Differences In Drop Cos And No-Drop Cos Values - Cisco Nexus 7000 Series Configuration Manual

Differences in Drop CoS and No-Drop CoS Values

same order in which it was sent. The LAN traffic allows dropping of packets and is delivered on a best-effort
basis. The LAN traffic can have a different level of priority and a chance of being delivered in a timely manner
during congestion. The IPC networks require very low latency. Therefore, a DCB network must support traffic
differentiation and provide quality of service (QoS).
In a DCB network, virtual links (VLs) are used to differentiate traffic classes. VLs, which are an extension
of CoS, enable traffic differentiation and are carried in the priority bits of the 802.1Q or S-Tag.CoS allows
forming of a physical link into multiple logical links so that the traffic in the CoS does not affect the traffic
on the other CoS.
A DCB network has eight CoS values. All traffic that enters the DCB cloud must be mapped into one of these
eight CoS values. Each frame in a DCB network belongs to a CoS. You can define the CoS by a set of
parameters that gives a specific behavior to the CoS.
The network qos policy defines the characteristics of each CoS value, which are applicable network wide
across virtual device contexts (VDCs) and switches. With a network qos policy, you can configure the
following:
• Pause behavior—You can decide whether a CoS requires the lossless behavior (the lossless behavior is
• Congestion Control Mechanisms—You can select either tail drop (TD, which drops frames without
• MTU—You can set the maximum transmission unit (MTU) or maximum payload length for CoS. The
• Protocols—You can select Fibre Channel over Ethernet (FCoE), iSCSI, or TCP as the protocol for a CoS
Differences in Drop CoS and No-Drop CoS Values
The table below shows the drop and no-drop CoS values for the different policy templates.
Table 37: Differences in Drop CoS and No-Drop CoS Values
Templates
default-4q-8e-policy
default-4q-7e-policy
default-4q-6e-policy
Cisco Nexus 7000 Series NX-OS Quality of Service Configuration Guide
136
provided by using a priority flow control (PFC) mechanism that prevents packet loss during congestion)
or not. You can configure drop (frames with this CoS value can be dropped) and no drop (frames with
this CoS value cannot be dropped). For the drop and no drop configuration, you also need to enable PFC
per port. For more information about PFC, see "Configuring Priority Flow Control."
differentiation based on the per-VL occupancy) or Weighted Random Early Detection (WRED, which
drops frames without differentiation based on the per-VL occupancy and the probability factor) only for
a drop CoS. By default, TD is enabled for drop CoS in the default policies. Any of the burst-optimized
or mesh-optimized thresholds for TD and WRED can be selected for the packet-drop algorithm based
on the traffic pattern expected for the CoS. If no congestion control algorithm is selected, and congestion
occurs, a hard tail-drop that is based on a single threshold occurs for the drop CoS.
MTU range is from 1500 to 9216. The MTU must be smaller than the system jumbo MTU in all VDCs.
The MTU must be the same for the CoS that is mapped to the same ingress queue. For more information
about the ingress queue, see "Configuring Queuing and Scheduling on F-Series I/O Modules."
value. The protocol value is used in the DCB Exchanges.
0–7
0–2, 4–7
0–2, 5–7
Configuring Network QoS
—
3
3, 4