 Alarm Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a7100.shtml

DS1, T1 and E1 Glossary
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800f2fa1.shtml

E3 Alarm Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a7591.shtml

E3 Background and Configuration Guidelines
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a72c0.shtml

E3 Error Events Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a7549.shtml

E3 Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a7598.shtml

E1 Alarm Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70f6.shtml

E1 Alarm Troubleshooting – Remote Alarm Indication
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70f8.shtml

E1 Alarm Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70f7.shtml

E1 Error Event Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70f9.shtml

E1 Error Events Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800f99bb.shtml

E1 Layer 1 Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800fb754.shtml

E1 Layer 1 Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70f3.shtml

E1 PRI Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800f9e3a.shtml

E1 PRI Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70fc.shtml

E1 Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70fb.shtml

Hard Plug Loopback Test
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a7599.shtml

Hard Plug Loopback Test for E1 Lines Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70fa.shtml

Hard Plug Loopback Tests for E1 Lines
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a008010059a.shtml

Loopback Tests for T1/56K Lines
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a754b.shtml

MIP Configuration Example
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a5d12.shtml

Troubleshooting Serial Line Problems
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a758d.shtml

T3 Alarm troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a0080344194.shtml

T3 Error Events Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a008034418d.shtml

T3 Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a008034bdd8.shtml

T1 Alarm Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00801069ff.shtml

T1 Alarm Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a70ff.shtml

T1 Error Event Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a4a64.shtml

T1 Error Events Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00801095a3.shtml

T1 Layer 1 Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a0080104552.shtml

T1 Layer 1 Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a4a65.shtml

T1 PRI Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00801509d7.shtml

T1 PRI Troubleshooting Flowchart
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a754a.shtml

T1 Troubleshooting
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a5f40.shtml

Understanding the show controllers E1 Command
http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a5cdc.shtml

Cisco IOS Hints  — http://blog.ioshints.info/2007/08/basic-bgp-troubleshooting.html

BGP – Cisco

http://www.cisco.com/en/US/tech/tk365/tk80/tsd_technology_support_sub-protocol_home.html

BGP – Cisco – Troubleshooting

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009478a.shtml

Cisco command cheat sheet — http://www.madrock.net/2008/07/cisco-command-cheat-sheet/