14-1. Voice Ports Inline power is provided to a powered device as follows: - A phantom-powered device can be detected as a switch port becomes active. - A powered device loops the transmit and receive pairs back so that the switch detects its own 340 kHz test tone. - Power is applied to the port if the device is present; no power is applied if a normal Ethernet device is connected. - Inline power is provided over pairs 2 and 3 (RJ-45 pins 1,2 and 3,6) at 48V DC.
Inline power is available on the following switch modules: - Catalyst 6000 Inline Power 48-port 10/100 Ethernet Switch Module (WS-X6348-RJ45V) - Catalyst 4000 Inline Power 48-port 10/100 Ethernet Switch Module (WS-X4148-RJ45V), with auxiliary DC power shelf and power entry module - Catalyst 3524XL
Power can also be provided through an external 48-port power patch panel (WS-PWR-PANEL): - No detection of a phantom-powered device is performed. - Powered devices can be connected to a wall power adapter and the power patch panel. The devices use the patch panel as a backup power source. - Power is provided over pairs 1 and 4 (RJ-45 pins 4,5 and 7,8) at 48V DC.
A Catalyst switch can send instructions to a Cisco IP Phone on how to present frames from its voice and data ports. This is done through Cisco Discovery Protocol (CDP) messages. The switch and phone can communicate over an 802.1Q trunk, with voice traffic in a separate voice VLAN ID (VVID). Voice class of service (CoS) information can be propagated across the trunk. A Cisco IP Phone performs the following steps during initialization: 1. | Inline power is detected by the switch, if needed.
| 2. | The phone triggers a CDP exchange. The actual amount of required power is sent to the switch, while the VVID number is sent to the phone. The phone can also receive instructions on how to extend the QoS trust boundary.
| 3. | A special 802.1Q trunk is negotiated between the phone and the switch, if a VVID is to be supported. On Catalyst 4000 and 6000 switches, the trunk is negotiated through Dynamic Trunk Protocol (DTP) messages. On Catalyst 3500XL switches, DTP is not supported, so the trunk must be manually configured.
| 4. | A DHCP request is made.
| 5. | A DHCP reply is sent to the phone, containing the IP address and TFTP server address (DHCP option 150).
| 6. | The TFTP server is contacted for a phone configuration file. A list of Cisco CallManager servers is also obtained.
| 7. | Registration with a CallManager server is performed. A directory number (DN) is obtained so that calls can be placed and received.
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Configuration 1. | Use inline power for Cisco IP Phones.
- a. (Optional) Set the default power allocation:
COS | set inlinepower defaultallocation value
| IOS | N/A |
The amount of power allocated to a switch port is negotiated with the powered device. By default, 10.0 watts (0.24 amps at 42V DC) is supplied to each switch port. The default amount can be changed to value (2000 to 12500 mW, default 10000 mW).
TIP The default power allocation is based on the total power available from the switch power supplies. If you change the default allocation, be sure that you don't exceed the available power when all powered devices are active. Use the show environment power COS command to see the total power available. You can also use the show port inlinepower COS command to find switch ports that exceed the available power. Although the switch initially offers the default power allocation to the device, the amount of power can be changed to a value agreed upon by an exchange of CDP messages. - b. (Optional) Detect an inline-powered device:
COS | set port inlinepower mod/port {off | auto}
| IOS | (interface) power inline {auto | never}
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By default, the switch attempts to discover an inline-powered device on a switch port (auto). Use the off (COS) or never (IOS) keyword to disable inline power detection.
CAUTION After a powered device has been detected and power has been applied to a switch port, the switch waits four seconds to see that the device has initialized and the link is established. If not, power is removed from the switch port. If you unplug the powered device within the 4-second delay and plug a regular Ethernet device in its place, power will still be applied and the device could be damaged. Wait at least 10 seconds before swapping devices on a switch port.
| 2. | Establish VLANs with the IP Phone.
TIP A Cisco IP Phone can use an 802.1Q trunk to transport packets from two VLANs: the voice VLAN (voice packets) and the native VLAN (data packets, untagged). By default, a Cisco IP Phone transports both its voice packets and the data packets from a connected device over the native VLAN. All data is untagged. After a switch has been configured to instruct an IP Phone to support a VVID number, the switch and phone must use an 802.1Q trunk between them. On Catalyst 3500XL switches, trunk negotiation is not supported and the 802.1Q trunk must be manually configured. The IP Phone will automatically use an 802.1Q trunk on its end of the connection. For Catalyst 4000 and 6000 switches, a special-case 802.1Q trunk is negotiated with the IP Phone using CDP and the DTP. Once the phone is detected, the switch port becomes a vlan2-access port, supporting only the two voice and data VLANs. The port won't be shown in trunking mode from the show trunk command. In fact, it doesn't matter which trunking mode (auto, desirable, on, or off) is configured on the portthe special trunk will be negotiated through the DTP. Be sure that the trunk is not configured using the nonegotiate keyword, as DTP messages will not be sent or received and the trunk will not be automatically established. The Spanning Tree Protocol (STP) is automatically supported over the IP Phone trunk as well. The show spantree command displays the STP state for both of the VLANs on the trunk. - a. (Optional) Use a VLAN for data.
- (Optional) Identify the switch-port access VLAN: COS | set vlan vlan-id mod/ports
| IOS | (interface) switchport access vlan vlan-id
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- You can configure switch ports to support both PCs and IP Phones. For the case when a regular host (not an IP Phone) is connected to a switch port, the access VLAN should be set to vlan-id (1 to 1000 or 1025 to 4094). When a PC is connected, only the access VLAN is supported and no special trunking negotiations take place. See section "6-1: VLAN Configuration" for more information. - Identify the switch-port native VLAN: COS | set vlan vlan-id mod/ports
| IOS | (interface) switchport trunk native vlan vlan-id
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- Data from the access switch port on an IP Phone is carried over the native VLAN (untagged) of the special 802.1Q trunk. Therefore, you should identify the native VLAN number as vlan-id (1 to 1000 or 1025 to 4094). For a COS switch, the command is the same as setting the switch-port access VLAN.
- b. (Optional) Instruct the phone to transport data and voice.
- (Optional) Use an 802.1Q trunk with a voice VLAN: COS | set port auxiliaryvlan mod[/port] vlan-id
| IOS | (interface) switchport voice vlan vlan-id
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The IP Phone is instructed to use an 802.1Q trunk. Voice frames are tagged with VLAN vlan-id (1 to 4096 COS or 1 to 1001 IOS), whereas frames from the phone's data port are sent untagged (the native VLAN). The CoS value of the voice frames are carried in the 802.1p priority field. - (Optional) Use an 802.1Q trunk with no voice VLAN: COS | set port auxiliaryvlan mod[/port] dot1p
| IOS | (interface) switchport voice vlan dot1p
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The IP Phone is instructed to use an 802.1Q trunk and the 802.1p CoS priority field, but all voice frames are placed in the null VLAN (VLAN 0). Frames from the phone's data port are sent untagged (the native VLAN). This allows the voice priority information to be propagated, without requiring a separate voice VLAN. - (Optional) Use an 802.1Q trunk with no VLAN information: COS | set port auxiliaryvlan mod[/port] untagged
| IOS | (interface) switchport voice vlan untagged
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The IP Phone is instructed to send all voice frames untagged, over the native VLAN. As a result, no 802.1Q encapsulation is used, and no 802.1p CoS priority information can be propagated. - (Optional) Don't instruct the phone at all: COS | set port auxiliaryvlan mod[/port] none
| IOS | (interface) switchport voice vlan none
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The switch will not provide the IP Phone with a VVID to use. This is the default configuration. The phone will have no knowledge of a voice VLAN, and both voice and data frames are sent to the switch port over the same access VLAN.
- c. (Catalyst 3500XL only) Manually configure the 802.1Q trunk.
- Select 802.1Q encapsulation: COS | N/A | IOS | (interface) switchport trunk encapsulation dot1q
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The 802.1Q trunk encapsulation is forced on the switch interface. The IP Phone automatically uses and expects an 802.1Q trunk. - (Optional) Allow the voice and data VLANs on the trunk: COS | N/A | IOS | (interface) switchport trunk allowed vlan vvid,pvid
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- By default, all configured VLANs are allowed on the trunk. You can limit this to only the voice VLAN number (vvid) and the data VLAN number (pvid), to prevent any broadcast traffic on any other VLANs from using unnecessary bandwidth. - Enable the trunk: COS | N/A | IOS | (interface) switchport mode trunk
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- The interface is in the static access mode by default. The trunk mode must be started manually with this command.
| 3. | (Optional) Optimize the switch port for an IP Phone.
TIP A COS switch can perform the actions of the following configuration steps with a single command: set port host mod/port. Note that this command effectively disables trunking on the switch port; however, the switch port and the IP Phone still use a special form of 802.1Q trunking regardless. - a. Turn off EtherChannel support:
COS | set port channel mod/port mode off
| IOS | (interface) no channel-group
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Support for dynamic EtherChannel configuration using Port Aggregation Protocol (PAgP) is disabled, saving about 10 seconds of port startup time. See section "4-3: EtherChannel" for more information.
- b. Enable Spanning Tree PortFast:
COS | set spantree portfast mod/port enable
| IOS | (interface) spanning-tree portfast
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The switch port is tuned for a faster STP startup time by bypassing the listening and learning STP states. The port can be moved into the forwarding state immediately. See section "7-3: STP Convergence Tuning" for more information.
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Example A Catalyst switch is configured to support an IP Phone on a port. The switch supports inline power, but the switch port might connect to a regular PC or to a Cisco IP Phone. The port is set to automatically detect a device that supports inline power. The access or port VLAN ID (PVID) is set to VLAN number 55. If a PC is directly connected to the switch port, all data frames are transported over the access VLAN. If an IP Phone is connected, a two-VLAN 802.1Q trunk is negotiated. Data frames from a PC connected to the phone are carried untagged over the native VLAN 55 on the trunk. Voice frames to and from the phone are tagged and carried over the voice or auxiliary VLAN (VVID) 200 on the trunk. The switch port is also configured to minimize the port-initialization delays due to PAgP and STP. This is optional, but can keep the IP Phones from waiting for switch-port delays before phone configuration data is downloaded: COS | set port inlinepower 4/1 auto set vlan 55 4/1 set port auxiliaryvlan 4/1 200 set port host 4/1
| IOS | (global) interface fastethernet 0/1 (interface) power inline auto (interface) switchport access vlan 55 (interface) switchport trunk native vlan 55 (interface) switchport voice vlan 200 (interface) switchport trunk encapsulation dot1q (interface) switchport mode trunk (interface) no channel-group (interface) spanning-tree portfast
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Displaying Information About Voice Ports Table 14-1 lists some switch commands that you can use to display helpful information about voice ports. Table 14-1. Switch Commands to Display Voice Port InformationDisplay Function | Switch OS | Command |
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Inline power status | COS | show port inlinepower [mod[/port]]
| IOS | [View full width] (exec) show power inline [interface-id] [actual |  configured]
-OR- (exec) show cdp neighbor [interface-id] detail
| Access, native, and voice VLANs | COS | show port mod/port
-OR- show trunk mod/port detail
| IOS | (exec) show interface [interface-id] switchport
| Discovered device | COS | show cdp neighbor mod/port [detail]
| IOS | (exec) show cdp neighbor [interface-id] [detail]
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