Cisco UCS provides a configurable “Chassis Discovery Policy” that affects how chassis links (called “Server Ports”) are activated when discovering a new chassis. See this page on cisco.com.
After a recent discussion on Twitter, I decided to test out a few scenarios.
My configuration is a pair of UCS 6120 Fabric Interconnects, two UCS 5108 Chassis, with 2 links per IO Module.
Additionally, this particular lab is on a reasonably old version of UCSM code, 1.0(1f). I’m not in a position to upgrade it at the moment – I can re-test at a later date with more current code. I don’t expect the behavior to change, however.
I started with the default discovery policy of 1-link. I enabled one link per fabric interconnect to a chassis, and checked the status of the ports. The ports were in an “Up” Overall Status, which is proper – and means that the link is available for use. I then enabled a second link per fabric interconnect to the same chassis. The second activated link went into a “Link Up” state, with “Additional info” of “FEX not configured”. This means that the physical link has been activated, but is not in use – the IO Module (FEX) is not using the link. A re-acknowledgement of the chassis activates the second link.
I then changed the discovery policy to 4-links. I enabled one link per fabric interconnect to a different chassis, and checked the status of the ports. The ports went into the “Link Up” state, “FEX Not Configured” – in other words, the links are not use. While we can detect that the chassis is present, no traffic will flow as the FEX has not yet been configured to pass traffic. Furthermore, the chassis is an error state, in as much as the cabling doesn’t match the chassis discovery policy.
Reacknowledging the chassis activates the links, and removes the error condition, even without changing the discovery policy.
Finally, I tested the scenario of setting the chassis discovery policy to 2 links and activating only one link per Fabric Interconnect. As expected, the link enters the “link-up”, “FEX not configured” state. I then activated a second link per Fabric Interconnect. After a brief period, both ports per Fabric Interconnect enter the “Up” status.
In short, setting the Chassis Discovery Policy determines how many links must be present per Fabric Interconnect to an IO Module before the ports are activated and the chassis is put into service. If the policy is set at 1 link, and more than 1 link are activated, a simple re-acknowledgement of the chassis will activate the additional links. If the policy is set to a higher number – and that number of links are not present – the chassis will not be activated unless a manual re-acknowledgement is done.
Frankly, I don’t see a lot of value in this feature – unless you’re adding a large number of chassis that will all have identical numbers of uplinks. Even then, you’re saving yourself at most a few clicks of the mouse to re-ack the chassis that don’t match the policy. Why not simply leave the policy at default and re-ack any chassis that has more than 1 link per IOM? Presumably you’re going to do this before actually deploying any service profiles, so there’s no potential for disruption with the re-ack.
Thoughts or comments?