Configuration Notes and Feature Limitations for Symmetrical Flow Control
Note the following configuration notes and feature limitations before enabling symmetrical flow control.
- Symmetrical flow control is supported on all 1G,10G, 40G, and 100G data ports on ICX devices.
- Symmetrical flow control is not supported on
stacked ports or across units in a stack. If you are using symmetrical flow control
on stacked ports or across units in a stack be aware that:
- It is unrealistic to infer that lossless service exists across stacked units.
- Symmetrical flow control is not priority aware; oversubscription of one priority may cause the dropping of higher priority controls in stacked links. The loss of these priority controls results in a broken stack.
- The system depends on buffer resources to ensure quality of service. Under symmetrical flow control, persistent congestions may leave a buffer resource vulnerable to exhaustion. An example is where bandwidth of ingress ports is greater than egress ports — a packet receives on a 10G port, but then forwards the packet to a 1G port. If the buffers are exhausted, there is no guarantee of quality of service. The end result is an unstable system with flapping protocols.
- In a stacked environment, pause frames are not propagated from one stack unit to another, as a result they may hold buffers up to a core limit due to multiple port congestions. Under this condition, the stack may break.
- Not propagating pause frames also prevents head-of-line (HOL) blocking conditions for stacked ports, which are normally used as aggregation links. Stacked ports or trunks are flow control disabled for both transmit and receive, HOL blocking may occur when symmetrical flow control is enabled. This means that a peer can stop transmitting traffic streams unrelated to the congestion stream.
- To use this feature, 802.3x flow control must be enabled globally and per interface
on ICX devices. By default, 802.3x flow control is enabled, but can be disabled with
the
no flow-controlcommand. - The following QoS features are not supported together with symmetrical flow control:
Note: Although the above QoS features are not
supported with symmetrical flow control, the CLI will still accept these commands.
The
last command issued will be the one placed into effect on the device. For example,
if
trust dscp is
enabled after symmetrical-flow-control is enabled, symmetrical flow control will be
disabled and trust dscp will be placed into effect. Make sure you do not enable
incompatible QoS features when symmetrical flow control is enabled on the device.