Switch QoS: Queuing

In this blog, we will take a look at Cisco switch queuing architectures and configuration. Ethernet based switches have a lot of bandwidth, but congestion is a possibility on any platform. Local area network designs typically involve some form of oversubscription on uplinks to other switches. Large Cisco switch deployments consist of the following three layers:

• Access
• Distribution
• Core

Access layer switches aggregate Fast Ethernet or Gigabit Ethernet access ports and uplink the switches to the distribution layer. The Cisco campus network design guide recommends an oversubscription ratio of 20:1. A 24 port Gigabit Ethernet switch uplinked over 1 Gigabit Ethernet port would result in a 24:1 oversubscription ratio. Many Cisco switches have 10 Gigabit Ethernet uplink ports to limit the oversubscription at the access layer, but there is a certain amount of oversubscription in most switch network. Oversubscription will most certainly result in congestion during periods of heavy network utilization. When there congestion, there is the possibility of dropping high priority traffic so queuing is highly encouraged at the access layer.

Distribution layer switches aggregate access layer switches and provide connectivity between the access layer switches and the core switches. Some switch based deployments are not large enough to warrant a core layer and utilize a collapsed core model. Distribution layer uplinks to core switches usually employ a 4:1 oversubscription factor which can result in packet loss.

Core switches are usually designed to be non-blocking architectures. Due to the complexity level of the internal switch architecture, it is recommended to utilize QoS queuing even when a switch is non-blocking in nature (no oversubscription).

The “Catalyst® QoS: Quality of Service in Campus Networks” Cisco Press book does a good job explaining the “VERY” limited QoS capabilities of the Cisco Catalyst 2900XL, 3500XL, 4000, 5000, 2948G, and 2980G platforms. Most of these switches are at or near the end of support date in Cisco’s “End-of-Sale and End-of-Life Products” (link below). We will not be covering these switches in this blog series.

The first queuing algorithm used on QoS capable switches was weighted round robin (WRR). Since weighted round robin is based on round robin scheduling, let’s take a look at round robin scheduling first. A T1 interface consists of 24 individual DS-0 channels in which each of the 24 channels are multiplexed together in a round robin manner. A sample is taken from each DS-0 channel (1-24) every time the T1 framing process occurs. WRR is very similar to round robin, but WRR will apply a weight to each queue, therefore allowing some queues to receive more bandwidth than other queues. The 2950 and 3550 switch platforms have a default WRR bandwidth allocation as follows:

Queue# Bandwidth CoS mapping
1 25 0,1
2 25 2,3
3 25 4,5
4 25 6,7

The quantity of 25 is a configurable parameter through the wrr-queue bandwidth command. Each queue can be configured with a value of 1 – 255, but it is best practice to have the sum of all queues equal to 100. Each queue will receive their bandwidth divided by the total configured bandwidth value during periods of congestion. As long as the total bandwidth is configured to equal 100, we can think of the configured bandwidth as a percentage of interface bandwidth.

In the next blog, we will begin discussing various switch configurations for queueing. We will also investigate the queuing operation performed in the Cisco switches when auto qos is used.

REFERENCES

Switches - End-of-Sale and End-of-Life Products
http://www.cisco.com/en/US/products/hw/switches/prod_category_end_of_lif...
Campus Network for High Availability Design Guide
http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/HA_campus_DG...

Implementing Cisco Quality of Service
http://www.globalknowledge.com/training/course.asp?pageid=9&courseid=757...

Advanced Cisco Quality of Service
http://www.globalknowledge.com/training/course.asp?pageid=9&courseid=936...

Enterprise QoS Solution Reference Network Design Guide
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRN...

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