CCNP BCMSN Notes – IP Telephony


Power Over Ethernet (PoE)

Two solutions exist to supply PoE:

          Cisco Inline Power (ILP) – Cisco proprietary solution developed before IEEE 802.3af

          IEEE 802.3af – Standard

IEEE 802.3af

An 802.3af PoE switch applies a small voltage across the wire and checks for 25K Ohm resistance to determine if a PoE device is connected.

Depending on the resistance presented at differing test voltages, the switch can determine which power class a device belongs to:

          Class 0 – 15.4W (default)

          Class 1 – 4.0W

          Class 2 – 7.0W

          Class 3 – 15.4W

          Class 4 – Reserved for future use

The power class determines how much of the switch’s power budget is allocated to the interface.

Power is supplied over pairs 1,2 and 3,6 or pairs 4,5 and 7,8.

Cisco ILP

A Cisco ILP switch transmits a 340kHz test tone on the Tx pair to detect a PoE device; if a Cisco ILP-capable device is present, the tone will be echoed back.

Power is supplied over pairs 1,2 and 3,6.

Cisco ILP detects a device’s power requirement via CDP.

Configuring PoE

All capable switch ports will attempt PoE by default (auto).


PoE can be verified with show power inline.

Voice VLANs

Trunks to IP phones are automatically negotiated by Dynamic Trunking Protocol (DTP) and CDP.

Configuring a voice VLAN:


      none (default) – No trunk is formed; voice and data traffic traverse the same access VLAN

      vlan – Forms an 802.1Q trunk with designated voice VLAN and native (access) VLAN for data; 802.1p CoS bits in 802.1Q header provide independent QoS

      dot1p – Forms an 802.1Q trunk with voice in VLAN 0, data in native VLAN; 802.1p CoS bits used

      untagged – Forms an 802.1Q trunk with voice and data both untagged; 802.1p is not used

Voice QoS

QoS models:

        Best-Effort Delivery – No QoS

        Integrated Services Model – Bandwidth is reserved along a path via Resource Reservation Protocol (RSVP); defined in RFC 1633

        Differentiated Services Model – QoS is handled dynamically per-hop, based on protocol headers and defined policies

Layer 2 DiffServ QoS

Layer 2 frames transported in a trunk receive a designated Class of Service (CoS) value in the trunk header (802.1p bits).

802.1Q native VLAN frames are not tagged and thus are treated with the default CoS.

ISL trunks duplicate the same priority scheme as 802.1p.

Layer 3 DiffServ QoS

The IP Type of Service (ToS) header field originally defined a 3-bit IP precedence value and a 4-bit ToS value.

The DiffServ QoS model reinterprets this field as a 6-bit Differentiated Services Control Point (DSCP), composed of a 3-bit class selector and a 3-bit drop precedence.

         Class 0 – Best effort forwarding

         Classes 1-4 – Assured Forwarding (AF) with drop preferences

         Class 5 – Expedited Forwarding (EF)

         Classes 6-7 – Network control

Configuring a Trust Boundary


mls qos trust device cisco-phone enables QoS trust only when a Cisco IP phone is detected via CDP.

switchport priority extend instructs the phone on how the trust boundary should be extended to a connected PC. The cos option overwrites all frames with the given CoS value.


Auto-QoS was developed to ease implementation of QoS.

Auto-QoS is a macro which automatically performs the following configurations:

       Enabling QoS

       CoS-to-DSCP mapping

       Ingress and egress queue tuning

       Strict priority queues for egress voice traffic

       Establishing an interface QoS trust boundary

Configuring Auto-QoS on an interface:


Any existing QoS configuration must be completely removed from an interface before Auto-QoS can be applied.

debug auto qos can be enabled before applying Auto-QoS to monitor the explicit commands being issued by the macro.

Verifying QoS

        show mls qos interface <interface>

        show interface <interface> switchport

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