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1、Cisco QoS Notes,By: Kevin. P. WatersCreated on: March 31st, 2010,Methods of Implementing QoS,Legacy CLIModular QoS (MQC)AutoQoS VoIPAutoQoS EnterpriseQoS Policy Manager (QPM)Used for Administration & Monitoring multiple interfaces provisioned for QoS across the Network (not just on individual device
2、s like AutoQoS does).,AutoQoS,AutoQoS VOIPCreates a QoS policy to prioritize Voice Over IP traffic ONLY, cannot be used to discover and implement QoS for other traffic types.AutoQoS EnterpriseUses Network-Based Application Recognition (NBAR) to discover traffic types on the Network and create a QoS
3、Policy based on best-practices for each flow.,Steps for implementing QoS,1. Identify Traffic on the NetworkUse a Network Analyzer to identify the different protocols and applications used on the network, and their requirements.,Steps for implementing QoS,2. Divide Traffic into Classes Voice: Highest
4、 Priority Mission-Critical: Transactional (Database) Best-Effort: Email, Web Browsing, FTP, etc Scavenger: P2P Apps, less than Best-Effort3. Define QoS PolicyHow much bandwidth to reserve for a particular classWhich traffic to prioritize and give preferential treatment toDefine a policy for how to m
5、anage congestion,Classification & Marking,IP PrecedenceDeprecated standard for marking packets at Layer 3 for QoS, superseded by DSCP; uses the ToS byte in the IP header.IP ToS Byte8-Bit Field within the IP Header of a packet, mainly used for marking packets with IP Precedence values.,Classification
6、 & Marking,What is Classification?The ability for a network device to identify different traffic types and divide them into different classes based on Business Requirements. Classification occurs on a devices inbound (Ingress) interface.Classification ToolsNetwork-based Application Recognition (NBAR
7、)Policy-Based Routing (PBR)Access Control Lists (ACLs),Marking,Methods of Marking:Class of Service (COS)Frame Relay DE BitMPLS Experimental (EXP) bitsIP PrecedenceDifferentiated Services Code Point (DSCP)In best practices you should limit the number of traffic classes for provisioning QoS to about 4
8、 or 5 classes. If more is needed, usually no more than 11 different classes are necessary.An 11-Class QoS model might be benefit a large enterprise that requires more granularity for classes.,Class of Service (COS),What is CoS?Turning on bits in the 802.1P (user priority) field within the 802.1Q Hea
9、der (or Cisco ISL Header) of an Ethernet Frame. Supported values are 0-5, 7 and 6 are reserved and typically are not used to classify traffic. CoS 5 should be treated for high priority (i.e: Voice) traffic.,Class of Service (COS),Limitation of COSDevices that receive packets on non-trunking or Ether
10、net ports will not preserve the L2 headers and the 802.1Q (or ISL) field, by stripping them of their priority markings. Therefore, CoS Markings should be mapped to mechanism which preserves the CoS as it transits other network devices, such as mapping CoS Values at Layer 2 to IP Precedence or DSCP v
11、alues within header of packets at Layer 3 (IP).,Marking with MQC,set cos Sets the COS bit on traffic class, within a policy-mapset ip precedenceSets the IP Precedence for a class of trafficset dscp Sets the DSCP for a class of traffic,Differentiated Services (DiffServ),DiffServ Field Formerly known
12、as the ToS Byte of an IP PacketDS Code Point (DSCP) The six left-most bits of the DiffServ Field. Packets can be divided into different classes or Behavior Aggregates (BA) and given preferential forwarding based on the bits set.Network devices, such as routers, switches, and IP Phones recognize DSCP
13、 markings on received packet(s) and can quickly determine the Forwarding and Queuing Method to use based on them. This is known as Per-Hop Behavior.With DSCP, packets can be marked with 64 different values (0-63).,Per-Hop Behaviors,Expedited Forwarding (EF)DSCP Value: 46 (101110)Backwards compatibil
14、ity with IP Precedence 5 (101)Ensures minimal departure of packetsGuarantees a maximum limit of bandwidthMarks packets with highest priority and zero drop rateIdeal for Voice traffic (audio, not signaling),Per-Hop Behaviors,Assured Forwarding (AF):Commonly used for Mission Critical trafficConsists o
15、f four classes and Three Drop Preference Levels.Guarantees a minimum amount of bandwidthAF Classes: AF1 = Lowest Priority AF2 & AF3 = Medium Priority AF4 = Highest Priority,AF DSCP Values,Drop Preference bits bolded in Red,Per-Hop Behaviors,What are the Drop Preference Levels for?The Drop Preference
16、 is used as a tie-breaker between packets of the same class during congestion. For example, If the router receives two packets of class AF1, it will check which packet has a higher drop preference set and discard that one in favor of the packet with the lower preference. Drop Preference is ignored between packets of different classes. If a packet marked with AF11 (Low Drop) and a packet with AF43 (High Drop) arrive at the router, the first one will be dropped because it is in the lower class, even though the other packet has a higher Drop Preference.The higher class is always favored.,
