AfNOG2005 Troubleshooting BGP

8/6/2019 AfNOG2005 Troubleshooting BGP

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Troubleshooting Connectivity


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979797 2005, Cisco Systems, Inc. All rights reserved.APRICOT2005

Troubleshooting Connectivity Example IV

Checklist:Assume AS1 has done everything in thistutorial so far

L2 problem? Route Flap Damping?

All the configurations look fine, the Looking Glassoutputs look fine, life is wonderful Apart from thoseannoying traffic swings every hour or so

Since BGP is configured fine, and the net has beenstable for so long, can only be an L2 problem, orRoute Flap Damping side-effect



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g yExample IV

L2 upstream somewhere has poorconnectivity between themselves and therest of the Internet

Only real solution is to impress uponupstream that this isnt good enough, and getthem to fix it

Or change upstreams



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g yExample IV

Route Flap DampingMany ISPs implement route flap damping

Many ISPs simply use the vendor defaults

Vendor defaults are generally far too severe

There is even now some real concern that the morelenient RIPE-229 values are too severe

www.cs.berkeley.edu/~zmao/Papers/sig02.pdf

Again Looking Glasses come to the operators

assistance


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g yExample IV

Most Looking Glasses allow the operators tocheck the flap or damped status of theirannouncements

Many oscillating connectivity issues are usually caused

by L2 problemsRoute flap damping will cause connectivity to persistvia alternative paths even though primary paths havebeen restored

Quite often, the exponential back off of the flapdamping timer will give rise to bizarre routing

Common symptom is that bizarre routing will often clearaway by itself


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Troubleshooting Summary

Most troubleshooting is about:

ExperienceRecognising the common problems

Not panicking

Logical approachCheck configuration first

Check locally first before blaming the peer

Troubleshoot layer 1, then layer 2, then layer 3, etc


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Troubleshooting Summary

Most troubleshooting is about:

Using the available tools

The debugging tools on the router hardwareInternet Looking Glasses

Colleagues and their knowledge

Public mailing lists where appropriate


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Closing Comments

Presentation has covered the most commontroubleshooting techniques used by ISPs today

Once these have been mastered, more complexor arcane problems are easier to solve

Feedback and input for future improvements isencouraged and very welcome


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Troubleshooting BGPPhilip SmithPhilip Smith

AfNOGAfNOG

20052005

Maputo, MozambiqueMaputo, Mozambique

AfNOG2005 Troubleshooting BGP

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AfNOG2005 Troubleshooting BGP