Stability Issues in OSPF Routing Basu and Riecke

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Stability Issues in OSPF RoutingBasu and Riecke

• George Porter
• NETREAD, Spring 2002

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Motivation/Overview

• What is OSPF-TE? Stability?
• Processor, route flaps, convergence
• Subsecond HELLO timers
• Link State Advertisement (LSA) synchronization
  avoidance

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OSPF-TE

• Open Shortest Path First, Traffic Extensions
• Link State Advertisements (LSA) sent
  periodically, and when network changes
• Flooding used to distribute
• RSVP soft state
• CR-LDP (constraint based) hard state

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Issues in stability

• Convergence time
• Processor load
• Number of route flaps
• Node failures
• Message loss
• Triggering thresholds
• Subsecond Hello timers
• Refresh Packets

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Multiple node failures

• Processor
• 1 failure no effect
• 5 failures 50
• 10 failures 100
• Route flaps
• OSPF affected by as few as 5 failures

• Convergence times
• OSPF robust in this regard
• TE overhead about 20

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Message Loss

• Processor
• 2 and 10 show little effect (at 10, OSPF
  thinks it has fewer neighbors, and so traffic
  goes down)
• 5 utilization is affected

• Route flaps
• Little effect at 2 and 5
• At 10, there is a huge amount of flapping

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Convergence time

• Hello messages are usually sent every 10 seconds
• Propagation time given Hello timer of
• 10 sec ? 33 sec
• 500ms ? 3 sec
• 250ms ? 1 sec
• Processor load for subsecond Hellos is basically
  the same (as long as over 275 ms)

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Refresh Synchronization

• Based on the LSA timers, periodic bursts of
  traffic can develop
• Proposal
• Introduce randomization
• Zinin mechanism (but why? They said it didnt
  have much of an effect)

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Summary

• OSPF with TE is pretty stable
• TE has little effect on convergence time
• OSPF resilient to message loss
• Setting HELLO timers to lt1 sec is a good idea (if
  over, say, 300ms)
• Randomization can prevent bursts of LSA traffic