Solved: BGP over PoS and pos delay triggers path command

cybrsage · ‎08-05-2008

I am trying to reduce my convergence time for BGP running over a PoS line. The timers are set to 30 and 90, and the provider will not change them. 90 seconds is WAY to long to wait.

I have heard that the pos delay triggers path command will cause BGP to bypass its normal hold timer and start to converge after the amount of time set in that command...provided I also had Fast-External-Failover turned on (which is on by default).

So, for instance, if I used pos delay triggers path 120 and the circuit went down (creating a POS PATH AIS), will BGP still wait the 90 seconds, or will it only wait 120ms?

Just trying to wrap my head around this one.

Thanks, and points will be liberally handed out!

Giuseppe Larosa · ‎08-06-2008

Hello,

here a link the pos delay

http://www.cisco.com/en/US/docs/ios/12_1/12_1ex/feature/guide/e_posde.html

pos delay defends the router from a flapping interface by waiting some msec before accepting the SDH alarm and putting the inteface down.

if you use fast-external-fallover the BGP convergence time is the time to detect that the link is down and shouldn't be the hold time.

No more then 3 times the L2 keepalive interval (30 seconds) in a normal link in a POS you can take advantage from SDH alarms.

Hope to help

Giuseppe

View solution in original post

royalblues · ‎08-05-2008

I think the POS delay trigger actually tell the router to wait a certain amount of time before bringing down the line protocol when a alram trigger is received. This might actually cause BGP to converge slowly

If you want to reduce the BGP keepalive and hold intervals, you can reduce it to a much lower value. When the BGP adjacency comes up, the lowest one among the neighbors is chosen. You can also twea bgp convergence by manipulating BGP scan time and BGP advertisement interval

Other option to consider would be to use BFD with BGP, if your platform supports the same and the neighbors are directly connected.

HTH

Narayan

cybrsage · ‎08-05-2008

That is not what I want to hear! :(

I will give it a test and sometime this week or next week and let you know the results.

Giuseppe Larosa · ‎08-06-2008

Hello,

here a link the pos delay

http://www.cisco.com/en/US/docs/ios/12_1/12_1ex/feature/guide/e_posde.html

pos delay defends the router from a flapping interface by waiting some msec before accepting the SDH alarm and putting the inteface down.

if you use fast-external-fallover the BGP convergence time is the time to detect that the link is down and shouldn't be the hold time.

No more then 3 times the L2 keepalive interval (30 seconds) in a normal link in a POS you can take advantage from SDH alarms.

Hope to help

Giuseppe

cybrsage · ‎08-06-2008

Thank you for the link, it gave me all I needed. This is the specific part:

Setting Path Level Triggers as Alarms and Enabling POS Alarm Delay for Path Level Triggers

The following alarms are considered path level triggers: path alarm indication signal and path remote defect indication. These alarms are not triggers unless the pos delay triggers path command is entered.

Without using this command, path alarms are not triggers. With it, they are.