CCNA Collaboration – Notes, Chapter 3

CICD – Ch3 – Cisco IP Phone


  • IP Phones require the following
    • POE – Power Over Ethernet
    • Voice VLAN
    • DHCP
  • Phone has 3 port switch built into it
    • Port 1, connects to switch
    • Port 2, phone ASIC
    • Port 3, connects to PC


Power Over Ethernet, POE

  • Phones must receive power from a source
    • Switch POE
    • Power patch panel
    • POE injector
    • Power brick
  • POE is the ability to send electricity over ethernet
    • Centralized power distribution
      • Switches are generally on some type of backup power (UPS, generator)
    • Don’t need a power outlet at the phone
      • Outlets may not be where phones are being places
  • Standard, IEEE
    • 802.3af
      • 15-25 watts
    • POE+
      • 802.3at, 51 watts


Output from a switch

Home_Switch#sh power inline 


Interface AdminOper Power DeviceClass Max


--------- ------ ---------- ------- ------------------- ----- ----

Fa0/1 auto off0.0 n/a n/a 15.4 

Fa0/2 auto off0.0 n/a n/a 15.4 

Fa0/3 auto off0.0 n/a n/a 15.4 

Fa0/4 auto off0.0 n/a n/a 15.4 

Fa0/5 auto off0.0 n/a n/a 15.4 

Fa0/6 auto on 12.0IP Phone 7945 3 15.4 

Fa0/7 auto off0.0 n/a n/a 15.4 

Fa0/8 auto off0.0 n/a n/a 15.4 



Voice VLAN

  • Cisco recommends having a dedicated vlan for voice
  • VLAN = Broadcast domain = IP Subnet
  • Trunk
    • Allow multiple vlans across a single physical interface
    • Also known as, tagging
    • 802.1q = standard
    • ISL = Cisco Proprietary
  • Voice vlan allows interface to become a multi-vlan access port
    • PC connects to phone, phone connects to switch
    • PC sends traffic untagged = access vlan
    • Phone sends traffic tagged = voice vlan
  • Phones receive voice vlan information through CDP neighbor
  • Configuration, switch

*Create layer 2 vlan on the switch

vlan <#>

name DATA

vlan <##>

name VOICE


spanning-tree bpduguard enable —> This command is not referenced in the book, but I mention it here as a best practice. This is a global command that will affect portfast enabled ports. BPDU Guard disables any interface that receives a BPDU into the interface. This is helpful is someone decides to create a loop by plugging in both ethernet ports on the phone into the switch


*Configure interface connected to a phone

interface <int> —> Go into the interface configuration

switchport access vlan <#> —> assign the access (data) vlan to the interface

switchport voice vlan <##> —> assign the voice vlan to the interface

spanning-tree portfast —> immediately bring interface into forwarding state, bypass spanning-tree listening and learning states

switchport mode access —> statically configure the interface as an access port. Default is to dynamically determine based on what plugged into the interface. Could either be trunk or access

Home_Switch(config)#vlan 20

Home_Switch(config-vlan)#name VOICE



Phone Boot Process

  1. Phone connects to ethernet, if switch supports POE, the phone powers on
  2. Switch delivers voice vlan to phone through CDP. Phone starts tagging traffic with correct vlan information
  3. Phone broadcasts a DHCP Request
    1. Broadcasts are contained within a layer 3 vlan. Configuration can be added to the layer 3 SVI (Switches Virtual Interface) to relay DHCP request to a DHCP server if the server lives on a different subnet. ip helper-address <ip>
    2. Asks for an IP on it’s voice vlan
  4. DHCP server respinds with DHCP Offerr
    1. Phone access the offer if there is no duplicate address
    2. Offer contains: Default gateway, DNS Information, domain name
    3. Required from DHCP: Option 150. Option 150 contains information on the TFTP server, more on this later.
  5. Phone contacts TFTP server and downloads configuration file. The file contains valid CME or CUCM servers
  6. Phone registers with CME or CUCM


Router DHCP Configuration

  • DHCP is required for phones (and endpoints for that matter) to get an IP address and be able to communicate on the network
  • Below is an example configuration that can be done on a Cisco router

Global config:

ip dhcp excluded-address <start> <end>

ip dhcp pool <name>

network <ip> <subnet>

default router <ip>

dns-server <ip>

option 150 ip <ip>

Interface config:

interface vlan <int>

ip helper-address <ip>

Actual configuration

Home_Switch(config)#int vlan 20

Home_Switch(config-if)#ip add

Home_Switch(config-if)#ip helper-address

Home_Switch(config-if)#no shut

Home_Switch(config)#ip dhcp excluded-address

Home_Switch(config)#ip dhcp pool VOICE




Home_Switch(dhcp-config)#option 150 ip 


Network Time Protocol – NTP

  • Provides a clocking source
  • Display the correct time and date on phones
  • Get the correct date and time for voicemails
  • Accurate Call Detail Records (CDR), explained in later chapters
    • Track calls on the network
  • Security features
  • Tag log messages
  • Stratum levels, how accurate is the time source
    • Level 1 is the best



ntp server <ip> —> where to get source of time from

clock timezone <timezone> —> What timezone is the device in

ntp master <stratum> —> Tells router to provide time

ntp server prefer

clock timezone EST -5

clock summer-time EDT recurring


Phone Registration

  • Phones use SCCP or SIP for signaling
  • SCCP, Skinny
    • Cisco proprietary voice signaling protocol to control phones
  • SIP, Session Initiation Protocol
    • IETF standard voice signaling protocol
    • Lightweight alternative to H.323
  • Phones identify themselves with MAC address
    • Talks to CME or CUCM (call processors)
    • Call processor will send XML file to phone with its configuration
    • Configuration includes: device language, firmware version, call processing IPs, ports #s, etc.
      • Softkey layout
  • Signaling protocol is used for majority of phone functionality
    • Dial tone, digit collecting, on/off hook conditions


Quality of Service – QOS

  • For VOIP to operate successfully, voice must have priority over data traffic
  • QOS definition: Ability for the network to provide better or special service to a set of users and application at the expense of other users and applications
  • Voice traffic is time sensitive
  • Voice should get first access to bandwidth
    • Router queues other traffic in time of congestion
  • Problems QOS is trying to solve
    • Lack of bandwidth
    • Delay
    • Fixed delay
    • Variable delay
    • Jitter (delay variation)
    • Packet loss
  • Voice Traffic Requirements
    • Voice is predictable, if you know which codec is being used you’ll be able to calculate how much bandwidth is required
    • These are the maximum thresholds, lower is better
      • End to end delay – 150ms
      • Jitter – 30ms
      • Packet loss – 1%
    • Video has same requirements, just requires more bandwidth

QOS Mechanisms

  • Best Effort – Default, no QOS
    • First come, first serve
  • IntServ – Reservation Model
    • Resource Reservation Protocol (RSVP)
    • Provides guaranteed bandwidth
    • Has scalability problems, each router must track the traffic flow
  • DiffServ – Most popular and flexible model
    • Configure every device to respond with a variety of QOS methods based on traffic classes
    • DSCP
    • Note: This CCNA does not go into the level of detail that I was expecting. I’ll write up another post that’ll be a more in-depth on QOS

QOS Tools

  • Classification and Marking – Identify and mark packets
  • Congestion Management – QOS Queuing strategies
  • Congestion Avoidance – Drop packets before congestion occurs
  • Policing and Shaping – Give hard or soft limits on how much of a specified traffic is allowed
  • Link Efficiency – compression mechanisms

CCNA Collab book goes into Link Efficency and Queuing Algorithms. If you want to know about the others, drop a comment and I’ll write some more details around the others

Link Efficiency

  • Payload compression
    • Compress app data from being sent across the WAN
  • Header compression
    • Eliminate redundant fields of the header
    • RTP Header Compression, compressed RTP (cRTP). Go from 40 bytes down to 2 bytes, 4 bytes with error correction
  • Link Fragmentation and Interleaving – LFI
    • Addresses serialization delay by chopping larger packets into smaller ones
    • Used on PPP or frame relay connections

Queuing Algorithms

  • WFQ – Weighted Fair Queuing
    • Tries to balance available bandwidth for all senders
    • Default on serial interfaces
  • CBWFQ – Class Based WFQ
    • Guarantees specific amounts of bandwidth for various traffic classes
  • LLQ – Low Latency Queuing
    • Add a priority queue
    • Similar to CBWFQ

Applying QOS

  • Input Actions
    • Classification
    • Marking
    • Policing
  • Output Actions
    • Congestion management
    • Marking
    • Congestion avoidance
    • Shaping
    • Policing
    • Compression
    • Fragmentation and Interleaving



  • Simplified mechanism to deploy QOS
  • Deploys template based on Ciso’s QOS best practice
  • Uses CDP to detect IP phone to apply QOS settings

AutoQOS Benefits

  • Reduced time to deploy
  • Configuration consistency
  • Reduced deployment cost
  • Allows manual tuning

AutoQOS, steps before deployment

  • Establish trust boundary – which endpoints do you trust markings from
  • Devices can mark traffic with different QOS classification
  • Ex: Phone marks all traffic as high priority (EF)
    • Note, DSCP was not covered in this book. I’ll write a future blog post
  • Phone has ability to strip marking PC’s set

AutoQOS Config

  • Single command under interface
  • Does not need to be applied on every device
    • This is according to the book. Real life, deploy QOS everywhere in a controller maner
  • Before commands are entered, check to make sure bandwidth statements are correct
  • AutoQOS uses a LLQ model

Global Config

Home_Switch(config)#auto qos ?

  srnd4QoS configurations based on solution reference network design 4.0


Home_Switch(config-if)#auto qos ?

  classifyConfigure classification for untrusted devices

  trust     Trust the DSCP/CoS marking

  video     Configure AutoQoS for video devices

  voip    Configure AutoQoS for VoIP

Home_Switch(config-if)#auto qos voip ?

  cisco-phone    Trust the QoS marking of Cisco IP Phone

  cisco-softphoneTrust the QoS marking of Cisco IP SoftPhone

  trust          Trust the DSCP/CoS marking

Home_Switch(config-if)#auto qos voip cisco-phone 

Home_Switch(config-if)#do sh run int fa0/7

Building configuration…

Current configuration : 226 bytes


interface FastEthernet0/7

 srr-queue bandwidth share 1 30 35 5

 priority-queue out 

 mls qos trust device cisco-phone

 mls qos trust cos

 auto qos voip cisco-phone 




Additional Output generated

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