[chassis] Add HLD for fabric port handling in chassis

[eswaranb]

This is a companion HLD for the other chassis HLDs - pulls/622 and pulls/639.

[ghost]

All CLA requirements met.

[minionatwork]

As we discussed on the review meeting, Is the direction to go Fabric ports as yet another ports and PortSyncD will manage it for fetching counters, and some show command to work?.

[eswaranb]

As we discussed on the review meeting, Is the direction to go Fabric ports as yet another ports and PortSyncD will manage it for fetching counters, and some show command to work?.

We looked into this some more and concluded that we cannot use portsyncd for fabric ports. This is because

1. Fabric ports don't have any hostifs. This means portsyncd cannot learn the port state via NETLINK messages like it does for front panel ports.
2. All fabric ports are enabled at startup by SAI. There is no configuration necessary for them at this point.

With that said, we can use FlexCounters to collect the counters for the queues that are mapped to the fabric ports. We are in the process of verifying this. Arista team will update this doc as soon as we can.

[rlhui]

As there can be many fabric links in the chassis, may be even more useful if there is a CLI to only display the links/fabric ports that have errors. Eg.. "show fabric counters errors"

[anshuv-mfst]

Please add @minionatwork and Srikanth to the reviewers.

[minionatwork]

Can port specified with bandwidth information and current utilization ?.

[ngoc-do]

I guess you are talking about fabric port data rates?

We can create another cli command showing current data rates of fabric ports on an asic.

show -n asicX fabric counters rates --period Y

Output could look like:

AsicX
Fabric port | InMbps | MaxInMbps | %InMbps | OutMbps | MaxOutMbps | %OutMbps
0
1
2
...

MaxInMbps typically gets from serdes link speed (e.g., 50 Khz).