why in ospf , lsa refresh time is set as 30 minutes

[binojbaby1984]

Hi all

i have doubt regarding the ospf lsa refresh time interval . why this is set as default by 30 minutes . is there any particular reason for this ? Please let me know

[Peter Paluch]

Hi,

RFC 2328 stipulates two facts:

• The maximum age of an LSA is 3600 seconds. After an LSA has reached an age of 3600 seconds, it must be flushed.
• Every LSA has to be reoriginated (that is, refreshed) every 1800 seconds.

The choice of 1800 seconds for the refresh interval is purely a very conservative estimate of the advance in which an LSA should be refreshed so that it can still be propagated to all other routers in an area or OSPF domain in time before the old LSA expires by reaching the age of 3600 seconds. In essence, the choice of the refresh interval at 30 minutes gives the network an additional 30 minutes to propagate the refreshed LSA before the old one expires.

In fact, current Cisco IOSes do not go by the exact 1800 second interval. The IOS has chosen various approaches to scheduling the LSA refreshing process, and it is very nicely described in this article by Petr Lapukhov:

http://blog.ine.com/2009/12/31/tuning-ospf-performance/

Scroll down to the part called LSA Group Pacing.

Best regards,
Peter

[rais]

RFC 2328 sets the LSRefreshTime under 'Architectural Constants' to 30 minutes.

However, this span could be anywhere from MinLSInterval [5 minutes] to MaxAge -1 [59 minutes] e.g. Junos sets the default refresh time to 50 minutes in order to relieve the router some processing and given the pretty stable networks of today.

Additionally, options are available to disable the refresh altogether per RFC 4136: OSPF Refresh and Flooding Reduction in Stable Topologies.

Thanks.

[Peter Paluch]

Hi,

Thank you for the additional and insightful info! Rated as deserved.

Best regards,
Peter

[rais]

Thanks Peter.

Best Regards,

Rais.