IP Phone

Power over Ethernet (POE):

  • PoE devices like IP phones receive power from the connected switch.
  • The switch can provide power in two different methods:
    • Cisco Inline power (ILP): Cisco proprietary.
    • IEEE 802.3af: Based on IEEE standard.
  • Switch should be capable of detecting PoE devices to offer power and bring the link UP.
  • In 802.3af, switch applies small voltage between Tx and Rx line and resistance is measured to check whether current is drawn. A 25K Ohm resistance will be measured if PoE device is connected.
  • There are 5 power classes that can be detected: 0 to 4. Class-0 being the default offers 15.4W.
  • Class-1 device needs 4W and class-2 device needs 7W and Class-3 is optional 15.4W and class-4 device needs upto 50W (under IEEE 802.3at)
  • In Cisco ILP, Power is supplied via data pair 2 and 3 (pins 1,2 and 3,6) at 48V DC.
  • For 802.3af, power can be supplied as above or over data pairs 1 and 4.
  • Switch uses CDP to detect power class in ILP. Switch can reduce the power on receiving CDP from connected cisco IP phone which has power required field.
  • Configuration:
    • (config-if)# power inline  { auto [ max <>] | static max <> | never}
    • “never” to disable PoE on that interface.
    • “show power inline”

Voice VLAN:

  • Cisco IP phone has 3-port switch setup. One to connected switch, one to connected PC and one to VoIP internal data.
  • Voice data traffic should be allocated to separate vlan called ‘voice vlan’ and Qos functionality has to be implemented for voice vlan traffic.
  • Switch configuration instructs how the connected Cisco IP phone should send data and voice traffic.
  • Voice VLAN applicable only on access port and not on trunk. Qos should be enabled before VVLAN.
  • (config-if)# switchport voice vlan <vlan id>
    • Data traffic is via native vlan (untagged)
    • Voice traffic is tagged with <vlan id>
    • Voice Qos set in 802.1p bits.
  • (config-if)# switchport voice vlan dot1p
    • Data traffic is via native vlan (untagged)
    • Voice traffic is tagged with VLAN =0
    • Voice Qos set in 802.1p bits.
  • (config-if)# switchport voice vlan untagged
    • Both Voice and data traffic via native vlan.
    • No 802.1p bits.
    • CDP exchange happens and the switch instruct IP phone to send traffic untagged.
    • (packets tagged with VLAN 1025)???
  • (config-if)# switchport voice vlan none
    • Both Voice and data traffic via native vlan.
    • No 802.1p bits and no CDP/DTP exchange between switch and IP phone.
  • ‘Show interface <type> switchport’ to view the voice and access VLAN.

Voice Qos:

  • By default, Cisco phone uses IP precedence value of 5 for voice traffic and 3 for voice control traffic.
  • Cisco IP phone can be considered as another switch and ‘trust boundary’ can be extended to phone.
  • Configuration:
    • Enable Qos on multilayer switch: “mls qos”
    • On the interface, configure trust parameter: ‘mls qos trust { cos | ip-precedence | dscp}’
    • If we configure, ‘mls qos trust device cisco-phone’ above qos value is trusted only if a cisco IP phone is detected via CDP. Else, qos parameter is not trusted.
    • ‘switchport priority extend { cos <value> | trust}
      • ‘trust’ option extends the trust boundary to PC. Applications can send packets with Cos bits set and the IP phone trust those values and pass unchanged.
      • ‘cos <value>’ makes the IP phone to overwrite the Qos bits from the PC to the specified value. If the PC is untrusted, cos should be overwritten by 0.
      • By default, the phones’ PC port is untrusted and overwrites cos values to 0.
    • Configure ‘mls qos trust cos’ to switch uplink port.
    • ‘Show mls qos interface <type>’
  • Auto-qos: Perform enabling qos, cos-dhcp mapping, ingress and egress queue, strict priority for voice egress traffic and trust boundaries.
    • (config-if)# auto qos voip { cisco-phone | cisco-softphone | trust }
      • ‘cisco-phone’ extends the trust boundary to the cisco IP phone detected by CDP.
      • ‘cisco-softphone’ . packets received with DSCP value of 24,26,46 are trusted.
      • ‘trust’: all packets received on the interface is trusted. Configured on switch uplink.
    • ‘Show auto qos interface <type>’
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