This document illustrates the order in which Quality of Service (QoS) features are executed when applied inbound or outbound to an interface on a router running Cisco IOS® software. QoS policies are configured with the modular QoS Command Line Interface (MQC). This document also discusses IP header marking, such as DSCP and IP Precedence, and the order in which the components of a QoS policy are evaluated by the router.
Readers of this document should have knowledge of:
Basic QoS methodologies
The example output in the Configurations section of this document was captured on a Cisco 7513 Series platform that runs Cisco IOS Software Release 12.2.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
Classification is the process of defining traffic classes that sort traffic into categories groups of flows. Classification defines the "match criteria" for each class of traffic that is to be treated by a QoS policy. More specifically, it defines the "traffic filter" that packets are checked against when a service-policy is applied.
Both distributed and non-distributed platforms match packets to a single class in a policy-map. Matching terminates at the first matching class. If two classes within a policy-map match the same IP precedence or IP address range, the packet always belongs to the first matching class. For this reason, class order within a policy-map is very important.
This classification approach is called "common classification" and has these benefits:
Accurate accounting and the avoidance of double-accounting problems that were seen before "common classification".
Reduces the impact of access control lists (ACLs) on the CPU since the ACL is checked once per class, rather than once per feature.
Faster lookup of packet headers because of caching.
Common classification is enabled automatically when you attach an input or output policy-map with the service-policy command.
This table illustrates the order of operation with common classification. It is important to understand from the table when classification occurs in the context of QoS features. On the inbound path, a packet is classified before it is switched. On the outbound path, a packet is classified after it is switched.
Note: Inbound Network-Based Application Recognition (NBAR) happens after ACLs and before policy-based routing.
Important changes have been implemented regarding feature ordering and remarked value usage. These changes include moving input CAR, input MAC, and IP precedence accounting functions to occur before MQC output classification:
Input rate-limiting, or CAR, applies to packets following the process switching path and destined to the router. Previously, only packets switched through the router using CEF could be rate-limited.
New IP precedence values set by input CAR or QPPB can be used for selecting a Virtual Circuit (VC) in an ATM VC bundle.
IP precedence, Differentiated Services Code Points (DSCP), and QoS group values set by input CAR or QPPB can be used for MQC output packet classification.
A frequent application of QoS is to remark a packet and then apply an action which considers the remarked value on the same interface or on the same router. You can configure both marking and other QoS actions with common classification.
You can remark packets with these QoS features:
set command with class-based marking
police command with class-based policing
This table indicates whether or not a remarked value is considered by a QoS action in a service-policy.
|Location of Policy||Value Used by the Outbound Policy Actions|
|Mark and apply QoS action in the same policy.||QoS actions use the original value of the packet when it is commonly classified. The packet will carry the new value when it is transmitted, and the next router uses the new value.|
|Mark with inbound policy and apply QoS action with outbound policy.||QoS actions use the new or remarked value when classifying traffic against the outbound policy.|
On the outbound path, common classification happens before any QoS features are applied. A result of this approach is that any QoS features applied on the outbound policy act upon the original priority value. If you need to take actions based on a remarked value on the same router, then you must mark the packets on the incoming interface and apply other QoS actions based on this new priority on the outgoing interface.
The configurations in this section uses this network diagram:
Note: The Multilayer Switch Feature Card (MSFC) is acting as a host.
This example demonstrates how the order of operations can affect packet marking.
|Separate Marking and Shaping Policy Configuration|
class-map match-all In_Mark match any policy-map In_Bound class In_Mark set ip precedence 5 !--- Use Private address below: interface FastEthernet4/0/0 ip address 10.20.3.2 255.255.255.0 ip route-cache distributed service-policy input In_Bound !--- Apply the input policy for class-based marking. class-map match-all Out_Shaper match ip precedence 5 ! policy Map Outbound_Shaper class Out_Shaper shape average 64000 256 256 !--- Use Private address below: interface Serial2/0/0 ip address 172.16.20.1 255.255.255.252 ip route-cache distributed service-policy output Outbound_Shaper !--- Apply the output policy for class-based shaping.
Complete these steps to confirm the marking and shaping policies:
Use the ping command to the 172.16.20.2 destination address. The ping matches the criteria of the class-map named "In_Mark".
msfc#ping 172.16.20.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 18.104.22.168, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/9/12 ms
Use the show policy-map interface fast 4/0/0 command to view the match counters of the input class-based marking policy. The classification mechanism is successfully matched on the IP packets, and remarked the IP precedence value to five.
7513#show policy-map interface fast 4/0/0 FastEthernet4/0/0 Service-policy input: In_Bound Class-map: In_Mark (match-all) 5 packets, 570 bytes 5 minute offered rate 0 bps, drop rate 0 bps Match: any QoS Set ip precedence 5 Packets marked 5 Class-map: class-default (match-any) 0 packets, 0 bytes 5 minute offered rate 0 BPS, drop rate 0 BPS Match: any
Use the show policy-map interface serial 2/0/0 command to view the match counters of the outbound class-based shaping policy. The classification mechanism is successfully matched on the remarked IP precedence value five in the packet header, and queued the packets to the correct class.
7513#show policy-map interface serial 2/0/0 Serial2/0/0 Service-policy output: Outbound_Shaper Class-map: Out_Shaper(match-all) 5 packets, 520 bytes 5 minute offered rate 0 BPS, drop rate 0 BPSMatch: ip precedence 5 queue size 0, queue limit 16 packets output 5, packet drops 0 tail/random drops 0, no buffer drops 0, other drops 0 Shape: cir 64000, Bc 256, Be 256 output bytes 520, shape rate 0 BPS Class-map: class-default (match-any) 0 packets, 0 bytes 5 minute offered rate 0 BPS, drop rate 0 BPS Match: any (1327)
You can see what happens when we configure a single service-policy that applies both shaping and marking to a class of traffic, as in this example.
|Single Marking and Shaping Policy Configuration|
class-map match-all prec5 match any ! policy-map shape_five class prec5 set ip precedence 5 shape average 64000 256 256 int serial1/0/0 service-policy out shape_five
The output from the show policy-map interface serial 2/0/0 command shows that the router remarked the five ping packets, but the packets were queued to the class-default class. The QoS classification mechanisms on this router did not consider the remarked value in the IP precedence field.
7513#show policy-map interface serial 2/0/0 Serial2/0/0 Service-policy output: shape_five Class-map: prec5 (match-all) 0 packets, 0 bytes 5 minute offered rate 0 BPS, drop rate 0 BPS Match: any queue size 0, queue limit 16 packets output 0, packet drops 0 tail/random drops 0, no buffer drops 0, other drops 0 QoS Set ip precedence 5 Packets marked 5 Shape: cir 64000, BC 256, Be 256 output bytes 0, shape rate 0 BPS Class-map: class-default (match-any) 5 packets, 520 bytes 5 minute offered rate 0 BPS, drop rate 0 BPS Match: any
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