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Quality of Service (QoS) - Key concepts

 Quality of Service Overview

QoS is crucial for managing bandwidth, latency, jitter, and packet loss, which impact application performance, especially for real-time services like voice and video. Key quality issues include lack of bandwidth, latency (packet delay), jitter (variability in delay), and packet loss (lost data during transmission). Technical Tip: Keep latency under 150 ms for real-time traffic and packet loss below 1%.

End-to-End Layer 3 QoS using MQC

DiffServ QoS Model

DiffServ is scalable and classifies traffic into classes, marking it for varying levels of priority. Technical Tip: Use DiffServ for efficient traffic management at the network edge for classification and marking, while relying on Per-Hop Behavior (PHB) in the core network.

CoS and DSCP Mapping

CoS marks traffic in Layer 2 headers, while DSCP marks it at Layer 3 for end-to-end traffic prioritization. Technical Tip: DSCP is more versatile for end-to-end QoS as CoS is limited to Layer 2 domains.

Classification and Marking

Classification identifies traffic based on multiple factors (Layer 2 to Layer 7), and marking assigns a QoS value for treatment. Technical Tip: Always classify and mark traffic close to the network ingress to optimize bandwidth.

Network-Based Application Recognition (NBAR)

NBAR classifies applications using Layer 7 inspection. Technical Tip: Use NBAR with Modular QoS CLI (MQC) for advanced traffic classification, and update NBAR with Packet Description Language Modules (PDLM) for new protocols.

Policing and Shaping

Policing drops or marks traffic that exceeds bandwidth limits, while shaping buffers excess traffic to smooth it out. Technical Tip: Use shaping for outbound traffic at network edges to comply with service provider limits, and policing to enforce strict traffic limits. Example:

policy-map POLICING_POLICY

 class CLASS1

  police 1000000 20000 exceed-action drop

Congestion Management and Avoidance

Congestion management uses queuing to prioritize traffic (e.g., CBWFQ), while avoidance tools like WRED preemptively drop packets to prevent congestion. Technical Tip: Use WRED to reduce tail drops and prevent global synchronization issues.

Queueing Algorithms

Different queuing methods include Priority Queueing (PQ), which gives absolute priority, and Class-Based Weighted Fair Queuing (CBWFQ), which allocates bandwidth to multiple queues. Technical Tip: Be cautious with PQ as it can starve lower-priority traffic.

Auto QoS

AutoQoS simplifies the setup of QoS for voice and video applications by automating configurations. Technical Tip: Enable AutoQoS before manual QoS setup to ensure proper interface settings. Use:

interface gigabitethernet0/1

 auto qos voip trust

Advanced QoS Concepts

RSVP

RSVP reserves bandwidth for real-time applications like voice and video. Technical Tip: Implement RSVP for applications that need guaranteed bandwidth, marking traffic with DSCP for Expedited Forwarding (EF).

Hierarchical QoS (HQoS)

HQoS manages traffic across different levels of the network, such as service provider and enterprise levels. Technical Tip: Use HQoS to manage traffic for different service levels efficiently, especially in environments with multiple service agreements.

QoS Configuration Example

class-map VOICE

 match ip rtp 16384 16383

policy-map VOICE_POLICY

 class VOICE

  set dscp ef

  priority 300

 class class-default

  bandwidth 1000

Key QoS Technical Tips

Classification and marking should be done at the network ingress. Use shaping to buffer excess traffic and avoid packet loss, and employ policing for strict rate enforcement. WRED prevents tail drops and mitigates global synchronization problems in congestion.

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