Understanding CoS Queues on the 40-port SFP+ Line Card on EX8200 Switches
The 40-port SFP+ line card is an oversubscribed 10-Gigabit Ethernet line card for the Juniper Networks EX8200 Ethernet Switches. The 40 ports on the line card are divided into 8 port groups, with each port group containing 5 ports. The ports in a port group share 10 gigabits of bandwidth. Because the port groups share bandwidth, class-of-service (CoS) ingress and egress queues are handled differently on the 40-port SFP+ line card than on other line cards for EX8200 switches.
This topic describes:
Ingress Queues on the 40-port SFP+ Line Card
Ingress packet classification and queuing occurs in two steps:
- Preclassification of Packets and Port Ingress Queuing
- Full Classification of Packets and Fabric Ingress Queueing
Preclassification of Packets and Port Ingress Queuing
Packets entering the ports on a port group are sent to one of two ingress queues. These ingress queues are used to schedule the traffic from the port group into the Packet Forwarding Engine.
- Low priority queue—Each interface in a port group has one low priority queue. Traffic on these queues is scheduled using the shaped deficit weighted round-robin (SDWRR) algorithm, with each interface’s queue in the port group having an equal weight.
- High priority queue—The interfaces in a port group share a single high priority queue. Traffic on this queue is scheduled by strict-high priority.
For the purpose of port ingress queuing, packets are classified only by behavior aggregate (BA) classification. To control which ingress queue the packets get sent to, you configure a BA classifier on the physical port and specify switch fabric priorities for the forwarding classes. On EX8200 switches, fabric priority determines the priority of packets ingressing the switch fabric. For the 40-port SFP+ line card, fabric priority also determines the priority of packets ingressing the port group.
By default, the fabric priority for all forwarding classes is low. To direct packets belonging to a forwarding class to the high priority ingress queue, set the fabric priority to high for that class.
Critical network-control packets are handled differently from other packets. Instead of using the BA classifier to classify them, the switch always sends critical network-control packets to the high priority queue. This handling ensures that these packets are not dropped because of congestion on the oversubscribed ports.
Full Classification of Packets and Fabric Ingress Queueing
When the packets from a port group reach the Packet Forwarding Engine, it performs full packet classification, along with other actions such as multifield (MF) classification, traffic policing, and storm control. It then schedules and queues the packets for ingressing the fabric. The fabric priority associated with the forwarding class determines whether packets are sent to the low priority or high priority fabric ingress queues.
Egress Queues on the 40-port SFP+ Line Card
As with all EX Series switch interfaces, each interface on the 40-port SFP+ line card supports eight egress CoS queues. You can map up to 16 forwarding classes to these queues.
All interfaces in a port group also share a single set of eight egress chassis queues at the Packet Forwarding Engine. Egress traffic is fanned out from the Packet Forwarding Engine chassis queues to the corresponding queues for the individual ports. For this reason, the interfaces in a port group must share the same scheduler map configuration. If you configure different scheduler map configurations for the different interfaces in a port group, an error is logged to the system log and the default scheduler map is used for the ports in the port group.
