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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
Cisco Employee

Hi Amit,

now I understand what you meant by "police the entire VPLS": you want to group several attachment circuits and apply a policer at the group level. This is supported, as Xander has already responded.

Rgds,

Aleksandar

Beginner

Hi Xander,

Consider the following diagram where colors, Purple and Green, represent 2 RSPs. The RSPs buy a large amount of bandwidth from Wholesale provider whose OLT/DSLAM is shared by RSPs. Each RSP provides Internet service, for example, to end-customers via S/C-tag service construct. However, the bandwidth assigned to end-customers is monitored by wholesale provider (in case the RSPs dont do any shaping downstream). The wholesale provider must also monitor the overall bandwidth of the RSPs in case they sign-up more customers than they should.

The BNGs could be connected to PE routers in a LAG with port-members on different LCs. Could you please provide more information/reference document on Shared Policy Instance?

ASR9K QoS Forum question.png

Regards,

Amit.

Cisco Employee

hi amit,

nice thanks for that visualization.

In this design, the only logical place to control the incoming BW is on the PE with the primary links.

But if the members are on different NPU's or different LC for that matter, it is hard to control the aggregated bw per bundle.

unless your bng loadbalance on a per destination addr (=subscr) only.

Assuming that you want to limit on the bw per bundle (and that gets multiplied then by the number of members in a full / equal distribution), you can use ingress shaping policing there.

Furthermore you can manage the BW egress out of the AC's to the OLT/DSLAM on that side also.

In this case assuming you have a vlan per BD, you may want to shape /police the overal aggregate towards the dslam where the SPI comes in.

All the AC's, representing different vlans, can be shaped together to a particular total value.

I think that that is what you're looking for in this design.

More info about SPI/shared policy instance here

xander

Beginner

Hi Alexander,

So if I understand this correctly, one policy-map to police/shape individual end-user traffic at EFP level, and a second policy-map to police/shape RSP's aggregated traffic at port-level because the ports connected to BNGs will be allocated to the respective RSPs only.

I think that makes sense to me. Thanks for taking the time to clarify this.

Regards,

Amit.

Cisco Employee

correct Amit, normally when you have one pmap X with some q'ing commands in it

and you attach it to two different interfaces, two unique sets of queues are created.

with the SPI basically we create 1 set of queues that both circuits use, hence shared .

cheers

xander

Beginner

Actually, one follow-up question - in case, policing is done at EFP and Port/bundle-level, which one takes effect first?

Cisco Employee

amit, policing is done before q'ing.

i have a good article for you on that also. google asr9000 feature order of operation.

regards

xander

Beginner

Hi Alexander,

Once again, thank you. I am putting the link here for anyone else looking for info.

https://supportforums.cisco.com/docs/DOC-32025

Regards,

Amit.

- Does the ASR9K have protocol recognition services such as NBAR?

- Does it have QoS mechanism such as UBRL on the 6500 or the 4500? Can it dp per-user rate limit?

-Hosam

How about NAT support? In your opinion can the ASR9K be a good edge for an enterprise to provide such services as:

- 10G backbone

- NATting

- NBAR services

- rate-limiting & QoS

Thanks.

HB

Cisco Employee

a9k does not have NBAR on the linecards, today you can use an SCE to do the DPI for you.

soon we'll be getting the VSM (services module) that will run a variaty of applications and DPI will be one of the planned functionalities (but likely not day 1).

It can do per user rate limiting (via policers on the "EFP", l2 subinterface or l3 subinterface)

10G not a problem, there is 40 and 100G too.

NAT is done by today the ISM (integrated services module), or you can use the to be launched new VSM which is faster.

regards

xander

Thanks for your thorough reply. Can you give ma a brief example on setting up the bandwidth rate-limit per user on an ASR9001? Lets say if we want each user on on matched subnets to have 1Mbps worth of bandwidth.

I'm not very familiar with IOS-XR and I'm trying to get hold of it.

Thanks

HB

Cisco Employee

Hi Hosam,

that model is achived with "regular" policy-maps. the syntax is almost the same as pmaps in IOS:

ipv4 access-list CUST1-SUBNET

10 permit ip 172.16.1.0 0.0.0.255 any

etc

class-map cust1

match ipv4 access-group CUST1-SUBNET

class-map cust2

match ipv4 access-group CUST2-SUBNET

policy-map POLICE-ME

class CUST1-SUBNET

police rate X (percent or bw)

class CUST2-SUBNET

police rate y (percent or bw)

cheers

xander

I appreciate you taking the time to answer my questions. So far I  think for us to go to the ASR9K we are looking at major cost factor  since it is modular and we have to put everything together.

1- Chassis

2- ISM for NAT44

3- SCE for DPI/Protocol anaylsys

Are all ASRs bult the same modular way?

Cisco Employee

not a problem at all Hosam and thanks for your interest!

all of the ASR9000 series is modular, but there is one chassis that is a bit more exceptional in the series, that is the 9001.

this one has modular bays, but it doesnt take linecards.

If you are in need of the ISM/VSM then you will need the 9904 (2 LC's), 9006 (4 LC's) or 9010 (8 LC's) minimally.

But the good news is, all ASR9000's function the exact same way in terms of forwarding, so once you know one

you know em all.

regards

xander

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