Advertising Equal Cost Multi-Path Routes in BGP

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Advertising Equal Cost Multi-Path Routes in
BGP

• Manav BhatiaSamsung India Software Operations,
• Bangalore IndiaJuly 17, 2003
• 57th IETF - Vienna
• draft-ecmp-routes-in-bgp-00.txt

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The Problem Statement

• Route Reflectors only reflect the routes which
  they think are the best !
• True Load Balancing not always possible
• In some cases leads to persistent route
  Oscillations
• Solutions in RFC 3345 place Network Design
  Constraints which is not a long term solution
  Protocol must be enhanced

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Route Reflectors View of the Best Path
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Solution
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BGP MED Oscillation - 1
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BGP MED Oscillation - 2
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BGP MED Oscillation - 3
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BGP MED Oscillation The Solution
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How to Advertise Multiple BGP Routes?

• If more than one route for a given NLRI is
  announced then it is considered as an implicit
  withdraw for the former advertisement
• New Optional Non Transitive attribute
  ECMP_NEXT_HOP introduced
• NEXT_HOP attribute not mandatory if ECMP_NEXT_HOP
  present
• Semantics of the BGP UPDATE message unchanged
• Modify Phase 2 and Phase 3 of the Decision Process

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ECMP_NEXT_HOP Attribute
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Advertising multiple routes with ECMP_NEXT_HOP
attribute
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Appending routes using ECMP_NEXT_HOP attribute
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Removing one of the Multiple Routes
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Pragmatic Considerations

• Advertising both Reachable and Unreachable
  Prefixes
• Multi-Protocol BGP
• Additional Load in the Network
• Impact on BGP Convergence
• Flapping BGP ECMP route
• May give higher degree of preference to a peer we
  know has more number of ECMP routes

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Next Steps

• Possibly combine with draft-lefaucheur-multiprotoc
  ol-nh-00.txt
• Get Feedback !