Link de-bounce timer operation when used on SFP+ ports on Nexus switches

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May 7, 2012 6:28 AM
May 7th, 2012
                              

This post explains the Link de-bounce timer operation when used on SFP+ ports on Nexus switches.

                        

Background information on de-bounce timer:

The port de-bounce time is the amount of time that an interface waits to notify  the supervisor of a link going down. During this time, the interface waits to see if the link comes back up. The wait period is a time when traffic is stopped. You can enable the de-bounce timer for each interface and specify the delay time in milliseconds. Default timer is 100 msec on Nexus 7000 (and other Nexus platforms).

Now, lets take a scenario where we have link de-bounce configured as 100 msec (default on N7k) and the link goes down:

Event 1 - Link goes down

Event  2 - Nexus HW (Line card) notifies NexusOS (SW) immediately, and we wait for the de-bounce time (100 msec in this case), and then notify  Ethernet port manager (EthPM) that link is down

Event 3 - If the link is not up within the 100 msec, EthPM brings the port down, as we  have passed the "de-bounce" window.

Link UP event:

when the link comes up on SFP+ ports it could take up to 1 sec to notify the link up  event. This has nothing to do with de-bounce feature, as de-bounce feature is there only to delay the link down event. This has to do with limitations in HW for SFP+ implementation.

Let us re-run the whole link down/up scenario again with the inherent delay in SFP+ link up notifications -

Event 1 - Link goes down

Event  2 - HW notifies SW immediately, and we wait for the de-bounce time (100 msec in this case), if the link doesn't come up during this time, we  notify the EthPM that link is down

Event 3 - Assume that link is up within 100ms time, but it takes up to 1 sec (because of the SFP+ implementation limitation) for SFP+ to transition to up state. By then, we have passed the 100 msec de-bounce window

Event 4 - EthPM  brings the port down, as SW notifies to do so. Hence, we see interface flaps. When we have the de-bounce timer set to 1000 msec or 1 sec, the link is up before the de-bounce timer expired. Hence, no link event are seen.

On the non-sfp+ e.g X2 (e.g. on C4900M, C6500), we don't have this problem, there is no inherent delay in bringing the port up.

The general recommendation is to configure 1 sec de-bounce timer on SFP+ ports that connects to DWDM equipments.

Background information on inherent delay in SFP+:

The delay in link up event in SFP+ implementation is due to a factor called - Electronic Dispersion Compensation (EDC) - it is a technology that can mitigate power penalties associated with optical link budgets. On traditional receivers without EDC (e.g. SFP, where there is no delay in bringing the port up) can recover an optical signal only if the dispersion is less than approximately one-half Unit Interval (UI) over the length of fiber. The IEEE 802.3aq standard, however, supports runs as long as 220m over OM1 and FDDI-grade fiber types and specifies that the receiver must be able to handle more than four UI of dispersion. Without EDC, we cannot possibly meet this requirement.

More information on EDC -

http://www.cisco.com/en/US/prod/collateral/modules/ps5455/prod_white_paper0900aecd806b8bcb.html

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