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