OSPF Enquiry on paths selection

Unanswered Question

Hi Chaps,

A question, i have Router A connects to Router B with 2 links, 1Gbps and 100Mbps WAN circuit. Both are running OSPF. Base on cost calculation, traffic vice-versa will traverse through 1Gb link and when it fails, it goes to 100Mbps link.

Will the traffic goes back to 1Gb link when the WAN circuit recover? I have an impression that OSPF will select a path with longer uptime with more stability.. (correct me if im wrong)

Thanks

I have this problem too.
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Nagendra Kumar ... Wed, 03/03/2010 - 19:51

Hi,

When your 1GB WAN circuit recovers, type 1 LSA will be generated which will trigger a full SPF calculation. This will result in installing the best path into routing tale which obviously will be your 1 GB link.

HTH,

Nagendra

KARUPPUCHAMY MA... Wed, 03/03/2010 - 21:35

Hi,

To avoid such a scenario and to make SPF calculation to be stablee we can use SPF Throttaling concept.

timers throttle spf spf-start spf-hold spf-max-wait

spf-start  : Initial delay to schedule an SFP calculation after a change, in milliseconds. Range is from 1 to 600000.

spf-hold  :  Minimum hold time between two consecutive SPF calculations, in milliseconds. Range is from 1 to 600000.

spf-max-wait  : Maximum wait time between two consecutive SPF calculations, in milliseconds. Range is 1 to 600000.

Throttle Timers Example

This example shows a router configured with the start, hold, and maximum interval values for the timers throttle spf command set at 5, 1,000, and 90,000 milliseconds, respectively.

router ospf 1
 router-id 10.10.10.2
 log-adjacency-changes
 timers throttle spf 5 1000 90000 
 redistribute static subnets
 network 21.21.21.0 0.0.0.255 area 0
 network 22.22.22.0 0.0.0.255 area 00

for more info just go through the below URL
http://www.cisco.com/en/US/docs/ios/iproute_ospf/configuration/guide/iro_short_path_ps6441_TSD_Products_Configuration_Guide_Chapter.html


Regards
Karuppu

Giuseppe Larosa Thu, 03/04/2010 - 05:18

Hello Charles,

>> I have an impression that OSPF will select a path with longer uptime  with more stability.. (correct me if im wrong)

only eBGP prefers oldest path in case of a tie

this is wrong, as explained by colleagues 1GB link recover will trigger a new SPF calculation

link instability and flapping are a different question that you can deal with different tools SPF throttling, BFD and so on.

Hope to help

Giuseppe

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