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Stability Issues in OSPF Routing Basu and Riecke

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What is OSPF-TE? Stability? Processor, route flaps, convergence. Subsecond HELLO timers ... OSPF-TE. Open Shortest Path First, Traffic Extensions ... – PowerPoint PPT presentation

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Title: Stability Issues in OSPF Routing Basu and Riecke


1
Stability Issues in OSPF RoutingBasu and Riecke
  • George Porter
  • NETREAD, Spring 2002

2
Motivation/Overview
  • What is OSPF-TE? Stability?
  • Processor, route flaps, convergence
  • Subsecond HELLO timers
  • Link State Advertisement (LSA) synchronization
    avoidance

3
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

4
Issues in stability
  • Convergence time
  • Processor load
  • Number of route flaps
  • Node failures
  • Message loss
  • Triggering thresholds
  • Subsecond Hello timers
  • Refresh Packets

5
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

6
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

7
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)

8
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)

9
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
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