This document provides details on how QOS is implemented in the ASR9000 and how to interpret and troubleshoot qos related issues.
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.
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.
The ASR9000 has the ability of 4 levels of qos, a sample configuration and implemenation detail presented in this picture:
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
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.:
police rate percent 25 burst 250 ms peak-burst 500 ms
For viewing the Bc and Be applied in hardware, run the "show qos interfaceinterface [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.
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
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.
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;
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