We're testing the reference system shown in the figure below.
- Four 2960 switches are used for transport;
- Equipment 1 and Equipment 2 exchange packets for synchronization;
- To reach synchronization Equipment 1 and 2 must exchange data with a very low jitter.
2960 Configuration details
Four our test puprose, we're using 100Mbit/s ports (22 and 23) as trunk.
In order to obtain minimum jitter We performed these configurations:
- We Enabled QoS;
- We Marked Synchronization packets with CoS 7 and DSCP 63;
- We marked other kind of traffic inserted in different ports) with CoS 0;
- We set "trust DSCP" on trunk ports;
- On the trunk ports we mapped traffic with CoS 7/DSCP 63 (and only this) on output queue 1;
- We enabled the expedite queue (priority-queue out).
With these settings we aim at forcing our synchronization packtes to precede other kind of traffic and go from Equipment 1 to Equipment 2 with minimum jitter.
Unfortunately we experienced high jitter when both synchronization packets and other traffic are sent through the systems.
What is wrong in our assumptions or configurations?
What is the real behaviour of the expedit queue?