DCBX

Traffic-class-to-priority mapping

In SR Linux, the traffic class (TC) index is defined by the forwarding class index, and each forwarding class is associated with an output queue. The traffic-class-to-priority mapping for the scheduler is defined per queue in the QoS scheduler policy.

The max TCs field in the configuration TLV is always set to 0 to advertise eight TCs (forwarding classes) whether or not all eight queues are explicitly configured in the scheduler policy.

Transmission Selection Algorithm (TSA) assignment

ETS TLVs specify bandwidth allocations for WRR traffic classes. Within the TLV, the TSA assignment entry allows a remote system to identify strict-priority queues and non-strict-priority queues. Each TC is assigned a value as follows:

• queues attached to the strict-priority scheduler are set to 0 to indicate that they do not participate in the allocation of bandwith percentages
• queues attached to the non-strict-priority WRR scheduler are set to 2 to indicate that they do participate in the allocation of bandwidth as a percentage of total interface bandwidth

Traffic class bandwidth assignment (as percentages) with PIR

The ETS TLV advertises the bandwidth allocated to each traffic class as a percentage of the total interface bandwidth. The sum of all percentages must add up to 100.

However, the distribution of bandwidth can be influenced by the PIR when it is also assigned to individual queues. If the PIR is set to a value below the bandwidth that the traffic class would otherwise receive based solely on weight allocation, the queue is allocated the lower PIR value, and the remaining bandwidth becomes available for other traffic classes.

The following table shows an example of how the PIR can affect the calculation of allocated bandwidth percentages:

Under-utilized bandwidth

The PIR configuration can sometimes lead to under-utilized interface throughput, specifically, if the sum of all PIRs is less than the interface speed. In this case, the sum of all ETS traffic classes does not add up to 100%. To comply with the specification in this case, the system redistributes the available bandwidth among all configured WRR queues.

Interoperability with LAG

Although the scheduling-related states are published at the LAG level, for the purpose of DCBX ETS, the TLVs must be sent at the individual LAG-member level. In this case, the same value of ETS TLV is assigned to each LAG member.

ETS TLV configuration example

To illustrate the ETS TLV behavior, consider the following QoS scheduler policy configuration example.

--{ candidate shared default }--[  ]--
# info with-context qos queues
    qos {
        queues {
            queue af2 {
                queue-index 2
            }
            queue best-effort {
                queue-index 0
            }
            queue expedited {
                queue-index 6
            }
            queue nc {
                queue-index 7
            }
        }
    }

--{ candidate shared default }--[  ]--
# info with-context qos forwarding-classes
    qos {
        forwarding-classes {
            forwarding-class af2 {
                output {
                    unicast-queue af2
                }
            }
            forwarding-class be {
                output {
                    unicast-queue best-effort
                }
            }
            forwarding-class ef {
                output {
                    unicast-queue expedited
                }
            }
            forwarding-class nc {
                output {
                    unicast-queue nc
                }
            }
        }
    }

--{ candidate shared default }--[  ]--
# info with-context qos scheduler-policies scheduler-policy policy-name
    qos {
        scheduler-policies {
            scheduler-policy policy-name {
                scheduler 0 {
                    priority strict
                    input 1 {
                        input-type queue
                        queue-name nc
                        peak-rate-percent 4
                    }
                    input 2 {
                        input-type queue
                        queue-name expedited
                        peak-rate-percent 8
                    }
                }
                scheduler 1 {
                    input 1 {
                        input-type queue
                        queue-name af2
                        peak-rate-percent 50
                    }
                    input 2 {
                        input-type queue
                        queue-name best-effort
                        peak-rate-percent 50
                    }
                }
            }
        }
    }

ETS configuration TLV example output

The following output show the ETS TLV state that is shown based on the preceding configuration applied to an interface.

--{ state }--[  ]--
# info from state qos interfaces interface eth-2/1 dcbx ets-tlv
    remote-credit-based-shaper 0
    local-credit-based-shaper 0
    remote-maximum-traffic-classes 8
    local-maximum-traffic-classes 8
    priority 0 {
        remote-priority-assignment-configuration 0
        remote-priority-assignement-recommendation 0
        local-priority-assignment 0
    }
    priority 1 {
        remote-priority-assignment-configuration 1
        remote-priority-assignement-recommendation 1
        local-priority-assignment 1
    }
    priority 2 {
        remote-priority-assignment-configuration 2
        remote-priority-assignement-recommendation 2
        local-priority-assignment 2
    }
    priority 3 {
        remote-priority-assignment-configuration 3
        remote-priority-assignement-recommendation 3
        local-priority-assignment 3
    }
    priority 4 {
        remote-priority-assignment-configuration 4
        remote-priority-assignement-recommendation 4
        local-priority-assignment 4
    }
    priority 5 {
        remote-priority-assignment-configuration 5
        remote-priority-assignement-recommendation 5
        local-priority-assignment 5
    }
    priority 6 {
        remote-priority-assignment-configuration 6
        remote-priority-assignement-recommendation 6
        local-priority-assignment 6
    }
    priority 7 {
        remote-priority-assignment-configuration 7
        remote-priority-assignement-recommendation 7
        local-priority-assignment 7
    }
    traffic-class 0 {
        remote-traffic-class-bandwidth-configuration 50
        remote-traffic-class-bandwidth-recommendation 50
        local-traffic-class-bandwidth-assignment 50
        remote-tsa-value-configuration 2
        remote-tsa-value-recommendation 2
        local-tsa-value 2
    }
    traffic-class 1 {
        remote-traffic-class-bandwidth-configuration 0
        remote-traffic-class-bandwidth-recommendation 0
        local-traffic-class-bandwidth-assignment 0
        remote-tsa-value-configuration 0
        remote-tsa-value-recommendation 0
        local-tsa-value 0
    }
    traffic-class 2 {
        remote-traffic-class-bandwidth-configuration 50
        remote-traffic-class-bandwidth-recommendation 50
        local-traffic-class-bandwidth-assignment 50
        remote-tsa-value-configuration 2
        remote-tsa-value-recommendation 2
        local-tsa-value 2
    }
    traffic-class 3 {
        remote-traffic-class-bandwidth-configuration 0
        remote-traffic-class-bandwidth-recommendation 0
        local-traffic-class-bandwidth-assignment 0
        remote-tsa-value-configuration 0
        remote-tsa-value-recommendation 0
        local-tsa-value 0
    }
    traffic-class 4 {
        remote-traffic-class-bandwidth-configuration 0
        remote-traffic-class-bandwidth-recommendation 0
        local-traffic-class-bandwidth-assignment 0
        remote-tsa-value-configuration 0
        remote-tsa-value-recommendation 0
        local-tsa-value 0
    }
    traffic-class 5 {
        remote-traffic-class-bandwidth-configuration 0
        remote-traffic-class-bandwidth-recommendation 0
        local-traffic-class-bandwidth-assignment 0
        remote-tsa-value-configuration 0
        remote-tsa-value-recommendation 0
        local-tsa-value 0
    }
    traffic-class 6 {
        remote-traffic-class-bandwidth-configuration 0
        remote-traffic-class-bandwidth-recommendation 0
        local-traffic-class-bandwidth-assignment 0
        remote-tsa-value-configuration 0
        remote-tsa-value-recommendation 0
        local-tsa-value 0
    }
    traffic-class 7 {
        remote-traffic-class-bandwidth-configuration 0
        remote-traffic-class-bandwidth-recommendation 0
        local-traffic-class-bandwidth-assignment 0
        remote-tsa-value-configuration 0
        remote-tsa-value-recommendation 0
        local-tsa-value 0
    }