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QoS Cheatsheet by


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QoS Cheatsheet by

  1. 1. QUALITY OF SERVICE PART 1 Quality of Service Models IP Type of Service (TOS)Best Effort No QoS policies are implemented PrecedenceIntegrated Services (IntServ)Resource Reservation Protocol (RSVP) is used to reserve bandwidth per-flow across all nodes in a path Ver HL TOS LenDifferentiated Services (DiffServ)Packets are individually classified and marked; policy decisions are made DSCPindependently by each node in a path Layer 2 QoS Markings Precedence/DSCP Binary DSCP Prec. Medium Name Type Ethernet Class of Service (CoS) 3-bit 802.1p field in 802.1Q header 56 111000 Reserved 7Frame Relay Discard Eligibility (DE) 1-bit drop eligibility flag 48 110000 Reserved 6 ATM Cell Loss Priority (CLP) 1-bit drop eligibility flag 46 101110 EF 5 MPLS Traffic Class (TC) 3-bit field compatible with 802.1p 32 100000 CS4 34 100010 AF41 IP QoS Markings 4 36 100100 AF42IP PrecedenceThe first three bits of the IP TOS field; limited to 8 traffic classes 38 100110 AF43Differentiated Services Code Point (DSCP) 24 011000 CS3The first six bits of the IP TOS are evaluated to provide more granular 26 011010 AF31classification; backward-compatible with IP Precedence 3 28 011100 AF32 QoS Flowchart 30 011110 AF33 No 16 010000 CS2 Software Queue 18 010010 AF21 Scheduler HW Yes 2 Queuing Hardware Queue Software Queue 20 010100 AF22 Decision Queue Full? Software Queue 22 010110 AF23 8 001000 CS1 Terminology 10 001010 AF11 1Per-Hop Behavior (PHB) 12 001100 AF12The individual QoS action performed at each independent DiffServ node 14 001110 AF13Trust Boundary Beyond this, inbound QoS markings are not trusted 0 000000 BE 0Tail Drop Occurs when a packet is dropped because a queue is full Congestion AvoidancePolicingImposes an artificial ceiling on the amount of bandwidth that may be Random Early Detection (RED)consumed; traffic exceeding the policer rate is reclassified or dropped Packets are randomly dropped before a queue is full to prevent tailShaping drop; mitigates TCPSimilar to policing but buffers excess traffic for delayed transmission; synchronizationmakes more efficient use of bandwidth but introduces a delayTCP Synchronization Weighted RED (WRED)Flows adjust TCP window sizes in synch, making inefficient use of a link RED with the added capability of recognizing prioritized traffic based DSCP Per-Hop Behaviors on its markingClass Selector (CS) Backward-compatible with IP Precedence values Class-Based WRED (CBWRED)Assured Forwarding (AF) Four classes with variable drop preferences WRED employed inside a class- based WFQ (CBWFQ) queueExpedited Forwarding (EF) Priority queuing for delay-sensitive trafficby Jeremy Stretch v2.0
  2. 2. QUALITY OF SERVICE PART 2 Queuing Comparison FIFO PQ CQ WFQ CBWFQ LLQ Default on Interfaces >2 Mbps No No <=2 Mbps No No Number of Queues 1 4 Configured Dynamic Configured Configured Configurable Classes No Yes Yes No Yes Yes Bandwidth Allocation Automatic Automatic Configured Automatic Configured Configured Provides for Minimal Delay No Yes No No No Yes Modern Implementation Yes No No No Yes Yes First In First Out (FIFO) Priority Queuing (PQ) LLQ Config Example High Class Definitions Tx ! Match packets by DSCP value Ring Medium class-map match-all Voice match dscp ef Normal Hardware ! Hardware Queue Queue class-map match-all Call-Signaling Low match dscp cs3 Packets are transmitted in the !order they are processed Provides four static queues which class-map match-any Critical-Apps cannot be reconfigured match dscp af21 af22 No prioritization is provided ! Default queuing method on high- Higher-priority queues are ! Match packets by access listspeed (>2 Mbps) interfaces always emptied before lower- class-map match-all Scavenger priority queues match access-group name Other Configurable with the tx-ring-limit interface config command Lower-priority queues are at risk policy-map Foo Policy Creation of bandwidth starvation Custom Queuing (CQ) class Voice Weighted Fair Queuing (WFQ) ! Priority queue policed to 33% Queue A 500 B/cycle priority percent 33 Flow 1 class Call-Signaling Queue B 4500 B/cycle ! Allocate 5% of bandwidth Flow 2 bandwidth percent 5 Hardware Queue C 1500 B/cycle Queue ... class Critical-Apps Hardware bandwidth percent 20 Rotates through queues using Flow n Queue ! Extend queue size to 96 packetsWeighted Round Robin (WRR) queue-limit 96 Queues are dynamically created class Scavenger Processes a configurable number ! Police to 64 kbps per flow to ensure fair processingof bytes from each queue per turn police cir 64000 Statistically drops packets from Prevents queue starvation but conform-action transmit aggressive flows more often exceed-action dropdoes not provide for delay-sensitive traffic No support for delay-sensitive class class-default traffic ! Enable WFQ Class-Based WFQ (CBWFQ) fair-queue Low Latency Queuing (LLQ) ! Enable WRED random-detect Queue A 512 Kbps Min Priority 512 Kbps Max interface Serial0 Policy Application Queue B 1024 Kbps Min Queue A 512 Kbps Min ! Apply the policy in or out Hardware service-policy output Foo Default Remainder Queue Queue B 1024 Kbps Min WFQ with administratively Hardware LLQ Config Example Default Remainder Queueconfigured queues show policy-map [interface] Each queue is allocated an CBWFQ with the addition of a Show interfaceamount/percentage of bandwidth policed strict-priority queue No support for delay-sensitive Highly configurable while still show queue <interface>traffic supporting delay-sensitive traffic Show mls qosby Jeremy Stretch v2.0