CCNP 3 v4 Module 8
Configuring Campus Switches to
Support Voice and Video
Applications
© 2003, Cisco Systems, Inc. All rights reserved.
1
Objectives
•
Accommodating Voice Traffic on Campus
Switches
•
Configuring IP Multicast
© 2003, Cisco Systems, Inc. All rights reserved.
2
Overview
• Campus networks carry a variety of data
with diverse purposes and impacts on
resources.
• Proper design and configuration efforts
will ensure that voice, video and data
traffic efficiently coexist on a single
Campus Infrastructure.
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3
Cisco Infrastructure
•
Cisco recommends an end-to-end single
vender (Cisco) solution.
•
This way, each new application such as
video, Web, or telephony represents just
another media type over the same
infrastructure.
– Tasks such as QoS configuration and
network upgrades are made easier by using
a single vendor.
© 2003, Cisco Systems, Inc. All rights reserved.
4
IP Telephony Integration
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5
Voice VLANs
•
Cisco Catalyst switches offer a "voice VLAN"
feature.
–
•
The voice VLAN, also known as an auxiliary VLAN,
provides automatic VLAN association for IP
phones.
Voice traffic is on a specific VLAN, and IP
subnet even though voice and data co-exist on
the same physical infrastructure.
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6
Voice VLANs
When a phone is connected to the switch, the switch sends necessary
voice VLAN information to the IP phone.
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7
Voice VLANs and Data VLANs
•
Placing phone traffic onto a distinct
VLAN allows the phone traffic to be
segmented from the data traffic.
•
QoS or security policies can be enforced
specifically for the traffic traversing the
phone VLANs without affecting the data
traffic.
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8
Connecting a PC to the IP Phone
•
To save switchport density and cable runs, a PC can be
connected to the integrated switch of the IP Phone.
•
In order for the device and the phone to communicate, one
of the following must be true:
–
They both use the same Layer 2 frame type.
–
The phone uses 802.1p frames and the device uses
untagged frames.
–
The phone uses untagged frames and the device uses
802.1p frames.
–
The phone uses 802.1Q frames, and the voice VLAN
equals the native VLAN.
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9
Connecting a PC to the IP Phone
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10
Voice Design Considerations
•
Deploying IP telephony in the enterprise campus requires
the implementation of various features particular to each
submodule.
•
Within the Building Access submodule, these features
support IP telephony:
–
Voice VLANs
–
802.1p/Q
–
Hardware support for multiple output queues
–
Hardware support for in-line power to IP phones
–
PortFast
–
Root Guard
–
Unidirectional Link Detection (UDLD)
–
UplinkFast
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11
IP Telephony on the Network
•
IP telephony places strict requirements on
the network infrastructure.
•
Most IP telephony installations are built
on an existing network infrastructure.
– To support voice traffic the network may
require enhancements and upgrades with
priority given to voice traffic.
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12
Campus Infrastructure Considerations
•
What features are required for each network device?
–
•
Can the physical plan support IP Telephony?
–
•
PoE on the switch or a separate inline power patch panel,
power bricks
Is adequate bandwidth available?
–
•
Cat5e minimum, available switchports and wall jacks
How will the phones be powered?
–
•
VLAN configuration, QoS, inline power
What other bandwidth intensive applications are running?
Will a VoIP implementation require an complete network
overhaul?
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13
Quality of Service
•
QoS is the application of features and
functionality required to actively manage
and satisfy networking requirements of
applications sensitive to loss, delay, and
delay variation (jitter).
•
QoS allows preference to be given to
critical application flows for the available
bandwidth.
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14
QoS and Voice Traffic
•
Congestion and latency can be caused by speed
mismatches, many-to-one switching fabrics and
aggregation.
•
When packets are dropped due to network
congestion, these packets must be retransmitted,
causing further congestion.
–
QoS ensures that prioritized voice traffic is not subject
to the existing network congestion and latency.
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15
Switchport Commands for VoIP QoS
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16
Switch Configuration Example
Switch(config)#interface fastethernet 0/4
Switch(config-if)#switchport voice vlan 110
Switch(config-if)#mls qos trust cos
Switch(config-if)#mls qos trust device cisco-phone
Switch(config-if)#ctrl-Z
Switch#show interfaces fastethernet 0/4
Switch#show mls qos interface fastethernet 0/4
FastEthernet0/4
trust state: trust cos
trust mode: trust cos
COS override: dis
default COS: 0
pass-through: none
trust device: cisco-phone
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17
Step-by-Step Configuration
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18
QoS by Network Layer
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19
Delay and Packet Loss
•
Delay (or latency) is the amount of time that it takes a packet
to reach the receiving endpoint from the sending endpoint.
–
This time period is termed the "end-to-end delay"
–
End-to-end delay can be broken into two areas:
•
Fixed network delay
•
Variable network delay
•
Fixed network delay includes encoding and decoding time
(for voice and video), as well as the amount of time required
to traverse the media en route to the destination.
•
Variable network delay refers to network conditions, such as
congestion, that may affect the overall time required for
transit.
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20
Types of Delay
•
Packetization delay – The amount of time that it takes to
segment data, sample and encode signals, process data,
and turn the data into packets
•
Serialization delay – The amount of time that it takes to
place the bits of a packet encapsulated in a frame, onto the
physical media
Propagation delay – The amount of time that it takes to
transmit the bits of a frame across the physical wire
Processing delay – The amount of time that it takes for a
network device to take the frame from an input interface,
place it into a receive queue, and then place it into the
output queue of the output interface
Queuing delay – The amount of time that a packet resides
in the output queue of an interface
Delay variation – Delay variation (or jitter) is the difference
in the end-to-end delay between packets.
•
•
•
•
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21
Classification and Marking
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22
Layer 2 Marking: 802.1p and CoS
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23
Layer 3 Marking: ToS, IP Precedence, DSCP
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24
Best Effort
•
Best-effort is a single service model in which
an application sends data whenever it must, in
any quantity, without requesting permission or
first informing the network.
•
Best-effort service is suitable for a wide range
of networked applications such as general file
transfers, e-mail and Web browsing.
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25
Differentiated Services
•
The Differentiated Services or DiffServ is
an IETF architecture standard.
•
This architecture specifies that each
packet is classified upon entry into the
network.
– The classification is carried in the IP packet
header, using either the IP precedence or
the preferred Differential Services Code
Point (DSCP).
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26
Precedence and DSCP
•
•
Represented using the first three (precedence) or
six (DSCP) bits of the Type of Service (ToS) field.
–
The first 3 DSCP bits are the class selector bits
–
The second 3 DSCP bits are the drop precedence bits
Classification can also be carried in the Layer 2
frame in the form of the Class of Service (CoS)
field embodied in ISL and 802.1Q frames.
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27
DSCP Code Points
Assured Forwarding - AF
Expedited Forwarding - EF
Class Selector - Priority
Drop Precedence - Priority
Internetwork Control
Class 6
110
48 – 55
Network Control
Class Selector Bits
Class 5
101
40 – 47 (46)
Class 7
111
56 – 63
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28
Layer 2 and 3 DiffServ
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29
Layer 2 and QoS
•
At the Datalink layer a raw Ethernet frame has
no fields to signify its QoS requirements.
•
If QoS marking is required, then ISL or
802.1Q/p must be used as these provide a
three-bit Class of Service (CoS) field.
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30
Layer 3 and QoS
•
At the Network layer an IP packet contains a one
byte Type of Service (ToS) field, of which the first
three bits form the IP-Precedence field and the
first six bits form the DSCP fields.
•
Either of these can be used to signify the QoS
requirements of an IP packet but not both.
•
DSCP has precedence
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31
QoS, CoS and ToS
CoS
ToS – IP Precedence
ToS – DSCP
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32
Modular QoS CLI (MQC)
•
The Modular QoS Command Line Interface or MQC is
central to Cisco’s model for implementing IOS based QoS
solutions.
•
The MQC breaks down the tasks associated with QoS into
modules that:
•
–
Identify traffic flows.
–
Classify traffic flows as belonging to a common class of
QoS.
–
Apply QoS policies to that class.
–
Define the interfaces on which the policy should be
enforced.
The modular nature of MQC allows the reuse of common
traffic classes and policies.
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33
Creating Class-maps
•
The class-map command is used to define a
traffic class.
•
The purpose of a traffic class is to classify
traffic that should be given a particular QoS.
•
A traffic class contains three major elements:
1. a name - cisco
2. a series of match commands - match
3. and if more than one match command exists in the
traffic class, how to evaluate these match commands
match-all | match-any
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34
Class-map Commands
switch(config)#ip access-list standard test
Switch(config)#class-map match-any cisco
Switch(config-cmap)#match access-group name test
Switch(config-cmap)#match interface fastethernet 0/1
•
On the Catalyst 3550 and 6500 the Modular
QoS CLI allows multiple traffic classes to be
configured as a single traffic class, such as
nested traffic classes, or nested class maps.
•
This nesting can be achieved with the use of
the match class-map command.
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35
Policy-maps
•
The policy-map command is used to create a
traffic policy.
•
The purpose of a traffic policy is to configure
the QoS features to be associated with the
traffic that has been classified in the traffic
class.
•
Traffic policy contains three elements:
1. Policy Name
2. Traffic class specified with the class command
3. QoS policies to be applied to each class
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36
Policy and Class-map Commands
Switch(config)#policy-map policy1
Switch(config-pmap)#class cisco
Switch(config-pmap-c)#bandwidth 3000
Switch(config-pmap-c)#exit
Switch(config-pmap)#class class-default
Switch(config-pmap-c)#bandwidth 2000
Switch(config-pmap)#exit
•
The service policy command is used to attach
the traffic policy to an interface.
Switch(config)#interface fastethernet 0/1
Switch(config-if)#service-policy output policy1
Switch(config-if)#exit
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Apply to outgoing packets
37
Classification at Access Layer
•
In order to be effective, QoS should be
implemented end-to-end within a network as
soon as possible at the network edge or
access layer.
•
Frames and packets can be marked as
important by using Layer 2 Class of Service
(CoS) settings in the User Priority bits of the
802.1p portion of the 802.1Q header
or
•
The IP Precedence/Differentiated Services
Code Point (DSCP) bits in the Type of Service
(ToS) Byte of the IPv4 header
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38
Trust – Do you trust me?
•
In order to take advantage of COS at the edge
then the access layer device must “trust” the
QoS devices/applications it is connected to.
•
The default action is for a switch with QoS
features activated not to trust edge devices that
have written CoS features into the frame.
–
•
Any frames that enter the switch will have their CoS
re-written to the lowest priority of zero.
If the edge device can be trusted then the switch
will switch the frame without changing the Cos
setting.
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39
Trusted vs. Untrusted Ports
Trusted
Untrusted
Trusted
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40
QoS Trust Boundaries
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41
Class of Service at the Switch
•
Depending on the switch model, it may be
necessary to first activate QoS:
switch(config)#mls qos
•
This command is required on both the Catalyst
3550 and the Catalyst 6500.
–
•
The Catalyst 2950 has QoS enabled by default.
The trust is configured on the switch port
using the command:
switch(config-if)#mls qos trust cos
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42
Remember Native VLAN?
•
If an untagged frame arrives at the switch port,
the switch will assign a default CoS to the
frame before forwarding it. (native VLAN)
•
By default untagged frames are assigned a
CoS of zero.
•
This can be changed using the interface
configuration command:
switch(config-if)#mls qos cos [cos-value]
–
Where [cos-value] is a number between 0 and 7.
–
Traffic that passes through the port will be
automatically tagged with the new CoS value.
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43
Override the CoS Field
•
In some cases it may be desirable not to trust
any CoS value that may be present in frames
sourced from an edge device.
•
For this reason, it is possible to use the
override parameter to tell the switch to ignore
any existing CoS value that may be in the frame
and apply the default value.
switch(config-if)#mls qos cos [cos-value]
Switch(config-if)#mls qos cos override
–
This will re-write the CoS value for any frame entering
the switch port to the default setting.
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44
MAC ACL to Assign DSCP
•
It is not always possible to classify the
CoS of a frame, based on an ingress port.
•
The ingress port may be attached to a hub
or a simple workgroup switch that does
not support QoS.
– This hub or switch may be connecting to
multiple workstations that all require
different CoS values.
– Differing types of devices may be on the
same subnet (IP ACL will not work)
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45
MAC ACL to Assign DSCP
•
Not all frames can be assigned a CoS based on
ingress port
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46
Configure a MAC ACL
•
However, in the QoS context, the permit and deny
actions in the access control entries (ACEs) have
different meanings than with security ACLs:
–
If a match with a permit action is encountered, known
as the first-match principle, the specified QoS-related
action is taken.
–
If a match with a deny action is encountered, the ACL
being processed is skipped, and the next ACL is
processed.
–
If no match with a permit action is encountered and all
the ACLs have been examined, no QoS processing
occurs on the packet.
Switch(config)#mac access-list extended [name]
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47
MAC ACL Example
Switch(config)#mac access-list extended receptionph
Switch(config-ext-macl)#permit host 000.0a00.0111 any
Switch(config-ext-macl)#exit
Switch(config)#
Switch(config)#class-map match-all ipphone
Switch(config-cmap)#match access-group name receptionph
Switch(config-cmap)#exit
Switch(config)#policy-map inbound-accesslayer
Switch(config-pmap)#class ipphone
Switch(config-pmap-c)#set ip dscp 40
Switch(config-pmap-c)#exit
Switch(config)#interface range fastethernet 0/1 - 24
config-if-range)#service-policy input inbound-accesslayer
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48
Using an IP ACL
•
Using the Modular QoS Command Line Interface (MQC) it is
possible to classify traffic based on its IP or TCP properties
•
In this FTP example, an IP ACL is used to identify the
packets:
Switch(config)#ip access-list extended 100
Switch(config-ext-nacl)#permit tcp any any eq ftp
•
Traffic is classified as “reducedservice” if it is permitted by
the access list.
Switch(config)#class-map reducedservice
Switch(config-cmap)#match access-group 100
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49
Policing and Marking
“out of profile”
•
Traffic policing involves placing a constraint on
the maximum traffic rate.
•
When the traffic rate reaches the configured
maximum rate, excess traffic is dropped or
remarked to a lower DSCP value
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50
Policing Flow Chart
Packets that exceed the limits are said
to be “out of profile” or nonconforming.
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51
Committed Access Rate (CAR)
•
CAR implements both classification services and policing
through rate limiting.
•
The classification services of CAR allow traffic flow limits
to be placed on incoming traffic.
•
These limits specify the average rate, rate-bps, and the
burst rate, burst-byte, that is permissible.
–
Traffic that is nonconforming either because it exceeds
the average rate or the burst rate specified can be
marked down in terms of DSCP.
–
Traffic is then dropped based on the new DSCP value as
part of congestion avoidance
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52
CAR Configuration
•
The policy-map command that enables
CAR is 'police' and is specified for a
given class of traffic.
Switch(config)#police [rate-bps] [burst-bps] [exceedaction {drop | policed-dscp-transmit}]
•
In order to mark down the DSCP value of
nonconforming traffic, the switch uses a
map to translate between the initial
DSCP value and the marked down DSCP.
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53
Configuring Classification using CAR
•
Create an IP standard ACL to permit
traffic, this will be used to match traffic.
•
Traffic that matches this ACL will receive
a DSCP value in the incoming packet is
trusted
•
In the following example, traffic that
exceeds an average traffic rate of 48000
bps and a normal burst size of 8000
bytes is marked down.
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54
CAR Example – drop
Switch(config)#access-list 1 permit 10.1.0.0 0.0.255.255
Switch(config)#class-map ipclass1
Switch(config-cmap)#match access-group 1
Switch(config-cmap)#exit
Switch(config)#policy-map flow1t
Switch(config-pmap)#class ipclass1
Switch(config-pmap-c)#trust dscp
Switch(config-pmap-c)#police 48000 8000 exceed-action drop
Switch(config-pmap-c)#exit
Switch(config-pmap)#exit
Switch(config)#interface gigabitethernet0/1
Switch(config-if)#service-policy input flow1t
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55
Scheduling
•
The process of assigning packets to one of
multiple queues, based on classification, for
priority treatment through the network is called
scheduling.
•
Examples of different scheduling techniques
are:
–
First In First Out - FIFO
–
Weighted Fair Queuing - WFQ
–
Class Based Weighted Fair Queuing - CBWFQ
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56
First In First Out
•
The simplest form of scheduling and the
default for interfaces 2 Mbps and faster.
•
The FIFO queue offers no preferential service
for traffic, packets are forwarded in the order
they are received.
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57
Weighted Fair Queuing
•
Weighted Fair Queuing (WFQ) classifies traffic entering the
queue based on traffic flows.
–
Classification can be based on source and destination
addresses, the protocol and TCP port numbers
•
Each flow is given its own queue.
•
WFQ services each of these queues on a round robin
basis.
–
•
Every flow of traffic has an equal share of the available
bandwidth
In some cases, the “weight” needs to be modified so that
WFQ does not share bandwidth on a round-robin basis,
but is influenced by the class or priority of the traffic in the
flow.
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58
Weighted Fair Queuing
Weighted fair queuing is activated on
a Layer 3 interface:
Router(config)#interface serial 0/0
Router(config-if)#fair-queue
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59
WFQ and IP Precedence
•
WFQ is IP precedence-aware.
•
WFQ can detect higher priority packets
marked with precedence and schedule
them faster.
– Higher priority packets are assigned a
lower weight and a greater share of the total
bandwidth
•
In order for WFQ to be truly fair, every
flow would have to have the same
precedence.
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60
Weight and Precedence
•
Weight is calculated inversely to precedence.
–
The higher the precedence, the lower the weight
W=K/precedence + 1
K = 4096 with Cisco IOS 12.0(4)T and earlier
releases, and 32384 with 12.0(5)T and later
releases.
•
Bandwidth is proportional to precedence.
–
Each flow will get precedence + 1 parts of the link
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 = 36
Therefore, precedence 0 traffic will get 1/36
of the bandwidth, precedence 1 traffic will get
2/36, and precedence 7 traffic will get 8/36.
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61
Class Based WFQ (CBWFQ)
•
Allows for user defined traffic classes
using match criteria including protocols,
ACLs, and input interfaces.
– CBWFQ provides for up to 64 classes -WFQ is limited to 7 classifications (queues)
•
Once a class has been defined according
to its match criteria, characteristics can
be assigned to it.
– To characterize a class, bandwidth, weight,
and maximum packet limit are specified.
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62
CBWFQ Class Characteristics
•
The bandwidth assigned to a class is the
guaranteed bandwidth delivered to that class
during congestion.
•
After a queue has reached its configured packet
limit, queuing of additional packets to the class
causes further packets to be dropped.
•
A default class can be configured with a
'bandwidth' policy-map class configuration
command, for all unclassified traffic
–
This traffic is put into a single FIFO or WFQ queue
and given treatment according to the configured
bandwidth.
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63
CBWFQ Example
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64
Configuring CBWFQ
Router(config)#mls qos
Router(config)#class-map prioritytraffic
Router(config-cmap)#match dscp 50
Router(config)#policy-map prioritybw
Router(config-pmap)#class class-default fair-queue
Router(config-pmap-c)#class prioritytraffic bandwidth
percent 40 queue-limit 200
Router(config)#interface gigabitethernet0/1
Router(config-if)#service-policy output prioritybw
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65
END PART 1
PART 1 STOP HERE
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66
Multicast Traffic
•
IP Multicast is an efficient means of delivering
bandwidth intensive content to many hosts over
a single IP flow.
–
•
Multimedia such as streaming video
IP Multicast is the transmission of an IP data
frame to a host group that is defined by a single
IP Multicast address.
–
Multicasting conserves bandwidth by replicating
packets only onto segments or individual switchports
where listening devices exist
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67
IP Multicast
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68
IP Multicast Characteristics
•
Delivers a multicast datagram to a destination multicast
address (also known as a multicast group) with the same
best-effort reliability as a regular unicast IP datagram
•
•
Allows group members to join and leave dynamically
Supports all host groups regardless of the location or
number of members
Supports the membership of a single host in one or more
multicast groups
Can carry multiple data streams to a single group address
•
•
•
Can use a single group address for multiple host
applications
•
Multicast server does not keep track of the number of
recipients
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69
Multicast at the Transport Layer
•
Multicast traffic is handled at the transport
layer using the User Datagram Protocol
(UDP).
•
Because of the simplicity of UDP, data
packet headers contain fewer bytes and
consume less network overhead than TCP.
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70
IP Multicast Group Membership
•
IP multicast relies on the concept of group
members and a group address.
–
•
The group address is a single IP Multicast address
that is the destination address of all packets sent
from a source.
Receiving devices join that group and listen for
packets with the destination IP address of the
group.
–
Essentially, the destination address is the group
since all multicast group members will receive data
at that destination address.
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71
IP Multicast Group Example
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72
Multicast Addresses
•
•
•
Multicast uses Class D IP address space.
– Class D = 224.0.0.0 – 239.255.255.255
Class D address consists of 1110 as the high-order bits in
the first octet, followed by a 28-bit group address.
– The last 28 bits of the IP address identify the multicast
group ID.
– Multicast addresses may be dynamically or statically
allocated.
Multicast IP addresses map directly to a range of MAC
addresses which allows an IP multicast group to be
translated to a group of hosts on an Ethernet LAN.
– Every host that is a member of that multicast group will
begin listening for traffic at the MAC address that
matches the IP multicast address.
http://www.iana.org/assignments/multicast-addresses
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73
Well-known Layer 3 Multicast Address
224.0.0.1
All multicast-capable hosts on the segment
224.0.0.2
All multicast-capable routers on the segment
224.0.0.4
All DVMRP routers on the segment
224.0.0.5
All OSPF routers
224.0.0.6
All OSPF designated routers
224.0.0.9
All RIPv2 routers
224.0.0.13
All PIM routers
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IP Multicast to MAC Address Mapping
5
01-00-5e identifies the frame as multicast
Only the MAC address range from 0100.5e00.0000 through
0100.5e7f.ffff is the available for carrying multicast frames.
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Multicast MAC Calculation
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The Missing 5 bits
•
Because the first 5 bits of the lower 28
bits are unused, not all multicast IP
address to multicast MAC address
mappings are unique.
– This means that there are 25 IP addresses
that will map to any one MAC address.
224 – 239. X±128 . X . X
0000.0
8 4 2 1.128
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IP to MAC Address Examples
224.10.8.5 = 0100.5e0a.0805
224.138.8.5 = 0100.5e0a.0805
225.10.8.5 = 0100.5e0a.0805
239.138.8.5 = 0100.5e0a.0805
239.138.24.5 = 0100.5e0a.1805
224.74.9.13 = 0100.5e4a.090d
As long as the last 23 bits do not change, you will always get the same
MAC address.
However, if we change any of the last 23 bits, we get a different MAC.
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78
Reverse Path Forwarding
•
•
Multicast-capable routers create distribution
trees that control the path that IP multicast
traffic takes through the network.
–
Multicast traffic is forwarded away from the source
rather than toward the receiver.
–
This is called Reverse Path Forwarding (RPF)
Multicast-capable routers create distribution
trees that control the path that IP multicast
traffic takes through the network, away from
the source.
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79
Reverse Path Forwarding
Traffic flows away from the source.
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80
Multicast Distribution Trees
•
Multicast distribution trees fall into the
categories:
1. Source based trees
2. Shared trees
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81
Source Distribution Trees
•
•
A source tree is the simplest form of a multicast
distribution tree.
–
A source tree has its root at the source and branches
forming a tree through the network toward the receivers.
–
“shortest path tree” (SPT)
An SPT is identified by a special notation of (S, G),
where S is the IP address of the source and G is the
multicast group address to which receivers belong.
–
Source trees are used for PIM Dense Mode (PIM-DM)
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82
Source Distribution Tree
(S,G) Notation
(192.168.1.1, 224.1.1.1)
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83
Shared Distribution Trees
•
Unlike source trees that have their root at
the source, shared trees use a single
common root placed at a chosen point in
the network.
– This shared root is called a "rendezvous
point (RP)."
– Multicast traffic is then forwarded from the
RP to reach all of the receivers.
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84
Shared Distribution Tree
Multicast traffic from the sources (hosts A and D)
travels to the RP (router D) and then down the
tree to the two receivers (hosts B and C).
(*, G) Notation
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85
Source Trees vs. Shared Trees
•
•
Source trees have the advantage of creating the optimal
path between the source and the receivers.
– This guarantees the minimum amount of network latency.
– However, the routers must maintain path information for
each source which can quickly drain the router’s
resources.
Shared trees consume less memory resources from the
router since fewer paths are created.
– However, since one shared distribution tree is used for
all source to receiver paths, the path any one source
uses may not be optimal.
– Multicast traffic must first get to the rendezvous point
and then from the RP to the receiver.
© 2003, Cisco Systems, Inc. All rights reserved.
86
Reverse Path Forwarding (RPF) Check
•
In multicast forwarding, the source sends traffic to a group
of hosts represented by a multicast group address.
•
The multicast router determines which direction is
upstream (toward the source) and which is downstream
(toward the receivers).
–
•
If there are multiple downstream paths, the router
replicates the packet down all appropriate downstream
paths (interfaces).
When a multicast packet arrives at a router, the router will
perform an RPF check on the packet.
–
If the check is successful, the router will forward the
packet. If the check fails, the packet is dropped.
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87
Reverse Path Forwarding Check
•
This RPF check is used to guarantee that the
distribution tree is loop-free.
•
RPF uses the unicast routing table to validate
from which interface upstream multicast traffic
should arrive.
–
When a packet arrives at one of the router’s
interfaces, the router compares the source address
to the unicast routing table.
–
If a packet has arrived on the interface leading back
to the source, the RPF check is successful and the
packet will be forwarded.
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88
RPF Check Example
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89
Multicast Protocols
•
•
In order to gain the benefits of using multicast
to send data, network devices must be
configured to support multicast.
–
Otherwise network devices will treat multicast
traffic like broadcast traffic.
–
By default, Layer 3 devices block multicast traffic.
Devices must be configured to support
multicast to ensure that the multicast traffic is
contained only to those network segments that
have group members.
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90
IP Multicast Protocols
•
Internet Group Management Protocol (IGMP)
–
and IGMP Snooping
•
Cisco Group Management Protocol (CGMP)
•
Protocol Independent Multicast (PIM)
–
PIM Dense Mode (PIM-DM)
–
PIM Sparse Mode (PIM-SM)
–
PIM Sparse-dense Mode
More on this later…
© 2003, Cisco Systems, Inc. All rights reserved.
91
Internet Group Management Protocol (IGMP)
•
•
IGMP is used to register individual hosts with a multicast
group that want to receive the multicast traffic.
–
There are three versions of IGMP (IGMPv1 - 3)
–
IGMPv1 is defined by RFC 1112, v2 is RFC 2236 and v3 is
RFC 3376.
IGMP uses “queriers” and “hosts”.
–
Querier is the router
–
The set of queriers and hosts make up the multicast group
•
The router (querier) sends query messages to discover
which hosts are members of the multicast group.
•
Hosts then send report messages in response to the query
message to inform the router of their membership.
http://www.networksorcery.com/enp/protocol/igmp.htm
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92
IGMPv1 and v2 Packet Format
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93
Joining a Multicast Group
•
•
•
IGMPv1 was designed to allow hosts to join a multicast
group.
Multicast routers send periodic membership queries to
determine if there is a host on a segment (router’s interface)
that belongs to a multicast group.
– The routers sends the membership query to the all hosts
multicast address, 224.0.0.1.
– Host respond by sending a report message of the groups
they want to receive multicast traffic for to the all routers
multicast address, 224.0.0.2.
– Only one host from the group responds to the query.
Hosts do not have to wait for a query message to send a
report message.
– When a host wants to join a group, it just sends the join
message (unsolicited Version 2 Membership Report).
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94
Maintaining Groups
Internet Group Management Protocol (IGMP) provides
communication between the local router and multicast hosts
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95
Response Suppression
•
In order to save bandwidth, only one host responds to the
query message.
–
•
This is called response suppression
When a host hears a query message it begins a
countdown timer.
–
The countdown timer can be between 0 and 10 seconds.
–
The countdown timer is selected randomly.
•
If the timer expires before the host hears a response, then
that host will send the report message.
•
If the host hears a response before the timer expires then
the host will not send (suppress) a report message.
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96
Leaving a Multicast Group – IGMPv1
•
With IGMPv1, there was no way for a host to
announce that it wanted to leave the group.
Hosts, left quietly.
–
Hosts that no longer need to be part of a multicast
group just ignore the query messages.
•
Eventually, no hosts will reply with a report
message when the router sends a query
message.
•
The router will then assumes that there are no
members attached to that interface and will
remove the group.
© 2003, Cisco Systems, Inc. All rights reserved.
97
IGMPv2
•
IGMPv2 includes the definition of groupspecific query.
–
•
This way, the router can send a query message to
any one particular group instead of sending it to the
all hosts address.
IGMPv2 also defines a leave group message
(leave report) which allows hosts to leave a
group more quickly.
–
This is known as "low leave latency" .
© 2003, Cisco Systems, Inc. All rights reserved.
98
IGMPv3
•
IGMPv3 enables a multicast host to indicate to
the router the groups from which it wants to
receive multicast traffic, as well as the unicast
addresses of the source.
•
IGMPv3 does this by sending two different
report messages:
–
Include Mode – send traffic from these sources
–
Exclude Mode – do not send traffic from these
sources
–
This is known as source filtering
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t5/dtigmpv3.htm
http://www.ciscosystems.cd/univercd/cc/td/doc/product/software/ios122s/122snwft/release/122s14/fs_xtrc.htm
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99
IGMP Snooping
•
•
•
The default behavior of a switch is to treat multicast traffic
like an unknown unicast. - Why?
– This means that multicast traffic will be sent out every port
of the switch/VLAN.
IGMP snooping is an IP multicast constraining mechanism
for switches.
– IGMP snooping runs on a Layer 2 switch.
– The switch snoops the content of the IGMP join and leave
messages sent between the hosts and the router.
When the switch sees an IGMP report message, the switch
creates a CAM entry for Layer 2 multicast group address for
the switchport that the report message was heard on.
– This way, multicast traffic is only forwarded out the
switchports that have hosts for that group.
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100
IGMP Snooping Configuration
•
IGMP Snooping is enabled globally on the switch by
default.
–
•
This means that IGMP snooping is enabled on all VLANs
by default.
If IGMP Snooping is disabled for some reason, you can reenable it using the global configuration command:
Switch(config)#ip igmp snooping
Switch(config)#ip igmp snooping vlan 10
immediate-leave
•
The second command allows a switchport to leave an
IGMP group as soon as it sees an IGMPv2 leave message
on that switchport.
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101
Multicast Routing
•
By default, a Layer 3 device will isolate multicast traffic to the
segment on which it was generated, not forwarding it across
the router to other network segments.
–
This is because most multicast traffic has a TTL of 1
•
Enabling IP multicast routing allows a Layer 3 device to
forward multicast packets based upon the configuration of
the Multicast routing protocol.
•
To configure multicast routing:
–
Enable multicast routing globally
–
Enable a multicast routing protocol at the interfaces
that are going to participate in multicasting
–
Configure the RP for sparse mode operation
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swmcast.htm
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102
Protocol Independent Multicast (PIM)
•
•
•
PIM is a multicast routing protocol that makes
packet-forwarding decisions independent of
standard or unicast IP routing protocols.
PIM uses the unicast routing tables to perform
multicast forwarding functions.
PIM has three forwarding modes:
–
Dense Mode – PIM DM
–
Sparse Mode – PIM SM
–
Sparse-Dense Mode
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103
PIM Example
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104
PIM Dense Mode
•
This mode uses a push model to flood multicast traffic to
every router in the network and then prune routers that do
not support members of that group.
•
Dense mode is typically used when:
•
–
There are active receivers on every subnet in the network
–
The volume of multicast traffic is high
–
Senders and receivers are in close proximity to each
other
Routers that do not have members of the group send a
prune message back towards the source.
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105
PIM Dense Mode Example
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106
PIM Sparse Mode
•
•
Sparse mode is used when receivers are widely
dispersed over a larger area, like a WAN.
–
This mode uses a pull model to deliver multicast
traffic.
–
Sparse multicast is most useful when there are few
receivers in a group and multicast traffic is
intermittent.
Sparse mode uses a shared tree distribution
system.
–
•
Sparse mode uses a shared distribution tree, also
called Core-Based Tree (CBT)
When a source begins to generate a flow, it is
directed to a rendezvous point.
Configuring a Rendezvous Point:
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swmcast.htm#wp1024288
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107
Sparse Mode Example
When a router determines that it
has receivers out its interfaces,
it registers with the rendezvous point.
The routers in the path will optimize
the path automatically to remove
any unnecessary hops.
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108
PIM Sparse-Dense Mode
•
PIM sparse-dense mode allows individual
groups to be run in either sparse or dense
mode depending on whether RP information is
available for that group.
•
If the router gleans RP information for a
particular group, it will be treated as sparse
mode; otherwise that group will be treated as
dense mode.
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109
Multicast Routing Configuration
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swmcast.htm
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110
Configuring Multicast Routing
Must be a routed port
Switch(config-if)#no switchport
pim
Router(config)#ip multicast-routing
Router(config)#int fa0/0
Router(config-if)#ip pim sparse-dense-mode
Router(config)#ip pim rp-address 192.168.1.254
Router(config)#ip pim autorp (Cisco only)
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111