Lag On Access Qos Consideration; Adapt Qos Modes - Alcatel-Lucent 7750 Configuration Manual

LAG on Access QoS Consideration

LAG on Access QoS Consideration
The following section describes various QoS related features applicable to LAG on access.

Adapt QoS Modes

Link Aggregation is supported on access side with access/hybrid ports. Similarly to LAG on
network side, LAG on access is used to aggregate Ethernet ports into all active or active/standby
LAG. The difference with LAG on networks lies in how the QoS/H-QoS is handled. Based on
hashing configured, a given SAP's traffic can be sprayed on egress over multiple LAG ports or can
always use a single port of a LAG. There are three user-selectable modes that allow operator to
best adapt QoS configured to a LAG the SAPs are using:
1. adapt-qos distributed (default)
2. adapt-qos link
3. adapt-qos port-fair
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In a distributed mode the SLA is divided among all line cards proportionally to the
number of ports that exist on that line card for a given LAG. For example a 100Mbps PIR
with 2 LAG links on IOM A and 3 LAG links on IOM B would result in IOM A getting 40
Mbps PIR and IOM B getting 60Mbps PIR. Thanks to such distribution, SLA can be
enforced. The disadvantage is that a single flow is limited to IOM's share of the SLA. This
mode of operation may also result in underrun due to a "hash error" (traffic not sprayed
equally over each link). This mode is best suited for services that spray traffic over all
links of a LAG.
In a link mode the SLA is given to each and every port of a LAG. With the example above,
each port would get 100 Mbps PIR. The advantage of this method is that a single flow can
now achieve the full SLA. The disadvantage is that the overall SLA can be exceeded, if
the flows span multiple ports. This mode is best suited for services that are guaranteed to
hash to a single egress port.
Port-fair distributes the SLA across multiple line cards relative to the number of active
LAG ports per card (in a similar way to distribute mode) with all LAG QoS objects
parented to scheduler instances at the physical port level (in a similar way to link mode).
This provides a fair distribution of bandwidth between cards and ports whilst ensuring that
the port bandwidth is not exceeded. Optimal LAG utilization relies on an even hash
spraying of traffic to maximize the use of the schedulers' and ports' bandwidth. With the
example above, enabling port-fair would result in all five ports getting 20Mbps.
When port-fair mode is enabled, per-Vport hashing is automatically disabled for
subscriber traffic such that traffic sent to the Vport no longer uses the Vport as part of the
hashing algorithm. Any QoS object for subscribers, and any QoS object for SAPs with
explicitly configured hashing to a single egress LAG port, will be given the full bandwidth
7750 SR Interface Configuration Guide