BGP Conversion Time

Unanswered Question
Oct 29th, 2006
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Guys,


Has anyone can tell me the convergence time for BGP. Basically, I have 2 CEs on redundancy. However, I find that the time taken to failover from primary to secondary link took 22secs. Is this a normal timing taken for conversion over MPLS on BGP?

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mheusinger Mon, 10/30/2006 - 00:05
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Hi,


the time could even be worse. eBGP takes up to 30 seconds to announce an update, iBGP up to 5 seconds. So you could get a convergence time of over a minute (2x eBGP + iBGP).

To reduce this use the command "neighbor advertisement-interval"

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a00800ca79d.html#wp1183687


You can also improve convergence time using BFD: "Decreasing BGP Convergence Time Using BFD"

http://www.cisco.com/en/US/products/ps6441/products_configuration_guide_chapter09186a00805387d7.html#wp1118014


Or by adjusting timers with "neighbor timers"

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a00800ca79e.html#wp1020454


You might want to investigate the convergence time improvement by adjusting the scan interval. This might be, however, somewhat risky in that it could lead to less stability, if the interval is choosen too small and the CPU load generated is too high. So use with care.


Hope this helps! Please rate all posts.


Regards, Martin

network.king Mon, 10/30/2006 - 02:53
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Hi ,


I think you have set the hold timer to 20 sec , and the convergence time of 22 sec is normal and its fine for the router's cpu and memory rather than trying to play with the timers.


Pls let us know if you have tried to reduce the timers and router's cpu is also fine.


regards

vanesh k

mheusinger Mon, 10/30/2006 - 06:06
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Hi Vanesh,


unfortunately your conclusion is not so straight forward. BGP sends triggered updates. So in principle you could a convergence time of few seconds only. However the situation is more complicated, because many timers are involved. A single measurement with no description of the precise settings does not allow for a conclusion which timer is set to which value. Even with all timers at default values you CAN get 22 sec.


Regards, Martin

globalnettech Tue, 10/31/2006 - 05:11
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Hello,


in addition to the other posts, and provided you run at least IOS 12.3(14)T, you might want to try the command 'neighbor x.x.x.x fall-over', which introduces a feature called fast peering session deactivation. Basically, when this feature is enabled, 'adjacency changes are detected, and terminated peering sessions are deactivated in between the default or configured BGP scanning interval.'

This might speed up the convergence...


Regards,


GNT


network.king Tue, 10/31/2006 - 22:14
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Hi Martin,


I think the post was about redundancy for the CE's .And there is no use of triggered updates as my BGP would go down on primary path and PE would note about the down state after the hold down time , thats why I said 20sec and once PE has confirmed abt the invalidity then comes the triggered updates between the PE routers for the fallback to happen and this may vary from 1 to 5 sec ...


pls correct me if anything is wrong.


regards

vanesh k

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