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OC-12 POS blades in GSR routers and DWDM Add Drop Mux's

I have several GSR routers with OC-12 POS blades using OSPF connected to a DWDM ring using Nortel's Optera 5200. It appears that whenever a line side protection switch takes place, that the POS blades react to this, and reconverge OSPF even though the protection switch is <50ms. Do the Cisco POS blades look at standard GR253 Sonet parameters to detect a switch and drop the Interface? Is there a listing of default parameters for the POS blades? I've seen reference all over the CCO to a Standards compliance document, but it can't be found when I follow the link. In theory, the POS blade should never detect a protection switch <50ms and should stay up.

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New Member

Re: OC-12 POS blades in GSR routers and DWDM Add Drop Mux's

If there is a protection switching on the path (in the DWDM network), AIS-P is sent to the POS line terminating equipment. By default,the POS interface detects path AIS, path RDI or B3 threshold crossing alarms, and brings down the interface until the alarm has been cleared for 10 seconds. This is because otherwise it would take three missed keepalives (3*10=30s) to detect an unprotected path failure.

One solution is to increase the OSPF Shortest Path First calculation interval. The default is 5 seconds. This time can be changed with the "spf-interval " IOS command.

Beginning from IOS 12.0(15.5)S there is another solution: you can cofigure a holdoff time before line alarms trigger the POS link down, using the "pos delay triggers path " command. The parameter is between 0 and 511, where 0 means disabling this feature. If you omit the parameter, it assumes a default of 100 ms, which enables protection switching on the path and the POS interface still remains up.

New Member

Re: OC-12 POS blades in GSR routers and DWDM Add Drop Mux's

If the POS delay trigger has not been set on the the card, then it will always detect the protection switch. Cisco has a white paper on DWDM and POS cards and they recommend setting the POS delay timer to either default, 100 ms or 51 ms and changing the clocking on the POS interface to internal.

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