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VoIP QUALITY OF SERVICE
Bob Franklin
Network DivisionUniversity Computing Service
Copyright 2008 University of Cambridge Computing Service
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CONTENTS QoS what is it and how does it help?
Overview of how the phone systemuses QoS
Configuration examples:
HP ProCurve 2610/2900/3500/5400
3Com 5500
Cisco Catalyst 3560/3750
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WHAT DOES QoS DO?
Traditionally everything best efforts
Stuff goes in, stuff comes out
Where more stuff goes in than can come out,buffering occurs (to
a point)
Normally you just wait for your data
Not good enough for VoIP (50ms or bust) QoS about handling
different types of traffic
differently
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BEST EFFORTS (NO QoS)
edge switch
central switch
best efforts queuing (default) no guarantees
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NETW
ORK PROBLEMS
Jitter
Loss
Latency
A VoIP packet has one attempt to arrive in 50ms.Too late or
missing and therell be a gap in the audio.
Order
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QoS QUEUING
classification
Hi
Lo
handling
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CLASSIFICATION IP packets have DSCP field
802.1Q tagged frames have 802.1p CoS field
i.e. needs VLAN tagged to phones Different traffic on the voice
VLAN has
different priorities VLAN priority not recommended
The CUDN uses the DSCP field (not CoS) DO NOT rewrite it (else
we may misclassify)
but rewriting CoS is OK, if you need to
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WIRESHARK
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VoIP PACKET TYPES
Voice payload most critical but not
much of it (up to 200Mbit/s per inst.) Any more of it and theres
a problem
Signalling less important
Traffic DSCP CoSBandwidth perhandset
Out of profileaction
Voice payload 46 5 128kbit/s Drop
Signalling 24 / 26 3 32kbit/s Mark down
Other 0 0 - -
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BONUS POINTS To avoid a problem affecting service
Dont trust other devices / VLANs
e.g. dont allow PCs to inject high priority(DSCP 46) traffic and
swamp the network
Limit bandwidth available to high prioritytraffic to prevent
pathological problem
Drop >128kbit/s voice traffic from a single port
Police down >32kbit/s signalling to bulk queue
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QoS NETW
ORK STRUCTURE Typically
Trust something in the core (e.g. DSCP)
Selectively trust at the edge and remarkpackets/frames
appropriately
(a bit like a firewall)
CUDN is a little oddWe do some filtering at the PoP
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AW
ORD OF CAUTION Switch QoS is notoriously changeable
What works on one version may not work
on the next (or be subtly different) Manufacturers usually
upgrade configurations
with software
Different models may do different things
A Cisco 2950 is nothing like a 2960 (queuenumbers upside down,
for example!)
Check release notes and manuals
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HP SW
ITCHES
2610 (10/100 + 4x 10/100/1000)
2900 (10/100/1000 no PoE)3500/5400 (10/100/1000 PoE)
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HP QoS MODEL Simple 4 or 8 egress queue system
Based around CoS (0-7)
All other mechanisms (DSCP, VLAN priority,etc.) provide
alternatives to derive CoSvalue
Bandwidth for each queue uses staticWRR (Weighted Round Robin)
ratios
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HP QoS CLASSIFICATION 6 methods high to low precedence:
UDP/TCP port number
Device priority (destination/source IP)
DSCP (~TOS) value
VLAN priority
Interface (switch port) priority Incoming CoS/802.1p value
assumed to be 0 for untagged frames
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HP QUEUES - HIGH TO LOW
Guaranteed MinimumBandwidth (GMB)
CoS Priority Function 2610 2900/3500/5400
7High
Network control75%
20%
6 Internetwork control 15%
5Medium
Voice19%
10%
4 Interactive video 10%
3 Normal Call signalling 5% 10%0 Normal 30%
2Low
Transactional1%
3%
1 Bulk 2%
!
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HP CAVEATS 2610 - defaults to 2 queues
Needs qos-passthrough-mode typical to
adjust to 4 queues
2900 defaults to 4 queues
Can adjust to 8 with qos queue-config 8-
queues 3500/5400 fine as they are
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HP CLASSIFICATION Phones (7911, 7941, 7961)
Use tagged edge ports (with LLDP-MED)
CoS supplied by device per-frame using 802.1p Odd devices
(ATA-186, 7937)
Dont do LLDP-MED (yet?), only CDP
Use untagged edge ports
Use port priority to set CoS (rewrite 802.1p field) Other
traffic
Move into Low priority queue
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HP CONFIGURATIONhp2610(config)# qos-passthrough-mode typical
hp2900(config)# qos queue-config 8-queues
hp(config)# vlan 789 name botolph
hp2610(config)# vlan 789 qos priority 2
!hp2610(config)# vlan 789 qos priority 0hp(config)# vlan 2789
name tel-botolph
hp(config)# vlan 2789 voice
hp(config)# interface 1-23 name data-or-phone
hp(config)# vlan 789 untagged 1-23
hp(config)# vlan 2789 tagged 1-23
hp(config)# interface 24 name fax-machine
hp(config)# vlan 2789 untagged 24
hp(config)# interface 24 qos priority 5
hp(config)# interface 28 name pop
hp(config)# vlan 789 untagged 28
hp(config)# vlan 2789 tagged 28
Move regular data intolow priority queue
Prioritise all traffic on port(sets queue and 802.1p)
Use LLDP-MED when tagged
ATA-186 doesntsupport LLDP-MED
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CISCO SW
ITCHES
Catalyst 3560/3750 -/G/E
(10/100 + 1000 on G/E)(PoE on PS/PD models)
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CISCO QoS MODEL 4 queue model with 3 drop thresholds
Uses DSCP internally
Everything else maps to DSCP value
Classification via complex access lists
Based on DSCP, CoS, IP address masks, CoS,ingress port, TCP/UDP
port, etc.
Can using policy-maps to rewrite / shapebandwidth on a per-rule,
per-port basis
Mmmm delightful!
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CISCO auto qos Luckily, there is the auto qos command that
does most of the work for you:
Use in interface mode auto qos voip trust on interlinks
auto qos voip cisco-phone on edge ports
Uses CoS so use CDP and tagged voice VLAN
show running-config to see it all Test on a dummy switch
first!
or do it by hand (as we do!)
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CISCO CONFIGURATIONcisco(config)# vtp mode transparent
cisco(config)# vlan 789 / name botolph / exit
cisco(config)# vlan 2789 / name tel-botolph / exit
cisco(config)# interface range Gi0/1 24
cisco(config-if-range)# description
data-or-phonecisco(config-if-range)# switchport mode access
cisco(config-if-range)# switchport access vlan 789
cisco(config-if-range)# switchport voice vlan 2789
cisco(config-if-range)# auto qos voip cisco-phone
cisco(config-if-range)# exit
cisco(config)# interface Gi0/28cisco(config-if)# description
pop
cisco(config-if)# switchport trunk encapsulation dot1q
cisco(config-if)# switchport mode trunk
cisco(config-if)# switchport trunk allowed vlan 789,2789
cisco(config-if)# auto qos voip trust
cisco(config-if)# exit
Use CDP to control trust of CoS
CDP and/or
LLDP-MED
Trust CoS
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REFERENCES HP
Advanced Traffic Guide
http://www.hp.com/rnd/support/manuals/
Cisco
Enterprise QoS SRND Guide
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html
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3COM SW
ITCHES
5500
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