This blog will explore the shaped round robin (SRR) technology implemented on the Cisco 2960/2970/3560/3750 switches. The 2960/2970/3560/3750 switches will be referred to as the 3750 switch family throughout this blog.
The 3750 switch family supports a four queue output architecture similar to the 2950, 3550, and 4500 series switches. The 2950 and 3550 use queue 4 as a priority queue, while the 4500 switch uses queue 3 as the priority queue. The scheduling of queues on the 3750 switch family is unique and the exact opposite of the 2950 and 3550 switches. Queue 1 on the 3750 family of switches is the priority queue, while queue 4 is the lowest priority queue. The CoS to output queue mapping default is as follows:
Queue # CoS
The 3750 switch family also support two input queues. CoS 5 is mapped to input queue 3, while all other CoS input traffic is mapped to queue 1 by default. Each of the two queues is allocated 50% of the queue during periods of congestion.
Shaped round robin (SRR) is a scheduling service that is similar to the weighted round robin algorithm used on some of the other switch platforms discussed so far. Shaped round robin (SRR) supports shaped mode and shared mode with shared mode being the default. Shaped mode allows individual queues to be limited (shaped), but is only available on the egress queues. Shared SRR allows non-idle lower priority traffic queues to use the idle bandwidth of higher priority queues. SRR does a better job of protecting switches from very aggressive flows with its internal scheduling algorithm. WRR can experience queue starvation issues related to large, bursty traffic streams due to the bandwidth allocation mechanism of the weighted round robin (WRR) algorithm. The default shaped mode of SRR will rate limit queue 1 to 25% by default even when the link is idle.
WRR and SRR both support the configuration of a priority queue. When the priority queue is configured, the switch uses an expedite scheduler that will not process any other queue when there is traffic in the priority queue. The 25% shape default no longer applies to queue 1 when the priority queue is configured. The expedite scheduling could lead to a queue starvation condition if there is so much traffic in the priority queue that other queues do not get scheduled. The queue starvation issue is limited in most deployments because only class of service (CoS - IEEE 802.1p) value 5 is normally mapped to the priority queue (PQ). Cisco phones mark voice media (RTP) traffic as CoS 5 by default. A Gigabit Ethernet uplink would have to have a lot of 93kbps G.711 voice calls for this issue to arise. It is possible for a denial of service (DoS) or distributed denial of service (DDoS) attack to be launched from a PC or Cisco IP phone using CoS 5, but this issue could also be limited if traffic is policed on both the voice and access VLANs at the access layer switch.
In the next blog, we will discuss the weighted tail drop (WTD) congestion avoidance mechanism used on the 3750 family of switches.
3750 QoS Configuration Guide
3750 Product Literature
3560 Product Literature
Dennis Hartmann, CCIE No. 15651, is a consultant with www.highpoint.com and author of Implementing Cisco Unified Communications Manager, Part 1. Dennis is also a lead instructor at Global Knowledge. Dennis has various certifications, including the Cisco CCVP, CCSI, CCNP, CCIP, and the Microsoft MCSE. Dennis has various specializations including unified communications, data center, routing & switching, service provider (MPLS and optical). Dennis has worked for various Fortune 500 companies, including AT&T, Sprint, Merrill Lynch, KPMG, and Cabletron Systems. He lives with his wife and children in Hopewell Junction, New York.