Modular QoS Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.1.x
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Congestion management
features allow you to control congestion by determining the order in which a
traffic flow (or packets) is sent out an interface based on priorities assigned
to packets. Congestion management entails the creation of queues, assignment of
packets to those queues based on the classification of the packet, and
scheduling of the packets in a queue for transmission.
The types of traffic regulation mechanisms supported are:
Class-based Weighted Fair Queueing
(CBWFQ) allows definition of traffic classes based on customer match criteria.
With CBWFQ you can define traffic classes and assign guaranteed amount of
minimum bandwidth to them. CBWFQ also allows for a strict priority queue for
delay-sensitive traffic.
Bandwidth
Remaining
The CBWFQ algorithm derives the weight for each class from the bandwidth remaining value allocated to the class. The bandwidth remaining option specifies a weight for the class to the
CBWFQ . After the priority-queue is serviced, the leftover bandwidth is distributed as per bandwidth remaining ratio (BWRR) or percentage.
If you do not configure this command for any class, the default value of the BWRR is considered as 1 (one). In the case of
bandwidth remaining percent, the remaining bandwidth is equally distributed among other classes, to make it 100 percentage (100%).
Restrictions
The
bandwidth
remaining command is supported only for egress policies.
Configure
Minimum Bandwidth and
Bandwidth Remaining
Guidelines
The bandwidth, bandwidth remaining, shaping, queue-limit, and random-detect commands may be configured together in the same class. The priority command cannot be configured along with bandwidth, bandwidth remaining commands, but can be configured with shaping, queue-limit and random-detect commands in the same class.
Configuration
Example
You have to
accomplish the following to complete the
minimum bandwidth and
bandwidth remaining configuration:
Creating or
modifying a policy-map that can be attached to one or more interfaces
Specifying the
traffic class whose policy has to be created or changed
Allocating the
minimum bandwidth and
leftover bandwidth for the class
Low-Latency Queueing
with Strict Priority Queueing
The Low-Latency Queueing (LLQ) feature brings strict priority queuing (PQ) to the CBWFQ scheduling mechanism. Priority Queueing (PQ) in strict priority mode ensures that one type of traffic is sent, possibly at the expense of all others.
For PQ, a low-priority queue can be detrimentally affected, and, in the worst case, never allowed to send its packets if a
limited amount of bandwidth is available or the transmission rate of critical traffic is high.
Configure Low
Latency Queueing with Strict Priority Queueing
Configuring low
latency queueing (LLQ) with strict priority queuing (PQ) allows delay-sensitive
data such as voice to be de-queued and sent before the packets in other queues
are de-queued.
Guidelines
Only priority level 1 to 7 is supported, with 1 being the highest priority and 7 being the lowest. However, the default CoSQ 0 has the lowest priority among all.
Egress policing is
not supported. Hence, in the case of strict priority queuing, there are chances
that the other queues do not get serviced.
You can configure shape average and queue-limit commands along with priority.
Configuration
Example
You have to
accomplish the following to complete the LLQ with strict priority queuing:
Creating or
modifying a policy-map that can be attached to one or more interfaces
Specifying the
traffic class whose policy has to be created or changed
Specifying
priority to the traffic class
(Optional) Shaping the traffic to a specific bit rate
policy-map test-priority-1
class qos-1
priority level 7
!
class qos-2
priority level 6
!
class class-default
!
end-policy-map
!
interface HundredGigE0/6/0/18
service-policy input 100g-s1-1
service-policy output test-priority-1
!
Verification
Router# show qos int hundredGigE 0/6/0/18 output
NOTE:- Configured values are displayed within parentheses
Interface HundredGigE0/6/0/18 ifh 0x3000220 -- output policy
NPU Id: 3
Total number of classes: 3
Interface Bandwidth: 100000000 kbps
VOQ Base: 11176
VOQ Stats Handle: 0x88550ea0
Accounting Type: Layer1 (Include Layer 1 encapsulation and above)
------------------------------------------------------------------------------
Level1 Class (HP7) = qos-1
Egressq Queue ID = 11177 (HP7 queue)
TailDrop Threshold = 125304832 bytes / 10 ms (default)
WRED not configured for this class
Level1 Class (HP6) = qos-2
Egressq Queue ID = 11178 (HP6 queue)
TailDrop Threshold = 125304832 bytes / 10 ms (default)
WRED not configured for this class
Level1 Class = class-default
Egressq Queue ID = 11176 (Default LP queue)
Queue Max. BW. = 101803495 kbps (default)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
TailDrop Threshold = 1253376 bytes / 10 ms (default)
WRED not configured for this class
Traffic shaping allows
you to control the traffic flow exiting an interface to match its transmission
to the speed of the remote target interface and ensure that the traffic
conforms to policies contracted for it. Traffic adhering to a particular
profile can be shaped to meet downstream requirements, thereby eliminating
bottlenecks in topologies with data-rate mismatches.
Note
Traffic shaping is supported only in egress direction.
Configure Traffic
Shaping
The traffic shaping performed on outgoing interfaces is done at the Layer 1 level and includes the Layer 1 header in the rate
calculation.
Guidelines
Only egress traffic shaping is supported.
It is mandatory to configure all the eight qos-group classes (including class-default) for the egress policies.
You can configure shape average command along with priority command.
Configuration Example
You have to accomplish the following to complete the traffic shaping configuration:
Creating or modifying a policy-map that can be attached to one or more interfaces
Specifying the traffic class whose policy has to be created or changed
policy-map egress_policy1
class c5
shape average percent 40
!
class class-default
!
end-policy-map
!
interface HundredGigE0/6/0/18
service-policy input 100g-s1-1
service-policy output egress_policy1
!
Verification
Router# show qos interface hundredGigE 0/6/0/18 output
NOTE:- Configured values are displayed within parentheses
Interface HundredGigE0/6/0/18 ifh 0x3000220 -- output policy
NPU Id: 3
Total number of classes: 2
Interface Bandwidth: 100000000 kbps
VOQ Base: 11176
VOQ Stats Handle: 0x88550ea0
Accounting Type: Layer1 (Include Layer 1 encapsulation and above)
------------------------------------------------------------------------------
Level1 Class = c5
Egressq Queue ID = 11177 (LP queue)
Queue Max. BW. = 40329846 kbps (40 %)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
Guaranteed service rate = 40000000 kbps
TailDrop Threshold = 50069504 bytes / 10 ms (default)
WRED not configured for this class
Level1 Class = class-default
Egressq Queue ID = 11176 (Default LP queue)
Queue Max. BW. = 101803495 kbps (default)
Queue Min. BW. = 0 kbps (default)
Inverse Weight / Weight = 1 (BWR not configured)
Guaranteed service rate = 50000000 kbps
TailDrop Threshold = 62652416 bytes / 10 ms (default)
WRED not configured for this class
Shaper rates are rounded to approximately 4 Mbps and accuracy can be off by 5% in the worst case. A shaper is programmed per
member on a bundle interface.
Traffic policing allows you to control the maximum rate of traffic sent or received on an interface and to partition a network
into multiple priority levels or class of service (CoS).Traffic policing manages the maximum rate of traffic through a token
bucket algorithm. The token bucket algorithm uses user-configured values to determine the maximum rate of traffic allowed
on an interface at a given moment in time. The token bucket algorithm is affected by all traffic entering or leaving the interface
(depending on where the traffic policy with traffic policing is configured) and is useful in managing network bandwidth in
cases where several large packets are sent in the same traffic stream. By default, the configured bandwidth value takes into
account the Layer 2 encapsulation that is applied to traffic leaving the interface.
Traffic policing also
provides a certain amount of bandwidth management by allowing you to set the
burst size (Bc) for the committed information rate (CIR). See,
Committed Bursts and Excess Bursts.
The router supports the following traffic policing mode(s):
Single-Rate Three-Color (SR3C) in color-blind mode.
Two-Rate Three-Color (2R3C) in color-blind mode. See Two-Rate Policer .
Restrictions
Traffic policing
is supported only in ingress direction, and only color-blind mode is supported.
Policer marking is not supported.
Policers are configured in the interface at the core level and “show qos int <>” value is displayed at the NPU level.
For policers configured in a bundle interface where bundle members are from the same NPU but different cores (NPU cores),
each member sends the traffic up to the core level policer configuration, but “show qos int <>” displays the NPU level policer
output.
Example:
For bundle interface with two 10GE members (same NPU, but one interface from core0, one interface from core1) 2R3C policer
applied on bundle interface (1G confirm rate , 1G exceed rate – total 2G policer rate) will be shown on the “show qos int
<>” output):
For traffic in one out of two interfaces, the policed rate will be 1Gbps. For traffic on two interfaces, policed rate will
be 2Gbps.
Committed Bursts and Excess Bursts
Unlike a traffic shaper, a traffic policer does not buffer excess packets and transmit them later. Instead, the policer executes
a “send or do not send” policy without buffering. Policing uses normal or committed burst (bc) values and excess burst values (be) to ensure that the router reaches the configured committed information rate (CIR). Policing decides if a packet conforms
or exceeds the CIR based on the burst values you configure. Burst parameters are based on a generic buffering rule for routers,
which recommends that you configure buffering to be equal to the round-trip time bit-rate to accommodate the outstanding TCP
windows of all connections in times of congestion. During periods of congestion, proper configuration of the excess burst parameter enables the policer to drop packets less aggressively.
A single-rate
two-color (SR2C) policer provides one token bucket with two actions for each
packet: a conform action and an exceed action.
Figure 1. Workflow of
Single-Rate Two-Color Policer
Based on the committed information rate (CIR) value, the token bucket is updated at every refresh time interval. The Tc token
bucket can contain up to the Bc value, which can be a certain number of bytes or a period of time. If a packet of size B is
greater than the Tc token bucket, then the packet exceeds the CIR value and a default action is performed. If a packet of size B is less than the Tc token bucket, then the packet conforms and a different default action is performed.
Single-Rate Three-Color Policer
A single-rate three-color (SR3C) policer provides one token bucket with three actions for each packet: a conform action, an
exceed action and a violate action. The packet is marked based on the CIR value and the two associated burst size - committed
burst size (CBS) and excess burst size (EBS). If a packet does not exceed the CBS, it is marked as conformed packet. The packet
is marked as exceeded if it exceeds CBS, but not the EBS. If it exceeds the EBS as well, it is marked as violate packet.
Traffic policing is often configured on interfaces at the edge of a network to limit the rate of traffic entering or leaving
the network. The default conform action for single-rate two color policer is to transmit the packet and the default exceed
action is to drop the packet. Users cannot modify these default actions.
Configuration
Example
You have to
accomplish the following to complete the traffic policing configuration:
Creating or
modifying a policy-map that can be attached to one or more interfaces
Specifying the
traffic class whose policy has to be created or changed
The default conform action and exceed actions for single-rate three-color policer are to transmit the packet and the default
violate action is to drop the packet. User cannot modify these default actions.
Configuration
Example
You have to
accomplish the following to complete the traffic policing configuration:
Creating or
modifying a policy-map that can be attached to one or more interfaces
Specifying the
traffic class whose policy has to be created or changed
(Optional)
Specifying the marking action
Configuring the
policy rate for the traffic along with the peak-burst values
The two-rate policer manages
the maximum rate of traffic by using two token buckets: the committed token
bucket and the peak token bucket. The dual-token bucket algorithm uses
user-configured values to determine the maximum rate of traffic allowed on a
queue at a given moment. In this way, the two-rate policer can meter traffic at
two independent rates: the committed information rate (CIR) and the peak
information rate (PIR).
The dual-token bucket algorithm provides users with three actions for each packet—a conform action, an exceed action, and
an optional violate action. Traffic entering a queue with the two-rate policer configured is placed into one of these categories.
The actions are pre-determined for each category. The default conform and exceed actions are to transmit the packet, and the
default violate action is to drop the packet.
This figure shows how the
two-rate policer marks a packet and assigns a corresponding action to the
packet.
Figure 2. Marking Packets and Assigning
Actions—Two-Rate Policer
The router supports Two-Rate Three-Color (2R3C) policer.
Configure Traffic
Policing (Two-Rate Three-Color)
The default conform and exceed actions for two-rate three-color (2R3C) policer are to transmit the packet and the default
violate action is to drop the packet. Users cannot modify these default actions.
Configuration Example
You have to accomplish the following to complete the two-rate three-color traffic policing configuration:
Creating or modifying a policy-map that can be attached to one or more interfaces
Specifying the traffic class whose policy has to be created or changed
Router# show policy-map interface HundredGigE 0/6/0/18
NOTE:- Configured values are displayed within parentheses
Interface HundredGigE0/6/0/18 ifh 0x3000220 -- input policy
NPU Id: 3
Total number of classes: 8
Interface Bandwidth: 100000000 kbps
Accounting Type: Layer1 (Include Layer 1 encapsulation and above)
------------------------------------------------------------------------------
Level1 Class = ipv4-4
- - -
- - -
Level1 Class = ipv4-7
New qos group = 4
Policer Bucket ID = 0x102a3
Policer Stats Handle = 0x8a8089e8
Policer committed rate = 19980000 kbps (20 %)
Policer peak rate = 49860000 kbps (50 %)
Policer conform burst = 99584 bytes (100000 bytes)
Policer exceed burst = 199168 bytes (200000 bytes)
Level1 Class = class-default
Policer Bucket ID = 0x102a7
Policer Stats Handle = 0x8a7c8510
Policer committed rate = 29880000 kbps (30 %)
Policer conform burst = 4194304 bytes (default)
The minimum policer rate is 22 kbps for commit rate and a difference of 22 kbps is expected between commit and excess rates.
Any value lower than 22 kbps is rounded up to 22 kbps in the hardware.
A policer is programmed per NPU core on a bundle interface. So, all members on a bundle interface from the same core share
the policer.
Shaper and policer rates can be configured in units of per-thousand and per-million on bundle interfaces. This provides the
ability to provision shape and police rates down to 100 kbps on bundle or link aggregation (LAG) interfaces even with 100
GE bundle members.
For example, consider a 100GE interface and simple policy.
Interface HundredGig0/0/0/0
Service-policy output TEST
Policy-map TEST
Class C
Shape average per-thousand 5
End-policy
Per thousand represents 0.1% of the link bandwidth and per million represents 0.0001% of the link bandwidth.
Which means that for a 100G link, 5 parts per thousand is 0.5% of the link bandwidth. Hence, the shape average per thousand
of 5 in the above example enforces a shaper of 500 Mbps.
References for Modular QoS Congestion Management
Committed Bursts
The committed burst (bc)
parameter of the police command implements the first, conforming (green) token
bucket that the router uses to meter traffic. The bc parameter sets the size of
this token bucket. Initially, the token bucket is full and the token count is
equal to the committed burst size (CBS). Thereafter, the meter updates the
token counts the number of times per second indicated by the committed
information rate (CIR).
The following describes how
the meter uses the conforming token bucket to send packets:
If sufficient tokens are in
the conforming token bucket when a packet arrives, the meter marks the packet
green and decrements the conforming token count by the number of bytes of the
packet.
If there are insufficient
tokens available in the conforming token bucket, the meter allows the traffic
flow to borrow the tokens needed to send the packet. The meter checks the
exceeding token bucket for the number of bytes of the packet. If the exceeding
token bucket has a sufficient number of tokens available, the meter marks the
packet.
Green and decrements the
conforming token count down to the minimum value of 0.
Yellow, borrows the remaining
tokens needed from the exceeding token bucket, and decrements the exceeding
token count by the number of tokens borrowed down to the minimum value of 0.
If an insufficient number of
tokens is available, the meter marks the packet red and does not decrement
either of the conforming or exceeding token counts.
Note
When the meter marks a packet
with a specific color, there must be a sufficient number of tokens of that
color to accommodate the entire packet. Therefore, the volume of green packets
is never smaller than the committed information rate (CIR) and committed burst
size (CBS). Tokens of a given color are always used on packets of that color.
Excess Bursts
The excess burst (be)
parameter of the police command implements the second, exceeding (yellow) token
bucket that the router uses to meter traffic. The exceeding token bucket is
initially full and the token count is equal to the excess burst size (EBS).
Thereafter, the meter updates the token counts the number of times per second
indicated by the committed information rate (CIR).
The following describes how
the meter uses the exceeding token bucket to send packets:
When the first token bucket
(the conforming bucket) meets the committed burst size (CBS), the meter allows
the traffic flow to borrow the tokens needed from the exceeding token bucket.
The meter marks the packet yellow and then decrements the exceeding token
bucket by the number of bytes of the packet.
If the exceeding token bucket
does not have the required tokens to borrow, the meter marks the packet red and
does not decrement the conforming or the exceeding token bucket. Instead, the
meter performs the exceed-action configured in the police command (for example,
the policer drops the packets).
Two-Rate Policer
Details
The committed token bucket
can hold bytes up to the size of the committed burst (bc) before overflowing.
This token bucket holds the tokens that determine whether a packet conforms to
or exceeds the CIR as the following describes:
A traffic stream is
conforming when the average number of bytes over time does not cause the
committed token bucket to overflow. When this occurs, the token bucket
algorithm marks the traffic stream green.
A traffic stream is exceeding
when it causes the committed token bucket to overflow into the peak token
bucket. When this occurs, the token bucket algorithm marks the traffic stream
yellow. The peak token bucket is filled as long as the traffic exceeds the
police rate.
The peak token bucket can
hold bytes up to the size of the peak burst (be) before overflowing. This token
bucket holds the tokens that determine whether a packet violates the PIR. A
traffic stream is violating when it causes the peak token bucket to overflow.
When this occurs, the token bucket algorithm marks the traffic stream red.
For example, if a data stream
with a rate of 250 kbps arrives at the two-rate policer, and the CIR is
100 kbps and the PIR is 200 kbps, the policer marks the packet in the following
way:
100 kbps conforms to the rate
100 kbps exceeds the rate
50 kbps violates the rate
The router updates the tokens
for both the committed and peak token buckets in the following way:
The router updates the
committed token bucket at the CIR value each time a packet arrives at the
interface. The committed token bucket can contain up to the committed burst
(bc) value.
The router updates the peak
token bucket at the PIR value each time a packet arrives at the interface. The
peak token bucket can contain up to the peak burst (be) value.
When an arriving packet
conforms to the CIR, the router takes the conform action on the packet and
decrements both the committed and peak token buckets by the number of bytes of
the packet.
When an arriving packet
exceeds the CIR, the router takes the exceed action on the packet, decrements
the committed token bucket by the number of bytes of the packet, and decrements
the peak token bucket by the number of overflow bytes of the packet.
When an arriving packet
exceeds the PIR, the router takes the violate action on the packet, but does
not decrement the peak token bucket.