Campus Fabric QoS Considerations
QoS Classifications for Campus Fabric PE Ports
In a Campus Fabric system, forwarding decisions for PE ports are made in the CB. When a PE unit receives frames through its extended ports, the PE appends an E-tag to each of the frames and transmits them to an upstream port toward the CB. After the CB makes forwarding decisions, the frames are forwarded to destination modules in the Campus Fabric domain. If the destination module is a PE unit, the frames are sent with an E-tag. The E-tag carries an E-PCP field consisting of three bits, which allows the E-PCP to carry eight priority classifications.
QoS classifications are supported for PE ports and are encoded in the E-PCP. A priority classified at an edge device is honored in both upstream and downstream directions. When congestion occurs in SPX ports, frames are dropped based on their priorities.
QoS Features under Campus Fabric
The same QoS features available for devices in a normal configuration are supported on devices enabled as CB units in a Campus Fabric domain. Most QoS features available in a standard configuration are also available on devices enabled as PE units. Port-based features apply to CB local ports and PE ports.
QoS Feature Support on PE Units
| Feature | PE Units |
|---|---|
| 802 1p Quality of Service | Yes |
| DiffServ support | Yes |
| Port priority | Yes |
| Port trust DSCP | Yes |
| MAC address priority | No |
| QoS mappings | Yes |
| Global QoS egress queue scheduler | Yes |
| Port-based egress queue scheduler | Yes |
| Port-based egress buffer profile | Yes |
| Port-based ingress buffer profile | No |
| Priority-based flow control | No |
| Port-based symmetrical flow control | No |
| Egress queuing statistics | Yes |
| sFlow | Yes |
QoS for SPX Ports
To ensure system stability, most QoS user configurations are not allowed for SPX ports. The following limitations apply to QoS in a Campus Fabric domain:
- Port-based classification cannot be applied to SPX ports. SPX ports are set to honor the E-PCP.
- Port flow control is disabled to preserve cascade or uplink port bandwidth.
- Priority-based flow control is disabled.
- The egress queue scheduler runs in mixed SP WRR mode under
qos mechanism. A user-configured scheduler cannot be applied to SPX ports. - A user-defined ingress or egress buffer profile cannot be applied to SPX ports.
Performance Impact
SPX ports on PE and CB units carry an E-tag as part of the frame. The E-tag adds eight bytes of overhead, which should be taken into account when calculating port or system throughput.
Control Packets under SPX
Upper layer protocols, such as routing and multicast protocols, run in the CB. When a PE port receives control packets for these protocols, the PE unit assigns the appropriate priority to each packet before forwarding it to the CB. The same priority applies for each type of control packet when it is sent to PE ports downstream.