Switch QoS

Queuing on the 3550 platforms

In this blog, we will continue the switch based QoS coverage and discuss some of the QoS specifics of the 3550 switch platform. As this blog roll continues, we’ll discuss the queuing architectures and configurations of the the 4500, 6500, and 2960/2970/3560/3750 Catalyst family.

Before we proceed, let’s wrap up some items that were not mentioned in the last blog covering the 2950. The 2950 has the ability to police traffic on ingress (input) in increments of 1Mbps on Fast Ethernet interfaces (6 policers per port) and 8Mbps increments on the Gigabit Ethernet (60 policers per port) interfaces. Recall from the last blog that the 2950’s policers cannot mark in any way. The 2950 does not have any output policing support.

The 2950 has a DSCP understanding limitation. The 2950 only understands the following 14 DSCP values:

0
8 (cs1)
10 (AF11)
16 (cs2)
18 (AF21)
24 (cs3)
26 (af31)
32 (cs4)
34 (af41)
40 (cs5)
46 (ef)
48 (cs6)
56 (cs7)

Any DSCP values not understood by the 2950 switch will be processed as DSCP 0 and be mapped to queue 1 by default. The 2950 is the only Cisco switch I’m aware of with this limitation.

The 3550 series switch has a four queue architecture that is very similar to the 2950. The basic queuing configuration is near identical to the 2950 commands covered in the last blog with the some distinctions. The 3550’s wrr-queue bandwidth command always used a value of 1 to indicate queue 4 was the priority queue. The priority-queue out command is used on most switch platforms to turn on the priority queue, with exceptions on the 2950 and 4500. The following two commands are used to turn on the priority queue:

Switch(config-if)#wrr-queue bandwidth 10 20 70 1
Switch(config-if)#priority-queue out

Cisco’s other switches do not have the DSCP or ACL limitations discussed above on the 2950. All of Cisco’s switches support the range command in ACL configuration and all possible DSCP values. The 3550’s Gigabit Ethernet ports support the WRED congestion avoidance algorithm that was not supported on the 2950 switch. The 3550 supports 2 configurable thresholds in which the WRED mechanism can begin dropping traffic if the queue is experiencing congestion. The queuing nomenclature used to describe this product’s capabilities is 4Q2T or 1P3Q2T if the priority queue is turned on.

The 3550 supports up to 8 ingress policers on Fast Ethernet interfaces and 128 ingress policers per Gigabit Ethernet interface. Unfortuntely, the 3550 does not support egress (output) policing on either the Fast Ethernet or Gigabit Ethernet ports. The switch platforms do not support traffic shaping.

The 3550 also supports the use of a VLAN identifier in class maps to perform per VLAN policies. More information on the QoS architecture of the 3550 is available in the links below.

REFERENCES
2950 Data Sheet
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps628/product...

3550 Data Sheet
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps646/product...

Policing and Marking on the 3550 Switch
http://www.cisco.com/en/US/products/hw/switches/ps646/products_tech_note...
QoS SRND 3.3 – 3550 Section
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRN...

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