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QoS: Classification of Locally Sourced Packets

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QoS: Classification of Locally Sourced Packets

Table Of Contents

QoS: Classification of Locally Sourced Packets

Contents

Information on QoS: Classification of Locally Sourced Packets

Background Information and Benefits

Classification Scheme for Locally-Sourced Packets

Configuration Scenarios

Example 1: No Egress Policy Configured

Example 2: Egress Policy Without Queues Allocated for Precedence 6 and 7 Packets

Example 3: Egress Policy With Queues Allocated for IP Precedence 6 Packets

Example 4: Egress Policy With Queues Allocated for IP Precedence 6 Packets and Priority Packets

Restrictions

Information for the 3-port Gigabit Ethernet Line Card

Information for the 4-Port OC-12/STM-4 ATM ISE and 8-port OC03/STM1 ATM Line Cards

Information for OC-12 SRP Single Ring Line Cards

Information for 8-port Fast Ethernet 100baseFX, 8-port Fast Ethernet 100baseTX, and1-Port Gigabit Ethernet Line Cards

Additional References

Related Documents


QoS: Classification of Locally Sourced Packets


QoS: Classification of Locally Sourced Packets is an algorithm to govern queue assignments of egressing packets that originate from the Cisco 12000 Series route processors. These locally-sourced packets are directed to specific queues based on the IP precedent value of the packet, pak_priority flag setting, and the outbound QoS policy of the egress interface. This algorithm greatly reduces the possibility of locally-sourced packets being starved of bandwidth by high priority traffic on all Cisco 12000 Series line cards.

Feature History for QoS: Classification of Locally Sourced Packets

Release
Modification

12.0(28)S

This feature was introduced for all Cisco 12000 Series linecards


Contents

Information on QoS: Classification of Locally Sourced Packets

Configuration Scenarios

Additional References

Information on QoS: Classification of Locally Sourced Packets

There are no specific configuration tasks or command line interfaces related to this feature. To understand how locally-sourced packets are handled in response to typical QoS configuration scenarios, you must understand the following concepts:

Background Information and Benefits

Classification Scheme for Locally-Sourced Packets

Background Information and Benefits

Previously, the handling of locally-sourced traffic varied among the Cisco 12000 Series line cards. This feature establishes a common behavior for all line cards to prevent possible starvation of bandwidth to locally-sourced packets by heavy flows of high-priority data traffic such as Voice over IP or video.

Marking Vital Output Traffic from the Route Processor

Some internally generated packets, such as Layer 3 routing protocol updates are vital to the basic functioning of the router and should not be controlled by QoS features. Similarly, Layer 2 keepalives, or control packets in general, should not be subject to random dropping by the WRED mechanism.

Most vital traffic generated by a router is tagged with a pak_priority flag. Packets marked with pak_priority are not dropped by QoS features. All other packets are subject to QoS features enabled on the output interface.

Handling Vital Traffic Inbound to the Route Processor

A policing policy configured on an ingress interface should not drop important packets (such as, routing protocol updates). It is sometimes appropriate to handle router-generated traffic in the same way as user-generated traffic, that is, to classify the packets with respect to the enabled service policy, then apply the appropriate actions based on the class to which the packet is matched. An example of such packets are performance probes intended to measure the behavior incurred by packets of a specified class.

Classification Scheme for Locally-Sourced Packets

With QoS: Classification of Locally Sourced Packets, Route Processor packets are classified as Important Packets and Unimportant Packets. Important Packets have the pak_priority flag set for non-IP packets, or the IP precedence value set at 6 or 7 for IP packets. For MPLS packets, Important Packets are those with an EXP tag value of 6 or 7.

The following control packets have the pak_priority flag set (Important Packets):

ISIS

PIMV2

OSPF HELLO

RIP

ES-IS

DECNET

PPP (LCP, LQR)

HDLC keepalive

ATM ARP

ATM OAM

FR SVC (Q.933)

FR keepalive

FR eLMI status query

STP spanning tree

All other packets are classified as Unimportant Packets. If a Modular Quality of Service Command Line Interface (MQC) policy or legacy policy is not configured on the egress interface or subinterface, Important Packets egress out of the low-latency queue (LLQ). Unimportant packets egress out of the default queue (See the "Restrictions" section for exceptions). If an MQC policy or legacy policy is configured on the egress interface or subinterface, then Unimportant Packets egress according to the policy.


Note For ISE-based line cards, an MQC policy configured on the main interface can govern the behavior of the subinterfaces. (Such as, VLAN/DLCI groups.) Route Processor-generated packets directed towards the subinterface will conform with the main interface policy.


Non-IP and tag packets with the pak_priority flag set are directing by the following rules:

1. Take the queue allocated for Precedence 6 packets

2. Take the queue allocated for Precedence 7 packets if there is no Precedence 6 queue

3. Take the default queue if there is neither a Precedence 7 queue nor a Precedence 6 queue

Packets with the pak_priority flag set follow this sequence only for packets matched on IP Precedence. MPLS packets matched with match mpls experimental number 6 or match mpls experimental number 7 take the default queue.

Configuration Scenarios

This section explains how locally-sourced packets are handled in the following QoS egress interface configuration scenarios:

Example 1: No Egress Policy Configured

Example 2: Egress Policy Without Queues Allocated for Precedence 6 and 7 Packets

Example 3: Egress Policy With Queues Allocated for IP Precedence 6 Packets

Example 4: Egress Policy With Queues Allocated for IP Precedence 6 Packets and Priority Packets

Explanations in this section describe how PING packets with differing precedence values are handled by each configuration.

Example 1: No Egress Policy Configured

In the case where no egress policy is configured, all the following packets on all the Cisco 12000 Series line cards are directed to the LLQ:

IP packets with a Precedence value of 6 or 7

MPLS packets with an EXP tag value of 6 or 7

Packets with the pak_priority flag set

All of the remaining are Unimportant packets and are directed to the default queue.


Note Line cards based on Engine 0 have only a default queue on the interface until a policy is configured on the Egress side.


Example 2: Egress Policy Without Queues Allocated for Precedence 6 and 7 Packets

This scenario is true for all line cards based on Engine 0, Engine 2, Engine 3, and Engine 4+.

The following WRED configuration fragment detects packets with a precedence value of 1:

Class class-default
random-detect precedence-based
random-detect precedence 1 10 packets 20 packets 1

Because the LLQ is not indicated called with the priority keyword in the configuration, and there is no need for a queue allocated for packets with Precedence values of 6 or 7, the locally-sourced packets are directed as follows:

PING packets with Precedence values of 6 or 7 to the LLQ

PING packets with EXP tags of 6 or 7 to the LLQ

pak_priority packets to the LLQ

All other packets are directed to the default queue.

Example 3: Egress Policy With Queues Allocated for IP Precedence 6 Packets

The following WRED configuration fragment detects packets with precedence values of 1 and 6:

Class prec6
bandwidth percent 10
random-detect precedence-based
random-detect precedence 6 10 packets 20 packets 1
Class class-default
random-detect precedence-based
random-detect precedence 1 10 packets 20 packets 1

Because a queue is allocated for the Precedence 6 the locally-sourced packets are directed on the outbound side as follows:

PING packets with Precedence values of 6 to the queue allocated for Precedence 6 packets

PING packets with Precedence values of 7 to the LLQ

PING packets with EXP tags of 6 or 7 to the LLQ

pak_priority packets to the queue for Precedence 6 packets

All other packets are directed to the default queue.

Example 4: Egress Policy With Queues Allocated for IP Precedence 6 Packets and Priority Packets

The following WRED configuration fragment detects packets with precedence values of 1 and 6 as well as establishing a priority scheduling queue for the precedence 3 packets of class prec3:

Class prec6
bandwidth percent 10
bandwidth remaining percent 10
random-detect precedence-based
random-detect precedence 6 10 packets 20 packets 1
Class prec3
priority
Class class-default
random-detect precedence-based
random-detect precedence 1 10 packets 20 packets 1

Locally-sourced packets are directed on the outbound side as follows:

PING packets with Precedence values of 6 to the queue allocated for Precedence 6 packets

PING packets with Precedence values of 7 to the default queue (because prec3 class packets now occupy the LLQ)

PING packets with EXP tags of 6 or 7 to the default queue (because prec3 class packets now occupy the LLQ)

All other packets are directed to the default queue.


Note For line cards based on Engine 3 or Engine 4+, PING packets with MPLS EXP tag 3 are directed to the default queue.



Note For the line cards that do not support the match mpls experimental class-map configuration command, MPLS packets match to values set by the match ip precedence class-map configuration command. In Example 4, PING packets with MPLS EXP tag 3 coming from line cards based on Engine 0 and Engine 2 are directed to the priority queue because it matches the Prec3 priority class.


Restrictions

Individual differences among line cards are noted in the following sections:

Information for the 3-port Gigabit Ethernet Line Card

Information for the 4-Port OC-12/STM-4 ATM ISE and 8-port OC03/STM1 ATM Line Cards

Information for OC-12 SRP Single Ring Line Cards

Information for 8-port Fast Ethernet 100baseFX, 8-port Fast Ethernet 100baseTX, and1-Port Gigabit Ethernet Line Cards

Information for the 3-port Gigabit Ethernet Line Card

Product Number: 3GE-GBIC-SC=

This is an Engine 2 line card. If an egress policy is defined, use the queue from the policy. However, if the queue is not the LLQ, use the default queue. This is because there is only one non-LLQ per port supported on the 3-port Gigabit Ethernet line card and the non-LLQ is the default queue.

Information for the 4-Port OC-12/STM-4 ATM ISE and 8-port OC03/STM1 ATM Line Cards

Product Numbers: 4OC12X/ATM-MM-SC=, 4OC12X/ATM-IR-SC=, 8OC03/ATM/TS-MM-B=, 4OC12X/ATM-IR-SC=

These are Engine 3 line cards. If a policy map is not attached to a virtual circuit (VC), all Route Processor-generated traffic will egress from the default queue (only one queue is configured per VC for such cases). If a policy map includes a QoS configuration (such as a bandwidth command) and is attached to a VC, an LLQ is automatically opened on this VC whether or not we explicitly configured an LLQ using the priority command. For the second case all Route Processor-generated traffic will egress out either of the LLQ (for Important Packets) or from the default queue (Unimportant Packet). Classification of packets as Important Packet and Unimportant Packet is done exactly as explained in the "Classification Scheme for Locally-Sourced Packets" section. Important Packets are packets that have the pak_priority flag set, or IP packets with precedence values of 6 and 7. For MPLS packets, Important Packets are those with an EXP tag of 6 or 7. OAM cells are treated as Important Packets. Everything else is an Unimportant Packet.


Note All Route Processor-generated packets whether or not they are transmitted over Virtual Circuits, with or without policy map configured and regardless of their priority, egress without any Qadmit policy. This means that all Route Processor-generated packets are not dropped because of RED or qlimit thresholds.


Information for OC-12 SRP Single Ring Line Cards

Product Numbers: OC12/SRP-MM/SC-B=, OC12/SRP-IR/SC-B=, OC12/SRP-LR/SC-B=, OC12/SRP-XR/SC=

These are Engine 1 line cards. Important Packets flow out of the default queue, not the LLQ.

Information for 8-port Fast Ethernet 100baseFX, 8-port Fast Ethernet 100baseTX, and1-Port Gigabit Ethernet Line Cards

Product Numbers: 8FE-FX-SC-B=, 8FE-TX-RJ45-B=, GE-SX/LH-SC=, GE-GBIC-SC-B=

These are Engine 1 line cards. Important Packets flow out of the default queue, not the LLQ.

Additional References

For additional information related to QoS: Classification of Locally Sourced Packets, refer to the following references:

Related Documents