draft-holla-ospf-update-graceful-restart

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draft-holla-ospf-update-graceful-restart

• Sujay Gupta

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Introduction

• Graceful-Restart (RFC 3623) is deployed widely.
• RFC 3623 needs to be revisited.
• Optimize GR and Helper behavior.
• Provide better clarity w.r.t inter-operation.

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draft-holla-ospfv2-update-graceful-restart

• Goal To keep routing inconsistencies during GR
  to a minimum, in face of topology changes.
• Main theme Identify necessary and sufficient
  reasons for GR exit. GR must terminate only for
  necessary conditions and the termination must be
  ASAP.
• Compatible with existing GR mechanism.

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Draft Proposals - Topology change LSAs

• Current Behavior
• Strict-lsa-checking disabled
• Cannot take this policy decision apriori.
• May create extended routing holes and loops.
• Violates basic premise for Graceful restart and
  unbroken forwarding.
• Strict-lsa-checking enabled
• Very conservative.
• Helpers exit for any change in database contents.
• GR may never be completed in large and unstable
  topologies.

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Draft Proposals - Topology change LSAs

• Suggestion
• To strike balance between maintaining unbroken
  forwarding and avoiding false GR exits.
• Strict LSA checking is retained and is default.
• Helpers to react to Bad News immediately.
• Helpers to ensure that no new route is added with
  GR router as next hop.
• Bad news characterized explicitly by the scope
  of topology change LSA.

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Draft Proposals - Explicit negative feedback

• Current Behavior
• Helpers decision not to enter Helper mode or
  exit Helper mode is determined only through the
  router and Network LSA checks
• If a Router does not enter helper mode, until
  adjacency is re-established, the GR router will
  not be aware.
• If a Helper has completed re-establishing the
  adjacency, it can not signal an exit-helper event
  to the restarting router.
• Some of the consequences are
• Grace LSA may remain active in the topology for a
  long time.
• In some topologies, this can lead to prolonged
  routing inconsistencies.

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Draft Proposals - Explicit negative feedback

• Suggestion
• Provide explicit feedback mechanism for Helper to
  communicate with GR router.
• Ensures the most rapid way of reverting back to
  normal OSPF protocol, through the flushing of
  Grace LSAs.
• Faster convergence.

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Draft Proposals - Explicit negative feedback

• Suggestion
• Piggybacking an exit-Grace TLV in the
  link-local scope.
• Pros
• Achieves reliable link local signaling of
  feedback.
• Cons
• The use of LSA as a trigger sounds incorrect
• Alternatives
• Use a 1-way hello to signal helper exit.
• Pros
• Simple and intuitive
• Cons
• The 1-way in hello is interpreted as per OSPF
  protocol. Should not give a supplementary meaning
  to it.
• Use of 1-way forces restarting router to reset
  adjacency.

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Draft Proposals - Exit checks for GR router

• Use following checks along with Router and
  Network LSA checks
• Downward transitions from 2-way to 1-way.
• Dead interval expiry.
• Receipt of explicit negative feedback from a
  Helper.

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Miscellaneous clarifications needed in the RFC

• Treatment of multiple Grace-LSAs.
• Can restarting router be allowed to request for
  extension of Grace period?
• Can restarting router be allowed to request
  unplanned restart during a planned restart?
• How to interpret grace-period across multiple
  grace LSAs?

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Summary

• Proposals significantly improve convergence time.
• Identifies possible problem areas and solutions.
• All suggestions are backward compatible with RFC
  3623.

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Questions?

• Authors
• Ashok Chandrashekhar Holla Huawei Technologies,
  ashok_ch_at_huawei.com
• Anup Kumar T. Cisco Systems, anupt_at_cisco.com
• Sujay Gupta Huawei Technologies,
  sujayg_at_huawei.com