Quality of Service

Cisco IOS QoS features are a solution for delay, jitter, packet loss and bandwidth utilization in the IP network. Cisco IOS features including below toolsets;

Classification and Marking tools: allow traffic to be partitioned into multiple priority levels, or classes of service based on the input interface, ACLs, policy defined or NBAR for layer 7 recognition. This tool works by examining following parameters:



Layer 2 parameters—802.1Q Class of Service (CoS) bits, Multiprotocol Label Switching Experimental Values (MPLS EXP).

Layer 3 parameters—IP Precedence (IPP), Differentiated Services Code Points (DSCP), IP Explicit Congestion Notification (ECN), source/ destination IP address.

Layer 4 parameters— L4 protocol (TCP/UDP), source/destination ports

Layer 7 parameters— application signatures via NBAR

Policing and Markdown Tools: Policing tools (policers) determine whether packets are conforming to administratively-defined traffic rates and take action accordingly. Such action could include marking, remarking or dropping a packet.

For example, Traffic conforming to the defined rate of a given AF class is marked to the first Drop Preference level AF21. Traffic exceeding this rate is marked down to the second Drop Preference level AF22, then third Drop Preference level AF23 for violating traffic.

Scheduling and queuing Tools: Scheduling tools determine how a frame/packet exits a device. Devices have buffers that allow for scheduling higher-priority packets to exit sooner than lower priority
ones, which is commonly called queuing. Queuing algorithms are activated only when a device is experiencing congestion and are deactivated when the congestion clears. Main IOS software queuing tolls are Low Latency Queuing (LLQ) for real-time applications and Class-Based Weighted Fair Queuing (CBWFQ) to provides bandwidth guarantees for non delay-sensitive applications.


When queuing buffers begin overflowing from the top, packets may be dropped either as they arrive (tail drop) or selectively before all buffers are filled. Selective dropping of packets when the queues are filling is referred to as congestion avoidance. Congestion avoidance mechanisms work best with TCP-based applications because selective dropping of packets causes the TCP windowing mechanisms to “throttle-back” and adjust the rate of flows to manageable rates.

The principle IOS congestion avoidance mechanism is WRED, which drops packets randomly when queues becoming full, or drops packets based on traffic weight (IPP values by default or AF Drop Preference values for DSCP-based WRED).

Congestion avoidance mechanisms are complementary to queuing algorithms. Queuing algorithms manage the front of a queue while congestion avoidance mechanisms manage the tail of the queue.

Link-specific tools: Link-specific tools include the following:

Shaping tools—A shaper typically delays excess traffic above an administratively-defined rate using a buffer to hold packets and shape the flow when the data rate of the source is higher than expected. Technology of this scope includes GTS, FRTS, Committed Access Rate (CAR) and MQC-based TS.

Link Fragmentation and Interleaving tools—With slow-speed WAN circuits, large data packets take an excessively long time to be placed onto the wire. This delay, called serialization delay, can easily cause a VoIP packet to exceed its delay and/or jitter threshold. There are two main tools to mitigate serialization delay on slow (768 kbps) links: Multilink PPP Link Fragmentation and Interleaving (MLP LFI) and Frame Relay Fragmentation (FRF.12).

Compression tools—Compression techniques, such as compressed Real-Time Protocol (cRTP), minimize bandwidth requirements and are highly useful on slow links. To avoid the unnecessary consumption of available bandwidth, we can use cRTP on a link-by-link basis. cRTP compresses IP/UDP/RTP headers from 40 bytes to between two and five bytes (which results in a bandwidth savings of approximately 66%
for G.729 VoIP).

Transmit ring (Tx-Ring) tuning—The Tx-Ring is a final interface First-In-First-Out(FIFO) queue that holds frames to be immediately transmitted by the physical interface. The Tx-Ring ensures that a frame is always available when the interface is ready to transmit traffic, so that link utilization is driven to 100 % of capacity. The Tx-Ring may have to be tuned on certain platforms/interfaces to prevent unnecessary delay/jitter introduced by this final FIFO queue.

Call Admission Control tools: CAC tools fall into the following three main categories:

Local CAC—Local CAC mechanisms are a voice gateway router function, typically deployed on the outgoing gateway. The CAC decision is based on nodal information such as the state of the outgoing LAN/WAN link that the voice call traverses. Local mechanisms include configuration items to disallow more than a fixed number of calls.

Measurement-based—Measurement-based CAC techniques look ahead into the packet network to gauge the state of the network to determine whether or not to allow a new call. This usually implies sending probes to the destination IP address, which could be the terminating gateway or endpoint, or another device in between. The probes return loss and delay information experienced while traversing the network to the destination. The outgoing device then uses this information in combination with configured information to decide if the network conditions exceed a given or configured threshold.

Resource-based—There are two types of resource based mechanisms: those that calculate resources needed and/or available, and those that reserve resources for the call. Resources of interest include link bandwidth, DSPs and DS0 timeslots on the connecting TDM trunks to a voice gateway, CPU power and memory. Several of these resources could be constrained at one or more nodes that the call traverses to its destination.

Cisco CallManager Location-Based CAC is not mutually exclusive to the features listed above. While CallManager Location-Based CAC is deployed in the overall network to manage VoIP bandwidth availability for both Cisco IP Phones and voice gateways, local measurement-based or resource-based features may be deployed at the same time on the voice gateway to push back calls into the private Branch exchange (PBX) or publicly-switched telephone network (PSTN).

AutoQoS tools: AutoQoS tools are the result of Cisco QoS feature development coupled with Cisco QoS Design Guides based on large-scale lab-testing, it is an intelligent macro that allows an administrator to enter one or two simple AutoQoS commands to enable all the appropriate features for the recommended QoS settings for an application on a specific interface.

AutoQoS VoIP, the first release of AutoQoS, provides best-practice QoS designs for VoIP on Cisco Catalyst switches and Cisco IOS routers.

For Campus Catalyst switches, AutoQoS automatically performs the following tasks:

• Enforces a trust boundary at Cisco IP Phones.
• Enforces a trust boundary on Catalyst switch access ports and uplinks/downlinks.
• Enables Catalyst strict priority queuing for voice and weighted round robin queuing for data traffic.
• Modifies queue admission criteria (CoS-to-queue mappings).
• Modifies queue sizes as well as queue weights where required.
• Modifies CoS-to-DSCP and IP Precedence-to-DSCP mappings.

For Cisco IOS routers, AutoQoS is supported on Frame Relay (FR), Asynchronous Transfer Mode (ATM), High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), and FR-to-ATM links, AutoQoS automatically performs the following tasks:

• Classifies and marks VoIP bearer traffic (to DSCP EF) and Call-Signaling traffic (to DSCP CS3).
– Applies scheduling:
– Low Latency Queuing (LLQ) for voice
– Class-Based Weighted Fair Queuing (CBWFQ) for Call-Signaling
– Fair Queuing (FQ) for all other traffic
• Enables Frame Relay Traffic Shaping (FRTS) with optimal parameters, if required.
• Enables Link Fragmentation and Interleaving (LFI), either MLP LFI or FRF.12, on slow ( 768 kbps)
links, if required.
• Enables IP RTP header compression (cRTP), if required.
• Provides Remote Monitoring (RMON) alerts of dropped VoIP packets.

AutoQoS VoIP became available on Cisco IOS router platforms in 12.2(15)T.

In its second release, for Cisco IOS routers only, AutoQoS Enterprise detects and provisions for up to ten classes of traffic, including the following:

• Voice
• Interactive-Video
• Streaming-Video
• Call-Signaling
• Transactional Data
• Bulk Data
• Routing
• Network Management
• Best Effort
• Scavenger

AutoQoS Enterprise became available on Cisco routers in Cisco IOS 12.3(7)T, it consists of two configuration phases, completed in the following order:

• Auto Discovery (data collection)—Uses NBAR-based protocol discovery to detect the applications
on the network and performs statistical analysis on the network traffic.
• AutoQoS template generation and installation—Generates templates from the data collected during the Auto Discovery phase and installs the templates on the interface. These templates are then used as the basis for creating the class maps and policy maps for your network. After the class maps and policy maps are created, they are then installed on the interface.

Terms Definition:

NBAR: Network Based Application Recognition, NBAR is a Cisco proprietary technology that identifies application layer protocols by matching them against a Protocol Description Language Module (PDLM). The NBAR deep-packet classification engine examines the data payload of stateless protocols against PDLMs.

IP ECN: IP Explicit Congestion Notification, is used to indicate to TCP senders whether or not congestion was experienced during transit. In this way, TCP senders can adjust their TCP windows. IP ECN can be marked through a congestion avoidance mechanism such as weighted early random detection (WRED). First IP ECN bit (7th in the ToS byte) is used to indicate whether the device supports IP ECN and the second bit (last bit in the IP ToS byte) is used to indicate whether congestion was experienced.