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ASR9000/XR: Understanding QOS, default marking behavior and troubleshooting

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Introduction

This document provides details on how QOS is implemented in the ASR9000 and how to interpret and troubleshoot qos related issues.

 

Core Issue

QOS is always a complex topic and with this article I'll try to describe the QOS architecture and provide some tips for troubleshooting.

Based on feedback on this document I'll keep enhancing it to document more things bsaed on that feedback.

 

The ASR9000 employs an end to end qos architecture throughout the whole system, what that means is that priority is propagated throughout the systems forwarding asics. This is done via backpressure between the different fowarding asics.

One very key aspect of the A9K's qos implementation is the concept of using VOQ's (virtual output queues). Each network processor, or in fact every 10G entity in the system is represented in the Fabric Interfacing ASIC (FIA) by a VOQ on each linecard.

That means in a fully loaded system with say 24 x 10G cards, each linecard having 8 NPU's and 4 FIA's, a total of 192 (24 times 8 slots) VOQ's are represented at each FIA of each linecard.

The VOQ's have 4 different priority levels: Priority 1, Priority 2, Default priority and multicast.

The different priority levels used are assigned on the packets fabric headers (internal headers) and can be set via QOS policy-maps (MQC; modular qos configuration).

When you define a policy-map and apply it to a (sub)interface, and in that policy map certain traffic is marked as priority level 1 or 2 the fabric headers will represent that also, so that this traffic is put in the higher priority queues of the forwarding asics as it traverses the FIA and fabric components.

If you dont apply any QOS configuration, all traffic is considered to be "default" in the fabric queues. In order to leverage the strength of the asr9000's asic priority levels, you will need to configure (ingress) QOS at the ports to apply the priority level desired.

qos-archi.jpg

In this example T0 and T1 are receiving a total of 16G of traffic destined for T0 on the egress linecard. For a 10G port that is obviously too much.

T0 will flow off some of the traffic, depending on the queue, eventually signaling it back to the ingress linecard. While T0 on the ingress linecard also has some traffic for T1 on the egress LC (green), this traffic is not affected and continues to be sent to the destination port.

Resolution

 

The ASR9000 has the ability of 4 levels of qos, a sample configuration and implemenation detail presented in this picture:

 

shared-policy.jpg

 

 

Policer having exceeddrops, not reaching configured rate

 

When defining policers at high(er) rates, make sure the committed burst and excess burst are set correctly.
This is the formula to follow:

Set the Bc to CIR bps * (1 byte) / (8 bits) * 1.5 seconds

and

Be=2xBc

Default burst values are not optimal

Say you are allowing 1 pps, and then 1 second you don’t send anything, but the next second you want to send 2. in that second you’ll see an exceed, to visualize the problem.

 

Alternatively, Bc and Be can be configured in time units, e.g.:

     policy-map OUT

      class EF

       police rate percent 25 burst 250 ms peak-burst 500 ms

 

For viewing the Bc and Be applied in hardware, run the "show qos interface interface [input|output]".

 

 

Why do I see non-zero values for Queue(conform) and Queue(exceed) in show policy-map commands?

On the ASR9k, every HW queue has a configured CIR and PIR value. These correspond to the "guaranteed" bandwidth for the queue, and the "maximum" bandwidth (aka shape rate) for the queue.

In some cases the user-defined QoS policy does NOT explicitly use both of these.  However, depending on the exact QoS config the queueing hardware may require some nonzero value for these fields.  Here, the system will choose a default value for the queue CIR.  The "conform" counter in show policy-map is the number of packets/bytes that were transmitted within this CIR value, and the "exceed" value is the number of packets/bytes that were transmitted within the PIR value.

Note that "exceed" in this case does NOT equate to a packet drop, but rather a packet that is above the CIR rate on that queue.

You could change this behavior by explicitly configuring a bandwidth and/or a shape rate on each queue, but in general it's just easier to recognize that these counters don't apply to your specific situation and ignore them.

 

What is counted in QOS policers and shapers?

 

When we define a shaper in a qos pmap, the shaper takes the L2 header into consideration.

The shape rate defined of say 1Mbps would mean that if I have no dot1q or qinq, I can technically send more IP traffic then having a QIQ which has more L2 overhead. When I define a bandwidth statement in a class, same applies, also L2 is taken into consideration.

When defining a policer, it looks at L2 also.

In Ingress, for both policer & shaper, we use the incoming packet size (including the L2 header).

In order to account the L2 header in ingress shaper case, we have to use a TM overhead accounting feature, that will only let us add overhead in 4 byte granularity, which can cause a little inaccuracy.

In egress, for both policer & shaper we use the outgoing packet size (including the L2 header).

 

ASR9K Policer implementation supports 64Kbps granularity. When a rate specified is not a multiple of 64Kbps the rate would be rounded down to the next lower 64Kbps rate.

 

For policing, shaping, BW command for ingress/egress direction the following fields are included in the accounting.

 

MAC DA

MAC SA

EtherType

VLANs..

L3 headers/payload

CRC

 

Port level shaping

Shaping action requires a queue on which the shaping is applied. This queue must be created by a child level policy. Typically shaper is applied at parent or grandparent level, to allow for differentiation between traffic classes within the shaper. If there is a need to apply a flat port-level shaper, a child policy should be configured with 100% bandwidth explicitly allocated to class-default.

Understanding show policy-map counters

 

QOS counters and show interface drops:

 

Policer counts are directly against the (sub)interface and will get reported on the "show interface" drops count.
The drop counts you see are an aggregate of what the NP has dropped (in most cases) as well as policer drops.

 

Packets that get dropped before the policer is aware of them are not accounted for by the policy-map policer drops but may
show under the show interface drops and can be seen via the show controllers np count command.

 

Policy-map queue drops are not reported on the subinterface drop counts.
The reason for that is that subinterfaces may share queues with each other or the main interface and therefore we don’t
have subinterface granularity for queue related drops.

 

 

Counters come from the show policy-map interface command

 

 

Class name as per   configuration Class   precedence6
Statistics for this class   Classification statistics          (packets/bytes)     (rate - kbps)
Packets that were matched     Matched             :            31583572/2021348608           764652
packets that were sent to the wire     Transmitted         : Un-determined
packets that were dropped for any reason   in this class     Total Dropped       : Un-determined
Policing stats   Policing statistics                (packets/bytes)     (rate - kbps)
Packets that were below the CIR rate     Policed(conform)    :            31583572/2021348608           764652
Packets that fell into the 2nd bucket   above CIR but < PIR     Policed(exceed)     :                   0/0                    0
Packets that fell into the 3rd bucket   above PIR     Policed(violate)    :                   0/0                    0
Total packets that the policer dropped     Policed and dropped :                   0/0
Statistics for Q'ing   Queueing statistics  <<<----
Internal unique queue reference     Queue ID                             : 136

how many packets were q'd/held at max one   time

(value not supported by HW)

    High watermark  (Unknown)

number of 512-byte particles which are currently

waiting in the queue

    Inst-queue-len  (packets)            : 4096

how many packets on average we have to   buffer

(value not supported by HW)

    Avg-queue-len   (Unknown)

packets that could not be buffered   because we held

more then the max length

    Taildropped(packets/bytes)           : 31581615/2021223360
see description above (queue exceed section)     Queue(conform)      :            31581358/2021206912           764652
see description above (queue exceed section)     Queue(exceed)       :                   0/0                    0

Packets subject to Randon Early detection

and were dropped.

    RED random drops(packets/bytes)      : 0/0

 

 

Understanding the hardware qos output

 

RP/0/RSP0/CPU0:A9K-TOP#show qos interface g0/0/0/0 output

 

With this command the actual hardware programming can be verified of the qos policy on the interface

(not related to the output from the previous example above)


Tue Mar  8 16:46:21.167 UTC
Interface: GigabitEthernet0_0_0_0 output
Bandwidth configured: 1000000 kbps Bandwidth programed: 1000000
ANCP user configured: 0 kbps ANCP programed in HW: 0 kbps
Port Shaper programed in HW: 0 kbps
Policy: Egress102 Total number of classes: 2
----------------------------------------------------------------------
Level: 0 Policy: Egress102 Class: Qos-Group7
QueueID: 2 (Port Default)
Policer Profile: 31 (Single)
Conform: 100000 kbps (10 percent) Burst: 1248460 bytes (0 Default)
Child Policer Conform: TX
Child Policer Exceed: DROP
Child Policer Violate: DROP
----------------------------------------------------------------------
Level: 0 Policy: Egress102 Class: class-default
QueueID: 2 (Port Default)
----------------------------------------------------------------------

 

 

Default Marking behavior of the ASR9000

 

If you don't configure any service policies for QOS, the ASR9000 will set an internal cos value based on the IP Precedence, 802.1 Priority field or the mpls EXP bits.

Depending on the routing or switching scenario, this internal cos value will be used to do potential marking on newly imposed headers on egress.

 

Scenario 1

Slide1.JPG

Scenario 2

Slide2.JPG

 

Scenario 3

Slide3.JPG

 

Scenario 4

 

Slide4.JPG

 

Scenario 5

 

Slide5.JPG

 

Scenario 6

Slide6.JPG

 

Special consideration:

If the node is L3 forwarding, then there is no L2 CoS propagation or preservation as the L2 domain stops at the incoming interface and restarts at the outgoing interface.

Default marking PHB on L3 retains no L2 CoS information even if the incoming interface happened to be an 802.1q or 802.1ad/q-in-q sub interface.

CoS may appear to be propagated, if the corresponding L3 field (prec/dscp) used for default marking matches the incoming CoS value and so, is used as is for imposed L2 headers at egress.

 

If the node is L2 switching, then the incoming L2 header will be preserved unless the node has ingress or egress rewrites configured on the EFPs.
If an L2 rewrite results in new header imposition, then the default marking derived from the 3-bit PCP (as specified in 802.1p) on the incoming EFP is used to mark the new headers.

 

An exception to the above is that the DEI bit value from incoming 802.1ad / 802.1ah headers is propagated to imposed or topmost 802.1ad / 802.1ah headers for both L3 and L2 forwarding;

 

Related Information

ASR9000 Quality of Service configuration guide

 

Xander Thuijs, CCIE #6775

 

Comments
Beginner

HI Xander,

I was hoping you could help me understand the feature

hw-module fia-vqi-shaper <location> enhance

 We've used this command to eliminate FIA drops when traffic is going from 100GE to 10GE.  We applied this command on the 10GE line-cards and the issues went away correctly.

 

However, we recently ran into an issue in which we were seeing ingress FIA drops when the traffic was going from 10GE to 100GE.  We once again applied this command on the 100GE Typhoon line-card and the FIA drops were eliminated and performance improved.  My question is whether it's the "correct" procedure to apply this command on all Typhoon line-cards or whether this command was only intended for lower-speed line-cards (like 36x10GE).

 

Thanks always for the help.

Beginner

Hi Alexander!

 

Thank you for the great article.

 

I have problem with limiting bandwidth on ASR 9001 A9K-MPA-4X10GE ten gig interface.

I'm trying to apply the below policy-map:

 

policy-map POLICER-2GB
class class-default
police rate 2 gbps burst 6250000 bytes
conform-action transmit
exceed-action drop
!
!
end-policy-map

 

under interface:

 

interface TenGigE0/0/0/0
description Cust: X
mtu 9018
l2transport

 

in this way:

 

(config-if)#show commit changes diff

!! IOS XR Configuration 5.3.4
+ interface TenGigE0/0/0/0
+ service-policy input POLICER-2GB
!
end

 

but I receive - Failed to commit

 

interface TenGigE0/0/0/0
service-policy input POLICER-2GB
!!% Subinterface or Layer 2 configuration conflicts with existing configuration, or configuration in this commit: L3 configuration is not allowed under a L2 interface

 

Could you help me?

 

Kind regards,
Pawel

Beginner

Hi Xander!

I am currently implementing QoS on our ASR9k's (9010/RSP440, XR5.3.3, A9K-24X10GE-TR = 8 queues per port).

When I try to apply out QoS-Config I am getting this error-message:


!!% Given combination of p1, p2, p3, ..., pn queues are not supported at leaf-level of a queuing hierarchy: InPlace Modify Error: Policy BACKBONE_OUT: 'qos-ea' detected the 'warning' condition 'Given combination of p1, p2, p3, ..., pn queues are not supported at leaf-level of a queuing hierarchy'


Here is my config I wanted to commit (works on ASR920, our access devices - lightly modified in PQ and shape average)


policy-map BACKBONE_OUT_CHILD
 class QOS_GROUP_7
  bandwidth percent 1
 class QOS_GROUP_6
  bandwidth percent 1
 class QOS_GROUP_5
  priority level 1
  police rate percent 5
 class QOS_GROUP_4
  bandwidth percent 1
 class QOS_GROUP_3
  bandwidth percent 30
 class QOS_GROUP_2
  bandwidth percent 40
 class QOS_GROUP_1
  bandwidth percent 20
 class class-default
  bandwidth percent 1
 
policy-map BACKBONE_OUT
 class class-default
  shape average 9500 Mbps
   service-policy BACKBONE_OUT_CHILD

interface Bundle-Ether2301 (same on physical interface Te0/0/0/0)

 service-policy output BACKBONE_OUT


I also tried to apply the BOCKBONE_OUT on the interface with minimal BACKBONE_OUT_CHILD class of 1. Afterwards I added class by class to see how many are accepted (inplace modify). We can add up to 7 queues (with valid queue-id) until the error is coming up.

Xander, can you tell me why we are getting this error message? What are we doing wrong? Or is it just as designed; is there one queue we have to think of in the parent shaper?


Thanks in advance!

Thomas

Beginner
Hi!! .. I would like to ask something people.. somebody knows what is the default behaviour for the downstream traffic over a mpls egress port when the L3 traffic have a ip precedence mark . Does it mapped to experimental bits? Regards Javier

Dear Alexander,

Very good information

 

1. I have some questions.

For VoQ with control plan traffic such as BFD protocol , OSPF protocol 
in this case how to manage it.

2. It did help a lot in clarifying my confusions.

Ola, traffic that the RP or LC is injecting is directly enq'd to the port and no acl or qos is needed/available for such packets (this is new behavior in 4.x, in 39x you were able to apply qos for locally originated packets)

Thanks,
Manaschai.

Beginner

@xthuijs

Any possibility to match a specific TCP source port, encapsulated in PPPoE in Q in Q and passing through (L2) a ASR9k?

Cisco Employee
you know with XR 65 and the inclusion of CSCuj59068
you are able to compute FAT labels based on IP when it pertains to PPPoE/PPP headers!
this is to be done on the PE’s that carry the ppp traffic.
intermediate nodes, with their standard inner label hashing, will pick up this ip based derived label for distribution!!

cheers!
xander
Beginner

@xthuijs That's great, but in my use-case I'm neither using MPLS nor am I load-balancing. What I need is to match the layer 4 port encapsulated in pppoe in vlan in vlan. If this can't be done on the 9k, then I need to do it on the BNG (which is a box adjacent to the 9k but is actually a 1k) and set outgoing COS values based on that, so I can queue this on the 9k.

Cisco Employee
hi Lukas,
sorry I misunderstood your ask! :)
ah ok if it is for only qos marking purposes then yeah you can’t match on L3/L4 info, but you do have access to the COS values.
the COS would be set ideally by BNG and that you can use or remark as needed on the L2 adj node.
xander

Good afternoon

Sorry, English.

I am facing the same problem with link aggregate, 3x 10G interface is ignored the policy-map. Can anyone help?

 

Marcelo Macedo

 

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