Stability Issues in OSPF Routing

Authors: Anindya Basu and Jon G. Riecke

Complete Citation

Basu A, Riecke J. Stability Issues in OSPF Routing. ACM SIGCOMM 01, August 2001

Abstract

We study the stability of the OSPF protocol under steady state and perturbed conditions. We look at three indicators of stability, namely, (a) network convergence times, (b) routing load on processors, and (c) the number of route flaps. We study these statistics under three different scenarios: (a) on networks that deploy OSPF with TE extensions, (b) on networks that use subsecond HELLO timers, and (c) on networks that use alternative strategies for refreshing link-state information. Our results are based on a very detailed simulation of a real ISP network with 292 nodes and 765 links.

Annotations

• Effect of Subsecond HELLO Timers:
  

Figure (a) shows the propagation time comparisons: HELLO timers do result in significant improvements in propagation times.

Figure (b)shows the processor utilization:the extra HELLO messages add about a 1% extra load for the 500ms case and about a 2% extra load for the 250ms case. The authors conclude that the processor loads stay within reasonable ranges.

Figure (c)shows the number of route flaps: the number of route flaps increases a lot, which may make the network unstable.

Prioritized Treatment of Specific OSPF Version 2 Packets and Congestion Avoidanc

Authors: G. Choudhury

Complete Citation

G. Choudhury. Prioritized Treatment of Specific OSPF Version 2 Packets and Congestion Avoidanc. IETF rfc 4222. October 2005

Abstract

This document recommends methods that are intended to improve the scalability and stability of large networks using Open Shortest Path First (OSPF) Version 2 protocol. The methods include processing OSPF Hellos and Link State Advertisement (LSA) Acknowledgments at a higher priority compared to other OSPF packets, and other congestion avoidance procedures.

Annotations

A large network running OSPF protocol may occasionally experience the simultaneous or near-simultaneous update of a large number of link state advertisements, or LSAs. This event is called an LSA storm which may be initiated by an unscheduled failure or a scheduled maintenance event.

The LSA storm causes high CPU and memory utilization at the router such that incoming packets are delayed or dropped. If HELLO interval is reduced from seconds to sub-seconds, it will be more likely for Hello packets to be delayed beyond the dead interval during network congestion caused by an LSA storm. Thus, the network is driven to an unstable state.

-- YingxinJiang - 31 Oct 2007