CCNA Voice: IP Phone Concepts and Phone Registration

IP Phone Concepts and IP Phone Registration

Connecting/Powering Cisco IP Phones

Three sources of power for IP phones available:

• Catalst Switch PoE (pre-standard 802.3af)
• Power Patch Panel PoE (pre-standard 802.3af)
• Cisco IP Phone Power Brick (facility power)

Catalyst Switch PoE

Cisco Inline Power existed before official standard 802.3af was developed and used unused pairs in ethernet cable to deliver power

802.3at power standard created to increase maximum wattage fro 15.4W to 25.5W

Power Patch Panel

Patch panels are powered and inject power onto ethernet line as an intermediary

Lower cost than switch upgrades, but switches in use must otherwise support QoS and voice vlans or the switches need to be upgraded anyway, eliminating lower cost

Inline PoE Injector is even lower cost but requires dedicated power plug for each injector, not scalable

Cisco IP Phone Power Brick

Must be purchased separately from Cisco, one per phone, requires dedicated power plug for each phone brick

If IP phones have added modules (ie sidecar) then switch PoE is no longer sufficient and a brick is needed

Voice VLAN Concepts/Configuration

Cisco IP Phones support VLAN tagging and use CDP to discover voice vlan

PC can not understand tagged frames, so IP phone must strip tags before delivery to attached PC

Phone tags its own packets with voice vlan

VLAN Configuration

1.  Add voice vlan to switch
2. Configure IP phone switchport with mode access, access and voice vlan numbers
3. Enable port for spanning-tree portfast to allow IP phone to boot quickly

Cisco IP Phone Boot Process

1. IP phone receives power from the switch or one of several aforementioned power solutions
2. Switch delivers voice vlan info to phone using CDP
3. IP Phone sends DHCP request on voice vlan
4. DHCP server responds with IP address offer
5. IP phone receives DHCP option 150 with IP address and other normal info such as gateway and DNS
6. Option 150 directs IP phone to TFTP server address to pull configuration of the IP phone
7. Configuration includes call processing server IPs (CUCM or CME)
8. IP Phone attempts to register with a call processing server in order of the list in the configuration

Config files are named by phone, ie, SEP(MAC ADDRESS OF IP PHONE).cnf.xml

If this file does not exist on TFTP server, IP Phone requests XMLDefault.cnf.xml that has base configuration for auto-registration with CME/CUCM

 Configuring a Router as the TFTP Server

1. Create DHCP scope on router
2. Add network, default gateway, dns server (optional) and option 150 address pointing at the router's voice vlan IP
3. Ensure the configuration files are accessible on the router for the IP Phones to download 

NTP for Cisco Devices

Accurate clocks on devices are needed for the following reasons:

• Correct date/time displayed for users
• Correct date/time assigned to voicemail tags
• Accurate CDR records
• Many security features rely on accurate time
• Logs on routers/switches are accurate with correct time

clock set command can manually set time

Stratum of NTP server determines how far away the device is from a radio/atomic clock

ntp server (IP ADDRESS) configures the device to use a server for NTP

clock timezone (name) (UTC Offset) command configures time zone

To configure the device as an NTP server, command ntp master (stratum number) is used

IP Phone Registration

Required steps before registering:

1. IP Phone has received power
2. IP Phone has voice vlan information via CDP
3. IP Phone has DHCP address and option 150 address
4. IP Phone has downloaded its configuration from TFTP server

IP Phone configuration will list up to three call processing servers (CME/CUCM), IP Phone will attempt to register in order until it successfully registers with one

Registration is done with either SCCP or SIP depending on phone firmware

SCCP is Cisco proprietary, SIP is industry standard

Registration process is as follows;

1. IP Phone contacts call processing server, identifies itself by its MAC to the server
2. Server consults database and sends operating configuraton to the IP Phone including Directory Numbers, ring tones, softkey template, etc using SCCP or SIP
3. SCCP/SIP used to use phone from that point, when IP phone buttons are pressed, handset is lifted off-hook, etc

1.