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● By default, equal bandwidth is assigned to each port queue and each dot1p priority in a priority group.
● By default, equal bandwidth is assigned to each priority group in the dcb-map applied to an egress port. The sum of
auto-configured bandwidth allocation to dot1p priority traffic in all ETS priority groups is 100%.
● dot1p priority traffic on the switch is scheduled according to the default dot1p-queue mapping. dot1p priorities within the
same queue should have the same traffic properties and scheduling method.
● A priority group consists of 802.1p priority values that are grouped together for similar bandwidth allocation and scheduling,
and that share the same latency and loss requirements. All 802.1p priorities mapped to the same queue should be in the same
priority group.
○ By default:
￭ All 802.1p priorities are grouped in priority group 0.
￭ 100% of the port bandwidth is assigned to priority group 0. The complete bandwidth is equally assigned to each
priority class so that each class has 12 to 13%.
○ The maximum number of priority groups supported in ETS output policies on an interface is equal to the number of data
queues (4) on the port. The 802.1p priorities in a priority group can map to multiple queues.
● A dcb-map is created to associate a priority group with a dcb-map with scheduling and bandwidth configuration, and applied
on egress ports.
○ The ETS configuration associated with 802.1p priority traffic in a dcb-map is used in DCBx negotiation with ETS peers.
○ When a dcb-map is applied to an interface, ETS-configured scheduling and bandwidth allocation take precedence over
any auto-configured settings in the QoS output policies.
○ ETS is enabled by default with the default ETS configuration applied (all dot1p priorities in the same group with equal
bandwidth allocation).
ETS Operation with DCBx
In DCBx negotiation with peer ETS devices, ETS configuration is handled as follows:
● ETS TLVs are supported in DCBx versions CIN, CEE, and IEEE2.5.
● ETS operational parameters are determined by the DCBX port-role configurations.
● ETS configurations received from TLVs from a peer are validated.
● In case of a hardware limitation or TLV error, the DCBx operation on an ETS port goes down.
● ETS operates with legacy DCBx versions as follows:
○ In the CEE version, the priority group/traffic class group (TCG) ID 15 represents a non-ETS priority group. Any priority
group configured with a scheduler type is treated as a strict-priority group and is given the priority-group (TCG) ID 15.
○ The CIN version supports two types of strict-priority scheduling:
￭ Group strict priority: Allows a single priority flow in a priority group to increase its bandwidth usage to the bandwidth
total of the priority group. A single flow in a group can use all the bandwidth allocated to the group.
￭ Link strict priority: Allows a flow in any priority group to increase to the maximum link bandwidth.
CIN supports only the default dot1p priority-queue assignment in a priority group.
Hierarchical Scheduling in ETS Output Policies
ETS supports up to three levels of hierarchical scheduling.
For example, you can apply ETS output policies with the following configurations:
Priority group 1
Assigns traffic to one priority queue with 20% of the link bandwidth and strict-priority scheduling.
Priority group 2 Assigns traffic to one priority queue with 30% of the link bandwidth.
Priority group 3 Assigns traffic to two priority queues with 50% of the link bandwidth and strict-priority scheduling.
In this example, the configured ETS bandwidth allocation and scheduler behavior is as follows:
Unused
bandwidth usage:
Normally, if there is no traffic or unused bandwidth for a priority group, the bandwidth allocated to the
group is distributed to the other priority groups according to the bandwidth percentage allocated to each
group. However, when three priority groups with different bandwidth allocations are used on an interface:
● If priority group 3 has free bandwidth, it is distributed as follows: 20% of the free bandwidth to
priority group 1 and 30% of the free bandwidth to priority group 2.
● If priority group 1 or 2 has free bandwidth, (20 + 30)% of the free bandwidth is distributed to priority
group 3. Priority groups 1 and 2 retain whatever free bandwidth remains up to the (20+ 30)%.
236 Data Center Bridging (DCB)