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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