Queuing

Queuing is a congestion-avoidance function that prioritizes packet delivery. Queuing ensures discriminate packet discard during network congestion, and can delay a packet in memory until the scheduled transmission.

You can use queuing to manage congestion. Congestion management involves the creation of queues, assignment of packets to the queues based on the classification of the packet, and scheduling of the packets in a queue for transmission.

The system schedules packets for transmission according to their assigned priority and the queuing mechanism configured for the interface. The scheduler determines the order of packet transmission by controlling how the system services queues with respect to each other. The switch uses 16 CPU queues (used by traffic destined to the CPU), and eight unicast and eight multicast queues for each port. The deepest queue does not go beyond 60,000 packets.

A scheduler services the eight queues for each port, using a combination of strict priority and round-robin. Queue zero through five use round robin, and queues six and seven drain completely, or up to certain rate limits.

There are eight priorities on each egress port. Each Weighted Round Robin (WRR) queue has a minimum guaranteed weight to ensure fair bandwidth allocation for each queue in case all QoS queues are utilized and there is congestion. With the default profile, Class of Service (CoS) 0 to CoS 5 are WRR, and the default weights are 10, 20, 30, 40, 50, and 50 respectively. CoS 6 and CoS 7 are strict priority queues. The switch subjects CoS 6 and CoS 7 to traffic shaping at 50 per cent and five per cent of line rate respectively.

All packets destined for the CPU are sent as unicast packets.

Front panel ports also have 8 queues. When a front panel port is oversubscribed with both unicast and multicast packets, the bandwidth is divided so that the unicast packets receive 80% of the bandwidth and multicast packets 20% of the bandwidth.

If there are no multicast packets using the reserved 20%, the bandwidth is evenly distributed between COS levels 0 to 6. This results in approximately 3.3% more bandwidth for each level. If a specific COS level is not oversubscribed, the unused bandwidth is evenly distributed between the other COS levels. The CLI provides commands to map ingressing packets to specific COS levels.

Note

Unicast packets will be tail dropped on the ingress card. Multicast packets will be dropped at egress card.

Among the WRR queues, the approximate bandwidth percentage is relative to the remaining bandwidth after subtracting the Strict Priority queues. The following tables provide the approximate bandwidth percentage based on queue congestion.

Table 1. Bandwidth for Queue 0 to 5 congestion
Queue	Approximate bandwidth percentage*
0	5%
1	10%
2	15%
3	20%
4	25%
5	25%
* The approximate value assumes that all queues 0 to 5 are congested and similar packet sizes. Any queue that does not require bandwidth results in adding more bandwidth to the WRR queues. Percentage is relative to the total bandwidth minus the bandwidth consumed by Cos 6 and 7 (strict queues).

Table 2. Bandwidth for Queue 0 to 7 congestion
Queue	Approximate bandwidth percentage*
0	2.25%
1	4.5%
2	6.75%
3	9%
4	11.25%
5	11.25%
6	50%
7	5%
* The approximate value assumes that all queues 0 to 7 are congested and similar packet sizes.

CPU Queues

The following table outlines the CPU traffic and which queue it uses.

Table 3. CPU traffic queues
Queue description	Queue ID	Queue pps, burst size	Traffic type
Other	0	1400 pps, 100 kbps	Others
BC_Other	1	1400 pps, 100 kbps	Broadcast
HI_CPU looper	2	4000 pps, 100 kbps	IP_COS01
HI_CPU	3	4000 pps, 100 kbps	IP_COS23, Multicast Data
HI_CPU	4	4000 pps, 100 kbps	IP_COS4
HI_CPU	5	4000 pps, 100 kbps	IP_COS5, IGMP, PIM Unicast
HI_CPU	6	4000 pps, 100 kbps	ARP, RARP, ND Multicast and Unicast, MLD
HI_CPU	7	4000 pps, 100 kbps	IP_COS6, Telnet, SSH
mgmt	8	4000 pps, 200 kbps	IP_COS7, OSPF
Low rate looper	9	4000 pps, 6000 kbps	OSPF Multicast, PIM Multicast, RIP_v1, RIP_v2
Low rate 1	10	4000 pps,12000 kbps	ISIS
Low latency	11	4000 pps, 1000 kbps	ISIS Hello, LLDP, EAP, IST Control
Low latency	12	500 pps, 32 kbps	VRRP, CFM
Low latency	13	256 pps, 32 kbps	SLPP
Low latency	14	100 pps, 32 kbps	BPDU
Low latency	15	250 pps, 32 kbps	LACP, VLACP, TUNI-extract

Queue profiles

Table 4. Queue Profiles product support
Feature	Product	Release introduced
QoS per queue rate limiting	5320 Series	Fabric Engine 8.6
	5420 Series	VOSS 8.4
	5520 Series	VOSS 8.2.5
	5720 Series	Fabric Engine 8.7
	7520 Series	Fabric Engine 8.10
	7720 Series	Fabric Engine 8.10
	VSP 4900 Series	VOSS 8.1
	VSP 7400 Series	VOSS 8.0

This section identifies optional ways to customize the egress queues and scheduling depending on your need to override the default configuration. You can also enable egress queue rate limiting, if desired.

Use a queue profile to apply configured egress queue parameters and modify each queue individually. You can use the queue profile to configure a minimum weight for the queue and to enable rate limiting for the queue. The queue profile applies to all ports in the switch.

Note

If you enable rate limiting for all queues, the scheduler treats all queues as strict priority queues. If all queues are strict priority, the scheduler services the highest priority queue first until the maximum bandwidth is met, and then it services the next highest priority queue. Queue 0 is the lowest priority queue, which means that when over-subscribed, the lower priority queues are serviced last, or not at all.

The switch supports six queue profiles. The default queue profile, with the name default and ID 1, is automatically created during system startup and cannot be deleted.

Note

The egress queues with rate limiting enabled must be contiguous. For example, you can configure queues 3–6, but you cannot configure 3 and 6.

After you make a configuration change to a queue profile, you must apply the profile before the changes take effect.