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Understanding CoS Queues on EX8200 Line Cards That Include Oversubscribed Ports

 

Some line cards available for Juniper Networks EX8200 Ethernet Switches include oversubscribed ports that are combined in logical port groups that share bandwidth. These oversubscribed ports handle traffic differently than ports that provide continuous line-rate bandwidth. You might need to configure CoS queues differently for oversubscribed ports than for line-rate ports.

This topic describes:

Oversubscribed Ports on Line Cards

Oversubscribed ports on a line card are grouped into logical port groups. A port group collectively supports a certain bandwidth.

An EX8200 switch supports different line cards that provide line-rate and oversubscribed ports. Based on your requirement, you can choose the appropriate line card for an EX8200 switch. Line cards are field-replaceable units (FRUs) that can be installed in the line card slots in an EX8200 switch. In a line-rate EX8200 line card, each port in the line card supports the same amount of bandwidth and a single port can utilize that complete bandwidth. In an oversubscribed line card, a group of ports collectively support a certain total bandwidth and each port in that group can use either a portion or all of the available bandwidth. However, the total utilization of bandwidth by the ports in the group cannot exceed the bandwidth available for that group.

Because the port groups share bandwidth, class-of-service (CoS) ingress and egress queues are handled differently for these shared-bandwidth ports in logical port groups than they are for ports that individually support line-rate bandwidth. Some EX8200 line cards combine both port types, those that share bandwidth across port groups and those that individually support line-rate bandwidth.

EX8200 Line Cards That Include Oversubscribed Ports

Table 1 lists EX8200 line cards that include oversubscribed ports in logical port groups.

Table 1: EX8200 Line Cards That Include Oversubscribed Ports

Line Card Model

Name

Number of Oversubscribed Ports/Port Connector

EX8200-40XS

40-port SFP+

40 oversubscribed 10-gigabit SFP+ ports

EX8200-2XS-40P

40-port PoE+ with 4-port SFP and 2-port SFP+

40 oversubscribed 10/100/1000 Gigabit Ethernet ports with RJ-45 connectors, four small form-factor pluggable (SFP) ports (in which you can install 1-gigabit SFP transceivers) and two SFP+ ports

EX8200-2XS-40T

40-port RJ-45 with 4-port SFP and 2-port SFP+

40 oversubscribed 10/100/1000 Gigabit Ethernet ports with RJ-45 connectors, four SFP ports (in which you can install 1-gigabit small form-factor pluggable (SFP) transceivers) and two SFP+ ports

EX8200-48PL

48-port PoE+ 20 Gbps

48 oversubscribed 10/100/1000 Gigabit Ethernet ports with RJ-45 connectors

EX8200-48TL

48-port RJ-45 20 Gbps

48 oversubscribed 10/100/1000 Gigabit Ethernet ports with RJ-45 connectors

Ingress Queueing

Classification of packets occurs in two phases for the oversubscribed ports in the port groups.

Preclassification of Packets and Port Ingress Queuing

Packets entering ports are forwarded to one of the ingress queues. The ingress queues schedule traffic from ports into the Packet Forwarding Engine.

The ingress queues are:

  • Low-priority queue—Each interface in the line card 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 having equal weight. On EX4300 switches, traffic is queued using the weighted deficit round-robin (WDRR) algorithm.

  • High-priority queue—A set of interfaces in the line card shares a single high-priority queue. Traffic on this queue is scheduled by strict-high priority. The switch always sends critical network control packets on the high-priority queue.

  • Line-rate priority queue—The packets entering line-rate ports are forwarded to this queue. Traffic on this queue is scheduled by strict priority and is always given higher priority than the traffic on the high-priority queue. This queue is used only in the following oversubscribed lines cards for an EX8200 switch:

    • EX8200-2XS-40P

    • EX8200-2XS-40T

For the purpose of port ingress queuing on oversubscribed ports, packets are classified only by behavior aggregate (BA) classification. To control the ingress queue (high priority or low priority) to which packets are sent, 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 EX8200-40XS 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 and line-rate packets are handled differently from other packets. Instead of using the BA classifier to classify critical network-control packets, the switch always sends critical network packets to the high-priority queue. The line-rate packets are always sent to the line-rate priority queue. This difference in handling of network-control packets and line-rate packets ensures that these packets are not dropped because of congestion on the shared-bandwidth ports.

Full Classification of Packets and Fabric Ingress Queuing

When packets (apart from line-rate and critical network-control packets) from an oversubscribed port 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 ingress queues.

Egress Queues

On EX Series switches except EX4300 switches, each interface supports eight egress CoS queues. You can map up to 16 forwarding classes to these queues. An EX4300 switch interface supports 12 egress CoS queues.

In the EX8200-40XS line card, all interfaces in a port group share a single set of eight egress queues at the Packet Forwarding Engine. Egress traffic is fanned out from the Packet Forwarding Engine 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 in the system log and the default scheduler map is used for all ports in the port group.