WarmTransfer
network foundations · published

Troubleshooting IP phones that do not get a voice VLAN or DHCP options

Verified 2026-10-02 · 56 sources · tier 1–2

Also known as ip-phone-lands-on-data-vlan, ip-phone-no-tftp-address.

In Cisco's documented 8800 startup sequence, the phone configures its VLAN before it obtains an IP address, and it obtains the IP address before it contacts the TFTP server 45. In that sequence, the DHCP server both assigns the phone's IP address and directs the phone to a TFTP server 7.

How a phone learns its voice VLAN

When an 8800 phone is connected to a Cisco Catalyst switch, the switch tells the phone which voice VLAN to use during startup 3. Cisco 8800 phones use CDP to exchange the auxiliary VLAN ID, per-port power management details and QoS configuration with a Cisco Catalyst switch 4. They also support LLDP-MED on the SW (network) port for voice VLAN configuration, device discovery, power management and inventory management, and support plain LLDP on the PC port 16.

Cisco's VLAN design for a phone with a PC attached through it puts voice traffic on an auxiliary VLAN and the PC's data traffic on the native VLAN 2. Cisco states that this design addresses 3 concerns: shortage of IP addresses in one subnet, data traffic degrading voice quality, and security isolation 2.

Switch side (Catalyst 9300)

The Catalyst 9300 IOS XE 17.18.x guide states that voice VLAN is supported only on access ports, not on trunk ports 48. The same guide states that the 9500X and 9600X models do not support the Voice VLAN feature 48. For Catalyst 9300 voice VLAN to configure a Cisco IP phone, CDP must be enabled on the switch port the phone connects to, and CDP is enabled globally by default 51.

On Catalyst 9300, switchport voice vlan takes 4 options 50:

  • A vlan-id (1-4094) makes the phone send voice traffic in that VLAN with 802.1Q priority 5 50.
  • dot1p makes the phone use 802.1p frames tagged with VLAN 0 50.
  • untagged makes the phone send untagged voice traffic 50.
  • none lets the phone use its own configuration 50.

In the Catalyst 9300 IOS XE 17.7.x guide, LLDP is globally disabled by default and is enabled with lldp run 14. By default a Catalyst 9300 sends only plain LLDP until it receives LLDP-MED packets from the end device, and only then does it send LLDP with MED TLVs such as network policy 15. If an LLDP-MED network-policy profile is configured on an interface first, switchport voice vlan cannot be applied to that interface afterwards 30. If the voice VLAN is configured first, a network-policy profile can still be added 30.

Phone side (Cisco MPP)

On Cisco MPP desk phones, a manually configured VLAN ID is used only when neither CDP nor LLDP-MED sets the VLAN 26. If no VLAN ID is configured manually either, the MPP phone uses no VLAN 26. When both CDP and LLDP-MED are enabled on an MPP phone, the VLAN network policy is whichever was last set or changed by either discovery protocol 25. Cisco warns that when network devices give a phone inconsistent VLAN configuration through CDP and LLDP-MED, the phone can reboot repeatedly as it switches between VLANs 28.

WarmTransfer's reading of the sources is that a Cisco phone on an access port where neither CDP nor LLDP-MED delivers a voice VLAN, and with no manual VLAN set, sends untagged traffic 13. We infer that such a phone therefore lands in the port's access (data) VLAN and takes its DHCP lease and options from the data scope 13.

Yealink IP phones apply VLAN assignment in this order: LLDP or CDP first, then manual configuration, then DHCP VLAN 56. Yealink phones read DHCP VLAN settings from DHCP option 132 by default 53. That option is configurable, and up to 5 options can be listed separated by commas 53. After a Yealink phone gets a VLAN ID from the DHCP server, it releases the leased IP address and starts a new DHCP Discover tagged with the voice VLAN ID 55.

How a phone finds its TFTP or provisioning server

DHCP options 150, 66, 43 and 60

For 8800 phones on Cisco Unified CM, Cisco recommends DHCP custom option 150 with the TFTP server IP address as its value, and suggests DHCP option 66 if option 150 cannot be used 37. DHCP option 150, the TFTP Server Address option, is documented in RFC 5859 34. RFC 5859 is an Informational RFC that records existing usage, not a Standards Track specification 34. Option 150 carries 1 or more IPv4 TFTP or configuration server addresses, and its length field must be an integral multiple of 4 octets 32. RFC 5859 says that when option 150 and the TFTP Server Name option (66) are both present, option 150 SHOULD take priority over option 66 33.

RFC 2132 defines DHCP option 66 as the TFTP server name option, which identifies a TFTP server when the 'sname' field in the DHCP header has been used for DHCP options 36. RFC 2132 defines option 43 (Vendor Specific Information) as an opaque object for clients and servers to exchange vendor-specific information 35. Option 60 lets a client identify its vendor type and configuration 35.

For IPv6 phones, the Cisco Unified CM 15 guide says to pass the TFTP server IPv6 address in Cisco vendor-specific DHCPv6 information (sub-option type 1), not in option 150, which is IPv4 only 6.

Cisco MPP option and provisioning order

On Cisco MPP desk phones, the DHCP Option To Use parameter defaults to 66,160,159,150,60,43,125, which is the order in which the phone uses the options the DHCP server provides 24. WarmTransfer's reading of the sources is that, because this default list places 66 before 150, an MPP phone in a scope that carries both options with different values is likely to act on option 66, the opposite of the RFC 5859 recommendation 12.

Cisco MPP desk phones try automatic provisioning methods in this order: DHCP, then DNS SRV, then EDOS, then GDS (activation-code onboarding) or EDOS device activation 29. If the DHCP server gives an MPP phone no provisioning server information, the phone sends its MAC address and model to the Cisco EDOS Remote Customization server and its Customization State becomes Pending 27. For activation-code onboarding of MPP phones, Cisco says to allow activation.webex.com through the firewall 23. Because newer phones default to activate.cisco.com, Cisco recommends allowing both domain names 23.

Yealink phones get the provisioning server address in this priority order: Zero Touch, PnP Server, DHCP options, Phone Flash 54. For IPv4 DHCP the Yealink order is custom option, then option 66, then option 43, and for IPv6 it is custom option, then option 59 54.

Diagnosing it

Phone cannot obtain an IP address

Cisco lists 2 causes for an 8800 phone that cannot obtain an IP address during startup: the phone is not on the same network or VLAN as the DHCP server, or the switch port is disabled 31. On Catalyst 9300 switches, show interfaces interface-id switchport displays the voice VLAN configuration of a port 52.

Phone gets an address but not a TFTP server

On a Cisco 8800 phone running enterprise (CUCM) firmware, the screen message 'Enter activation code or service domain' is documented as caused by a missing TFTP address 1. Cisco's fix is to check DHCP option 150 or configure an alternate TFTP server manually 1. To check TFTP settings on an 8800 phone, go to Applications > Admin settings > Network setup > Ethernet setup > IPv4 setup > TFTP Server 1 46. When the phone uses DHCP it gets the TFTP address from the DHCP server, so Cisco says the administrator should check that the address is configured in option 150 46.

If an 8800 phone reaches the TFTP server or Cisco Unified CM by DNS name, a DNS server must be specified 10. Cisco's DNS check also says to confirm that a CNAME entry exists for the TFTP server and the Unified CM system, and that reverse lookups are configured 10. If the Cisco Unified CM or TFTP services are stopped, devices that depend on them cannot start up 47. Cisco's fix is to start the service from Cisco Unified Serviceability > Tools > Control Center - Feature Services 47.

Phone resets or reboots repeatedly

  • Cisco lists DHCP configuration errors as a cause of 8800 phone resets and recommends a DHCP lease time of 8 days for the phones 8.
  • Cisco attributes 8800 phone resets during heavy network use to a missing voice VLAN, and the fix is to put the phones on a separate auxiliary VLAN 44.
  • Inconsistent VLAN configuration delivered through CDP and LLDP-MED can make a phone reboot repeatedly as it switches between VLANs 28.
  • An 8800 phone may restart when it switches from external power to PoE, or from PoE to external power 43.

802.1X interactions

Cisco's procedure for 802.1X authentication problems on 8800 phones is to verify that the required components are configured and that the shared secret configured on the phone matches the one on the authentication server 11. Cisco documents that enabling IEEE 802.1X on a Catalyst 9300 access port that has a voice VLAN and a connected Cisco IP phone causes the phone to lose connectivity to the switch for up to 30 seconds 49.

When the 802.1X host mode is multi-domain, a voice VLAN must be configured for the IP phone 22. Cisco recommends MDA rather than CDP bypass for IP phones on dot1x-secured ports 22. With multidomain authentication on Cisco IOS, a voice device is authorized only if the AAA server sends the Cisco AV pair device-traffic-class=voice 21. If the AAA server does not send device-traffic-class=voice, the switch treats the voice device as a data device 19.

In multidomain mode, a port is error-disabled if more than 1 device tries to authorize in either the voice or the data domain 18. The port is also error-disabled if, while unauthorized, it sees more than 5 devices on the data VLAN or more than 1 voice device on the voice VLAN 18. Under Cisco IOS multidomain authentication, non-Cisco IP phones are allowed into both the data and voice VLANs 20. The data VLAN lets such a phone reach a DHCP server to get an IP address and the voice VLAN information 20.

Power negotiation and PoE

After an 8800 phone reboots, the switch locks to CDP or LLDP for power negotiation, using whichever protocol the phone sends first with a power TLV 40. If that protocol is then disabled, the phone cannot power its accessories because the switch ignores power requests in the other protocol 40. Cisco's Catalyst 9300 LLDP guide says LLDP must be disabled on interfaces where CDP is used for power negotiation 17.

When Power Negotiation is disabled on an 8800 phone, the phone can power accessories up to 15.4 W 42. Cisco recommends leaving Power Negotiation enabled, which is the default, on switches that support it 42. On Catalyst 9300, the initial PoE allocation for Class 0, 3 and 4 powered devices is 15.4 W, and a device that asks for more over CDP or LLDP after startup can be given up to 30 W 39.

When a Catalyst 9300 denies a PoE power request because of the power budget, it turns off power to the port, generates a syslog message and updates the LEDs 38. On Catalyst 9300, show power inline shows PoE status and available power, and show power inline police shows power-policing data 41. The command power inline never disables powered-device detection on a port 41.

See also

Applicability

Applies to: Cisco IP Phone 8800 Series, Cisco Catalyst 9300 Series Switches, Cisco IP Phone Multiplatform Firmware, Yealink IP Phones, Cisco Unified Communications Manager, and Cisco IOS. Deployments: on-premises and multi-tenant. Sources checked 2026-10-02. The 8800 behavior described here comes from Cisco's documented 8800 startup sequence 45. The voice VLAN switch behavior is documented for Catalyst 9300 switches running IOS XE 17.18.x 48. The LLDP and LLDP-MED switch behavior is documented in the Catalyst 9300 IOS XE 17.7.x guide 14. The PoE switch behavior is documented for Catalyst 9300 on IOS XE 17.11.x 39. The multidomain authentication behavior is documented for Cisco IOS Release 15M&T 21. The DHCPv6 TFTP guidance is from the Cisco Unified CM Release 15 guide 6. The option definitions come from RFC 5859 and RFC 2132 34 36.

What remains uncertain

How a Yealink phone resolves conflicting VLAN information received through LLDP and CDP at the same time is not covered by the sources below. Whether the Catalyst 9300 LLDP defaults described for IOS XE 17.7.x still apply in later IOS XE releases is not covered by the sources below. Whether the 8-day lease recommendation applies to phone families other than the 8800 series is not covered by the sources below.

See also

Precedes

Related to

Troubleshoots

Referenced by

Sources

  1. 1
    On a Cisco 8800 phone running enterprise (CUCM) firmware, the screen message 'Enter activation code or service domain' is documented as caused by a missing TFTP address; the fix is to check DHCP option 150 or configure an alternate TFTP server manually.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Startup Problems > Phone Not Registering · Checked 2026-10-02
  2. 2
    Cisco's VLAN design for a phone with a PC attached through it puts voice traffic on an auxiliary VLAN and the PC's data traffic on the native VLAN. This addresses three concerns: shortage of IP addresses in one subnet, data traffic degrading voice quality, and security isolation.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · VLAN Interaction · Checked 2026-10-02
  3. 3
    When an 8800 phone is connected to a Cisco Catalyst switch, the switch tells the phone which voice VLAN to use during startup.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · Phone Startup Sequence > Configure VLAN · Checked 2026-10-02
  4. 4
    Cisco 8800 phones use CDP to exchange the auxiliary VLAN ID, per-port power management details and QoS configuration with a Cisco Catalyst switch.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · Network Protocols table > Cisco Discovery Protocol (CDP) row · Checked 2026-10-02
  5. 5
    With critical voice VLAN on Catalyst 9300 (IOS XE 17.15.x), a port goes into critical authentication mode when ISE does not respond, and a phone sending traffic tagged with the voice VLAN is placed in the configured voice VLAN.
    Security Configuration Guide, Cisco IOS XE 17.15.x (Catalyst 9300 Switches) - Configuring IEEE 802.1x Port-Based Authentication · Critical Voice VLAN · Checked 2026-10-02
  6. 6
    For IPv6 phones, the Cisco Unified CM 15 guide says to pass the TFTP server IPv6 address in Cisco vendor-specific DHCPv6 information (sub-option type 1), not in option 150, which is IPv4 only.
    System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure TFTP Servers · Configure TFTP Servers > Proxy TFTP deployment / DHCP options · Checked 2026-10-02
  7. 7
    In the 8800 startup sequence, the DHCP server both assigns the phone's IP address and directs the phone to a TFTP server.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · Phone Startup Sequence > Access TFTP Server · Checked 2026-10-02
  8. 8
    Cisco lists DHCP configuration errors as a cause of 8800 phone resets and recommends a DHCP lease time of 8 days for the phones.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Phone Reset Problems > Phone Resets Due to DHCP Setting Errors · Checked 2026-10-02
  9. 9
    In Cisco's Check DHCP Settings procedure for 8800 phones, a missing DHCP Server value on the phone's IPv4 setup screen means the IP routing and VLAN configuration should be checked.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Troubleshooting Procedures > Check DHCP Settings · Checked 2026-10-02
  10. 10
    If an 8800 phone reaches the TFTP server or Cisco Unified CM by DNS name, a DNS server must be specified. Cisco's DNS check also says to confirm a CNAME entry exists for the TFTP server and the Unified CM system, and that reverse lookups are configured.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Phone Displays Error Messages > Phone Cannot Connect Using DNS; Troubleshooting Procedures > Verify DNS Settings · Checked 2026-10-02
  11. 11
    Cisco's procedure for 802.1X authentication problems on 8800 phones is to verify that the required components are configured and that the shared secret configured on the phone matches the one on the authentication server.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Troubleshooting Procedures > Identify 802.1X Authentication Problems · Checked 2026-10-02
  12. 12
    Because the MPP default DHCP Option To Use list places 66 before 150, an MPP phone in a scope that carries both options with different values is likely to act on option 66. That is the opposite of the RFC 5859 recommendation that option 150 take priority.inferred
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details · Network Protocols > DHCP (DHCP Option To Use default), read against rfc-5859-tftp-server-address-option Section 3 · Checked 2026-10-02
  13. 13
    A Cisco phone on an access port where neither CDP nor LLDP-MED delivers a voice VLAN, and with no manual VLAN set, sends untagged traffic. It therefore lands in the port's access (data) VLAN and takes its DHCP lease and options from the data scope.inferred
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details · Final Network Policy Resolution and QoS (combined with cisco-cat9300-1718-voice-vlans, Prerequisites for Voice VLANs) · Checked 2026-10-02
  14. 14
    In the Catalyst 9300 IOS XE 17.7.x guide, LLDP is globally disabled by default and is enabled with 'lldp run'.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Cupertino 17.7.x (Catalyst 9300 Switches) - Configuring LLDP, LLDP-MED, and Wired Location Service · Default LLDP Configuration table · Checked 2026-10-02
  15. 15
    By default a Catalyst 9300 sends only plain LLDP until it receives LLDP-MED packets from the end device. Only then does it send LLDP with MED TLVs, such as network policy.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Cupertino 17.7.x (Catalyst 9300 Switches) - Configuring LLDP, LLDP-MED, and Wired Location Service · LLDP-MED · Checked 2026-10-02
  16. 16
    Cisco 8800 phones support LLDP-MED on the SW (network) port for voice VLAN configuration, device discovery, power management and inventory management, and support plain LLDP on the PC port.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · Network Protocols table > LLDP-MED and LLDP rows · Checked 2026-10-02
  17. 17
    Cisco's Catalyst 9300 LLDP guide says LLDP must be disabled on interfaces where CDP is used for power negotiation.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Cupertino 17.7.x (Catalyst 9300 Switches) - Configuring LLDP, LLDP-MED, and Wired Location Service · LLDP and Cisco Discovery Protocol coexistence · Checked 2026-10-02
  18. 18
    In 802.1X multidomain mode, a port is error-disabled if more than one device tries to authorize in either the voice or the data domain. It is also error-disabled if, while unauthorized, it sees more than five devices on the data VLAN or more than one voice device on the voice VLAN.
    802.1X Authentication Services Configuration Guide, Cisco IOS Release 15M&T - IEEE 802.1X Multidomain Authentication · Restrictions for IEEE 802.1X Multidomain Authentication · Checked 2026-10-02
  19. 19
    Under 802.1X multidomain authentication on Cisco IOS, if the AAA server does not send device-traffic-class=voice, the switch treats the voice device as a data device.
    802.1X Authentication Services Configuration Guide, Cisco IOS Release 15M&T - IEEE 802.1X Multidomain Authentication · Information About IEEE 802.1X Multidomain Authentication · Checked 2026-10-02
  20. 20
    Under Cisco IOS multidomain authentication, non-Cisco IP phones are allowed into both the data and voice VLANs. The data VLAN lets the phone reach a DHCP server to get an IP address and the voice VLAN information.
    802.1X Authentication Services Configuration Guide, Cisco IOS Release 15M&T - IEEE 802.1X Multidomain Authentication · Information About IEEE 802.1X Multidomain Authentication · Checked 2026-10-02
  21. 21
    With IEEE 802.1X multidomain authentication on Cisco IOS, a voice device is authorized only if the AAA server sends the Cisco AV pair device-traffic-class=voice.
    802.1X Authentication Services Configuration Guide, Cisco IOS Release 15M&T - IEEE 802.1X Multidomain Authentication · Information About IEEE 802.1X Multidomain Authentication · Checked 2026-10-02
  22. 22
    When the 802.1X host mode is multi-domain, a voice VLAN must be configured for the IP phone. Cisco recommends MDA rather than CDP bypass for IP phones on dot1x-secured ports.
    802.1X Authentication Services Configuration Guide, Cisco IOS Release 15M&T - IEEE 802.1X Multidomain Authentication · Guidelines for Configuring IEEE 802.1X Multidomain Authentication · Checked 2026-10-02
  23. 23
    For activation-code onboarding of MPP phones, Cisco says to allow activation.webex.com through the firewall. Because newer phones default to activate.cisco.com, Cisco recommends allowing both domain names.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) Administration Guide - Provisioning · Provisioning > Activation Code Onboarding · Checked 2026-10-02
  24. 24
    On Cisco MPP desk phones, the DHCP Option To Use parameter defaults to 66,160,159,150,60,43,125. This is the order in which the phone uses the options the DHCP server provides.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details · Network Protocols > DHCP (DHCP Option To Use default) · Checked 2026-10-02
  25. 25
    When both CDP and LLDP-MED are enabled on a Cisco MPP desk phone, the VLAN network policy is whichever was last set or changed by either discovery protocol.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details · Final Network Policy Resolution and QoS · Checked 2026-10-02
  26. 26
    On Cisco MPP desk phones, a manually configured VLAN ID is used only when neither CDP nor LLDP-MED sets the VLAN. If no VLAN ID is configured manually either, the phone uses no VLAN.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details · Final Network Policy Resolution and QoS · Checked 2026-10-02
  27. 27
    If the DHCP server gives a Cisco MPP phone no provisioning server information, the phone sends its MAC address and model to the Cisco EDOS Remote Customization server and its Customization State becomes Pending.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) Administration Guide - Provisioning · Provisioning > Remote Customization (EDOS) · Checked 2026-10-02
  28. 28
    Cisco warns that when network devices give a phone inconsistent VLAN configuration through CDP and LLDP-MED, the phone can reboot repeatedly as it switches between VLANs.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details · Coexistence with CDP / Final Network Policy Resolution and QoS · Checked 2026-10-02
  29. 29
    Cisco MPP desk phones try automatic provisioning methods in this order: DHCP, then DNS SRV, then EDOS, then GDS (activation-code onboarding) or EDOS device activation.
    Cisco IP Desk Phone with Multiplatform Firmware (MPP) Administration Guide - Provisioning · Provisioning > DNS SRV (auto-provisioning mechanism order) · Checked 2026-10-02
  30. 30
    On Catalyst 9300, if an LLDP-MED network-policy profile is configured on an interface first, 'switchport voice vlan' cannot be applied to that interface afterwards. If the voice VLAN is configured first, a network-policy profile can still be added.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Cupertino 17.7.x (Catalyst 9300 Switches) - Configuring LLDP, LLDP-MED, and Wired Location Service · Network-Policy TLV / Restrictions · Checked 2026-10-02
  31. 31
    Cisco lists two causes for an 8800 phone that cannot obtain an IP address during startup: the phone is not on the same network or VLAN as the DHCP server, or the switch port is disabled.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Startup Problems > Cisco IP Phone Cannot Obtain IP Address · Checked 2026-10-02
  32. 32
    Option 150 carries one or more IPv4 TFTP or configuration server addresses. Its length field must be an integral multiple of 4 octets.
    RFC 5859: TFTP Server Address Option for DHCPv4 · Section 3 (TFTP Server Address Option definition) · Checked 2026-10-02
  33. 33
    RFC 5859 says that when option 150 and the TFTP Server Name option (66) are both present, option 150 SHOULD take priority over option 66.
    RFC 5859: TFTP Server Address Option for DHCPv4 · Section 3 · Checked 2026-10-02
  34. 34
    DHCP option 150, the TFTP Server Address option, is documented in RFC 5859 (June 2010). RFC 5859 is an Informational RFC that records existing usage; it is not a Standards Track specification.
    RFC 5859: TFTP Server Address Option for DHCPv4 · Status of This Memo; Section 1 (Introduction) · Checked 2026-10-02
  35. 35
    RFC 2132 defines DHCP option 43 (Vendor Specific Information) as an opaque object for clients and servers to exchange vendor-specific information. Option 60 (Vendor Class Identifier) lets a client identify its vendor type and configuration.
    RFC 2132: DHCP Options and BOOTP Vendor Extensions · Section 8.4 (Vendor Specific Information); Section 9.13 (Vendor class identifier) · Checked 2026-10-02
  36. 36
    RFC 2132 defines DHCP option 66 as the TFTP server name option. It identifies a TFTP server when the 'sname' field in the DHCP header has been used for DHCP options.
    RFC 2132: DHCP Options and BOOTP Vendor Extensions · Section 9.4 (TFTP server name) · Checked 2026-10-02
  37. 38
    When a Catalyst 9300 denies a PoE power request because of the power budget, it turns off power to the port, generates a syslog message and updates the LEDs.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Dublin 17.11.x (Catalyst 9300 Switches) - Configuring Power over Ethernet · Power Management Modes / Power Budget · Checked 2026-10-02
  38. 39
    On Catalyst 9300, the initial PoE allocation for Class 0, 3 and 4 powered devices is 15.4 W. A device that asks for more over CDP or LLDP after startup can be given up to 30 W.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Dublin 17.11.x (Catalyst 9300 Switches) - Configuring Power over Ethernet · Power Negotiation / IEEE classification · Checked 2026-10-02
  39. 40
    After an 8800 phone reboots, the switch locks to CDP or LLDP for power negotiation, using whichever protocol the phone sends first with a power TLV. If that protocol is then disabled, the phone cannot power its accessories because the switch ignores power requests in the other protocol.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · VLAN Interaction / Power Negotiation note · Checked 2026-10-02
  40. 41
    On Catalyst 9300, 'show power inline' shows PoE status and available power, and 'show power inline police' shows power-policing data. 'power inline never' disables powered-device detection on a port.
    Interface and Hardware Components Configuration Guide, Cisco IOS XE Dublin 17.11.x (Catalyst 9300 Switches) - Configuring Power over Ethernet · Monitoring Power Status; Configuring a Power Management Mode on a PoE Port · Checked 2026-10-02
  41. 42
    When Power Negotiation is disabled on an 8800 phone, the phone can power accessories up to 15.4 W. Cisco recommends leaving Power Negotiation enabled, which is the default, on switches that support it.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · Power and PoE guidance > Power Negotiation · Checked 2026-10-02
  42. 43
    An 8800 phone may restart when it switches from external power to PoE, or from PoE to external power.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Phone Reset Problems > Phone Does Not Power Up · Checked 2026-10-02
  43. 44
    Cisco attributes 8800 phone resets during heavy network use to a missing voice VLAN. The fix is to put the phones on a separate auxiliary VLAN.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Phone Reset Problems > Phone Resets During Heavy Network Usage · Checked 2026-10-02
  44. 45
    In Cisco's documented 8800 startup sequence, the phone configures its VLAN before it obtains an IP address, and it obtains the IP address before it contacts the TFTP server.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details · Phone Startup Sequence (Configure VLAN; Obtain IP Address; Access TFTP Server) · Checked 2026-10-02
  45. 46
    To check TFTP settings on an 8800 phone, go to Applications > Admin settings > Network setup > Ethernet setup > IPv4 setup > TFTP Server 1. When the phone uses DHCP it gets the TFTP address from the DHCP server, so the administrator should check that the address is configured in option 150.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Troubleshooting Procedures > Check TFTP Settings · Checked 2026-10-02
  46. 47
    If the Cisco Unified CM or TFTP services are stopped, devices that depend on them cannot start up. Cisco's fix is to start the service from Cisco Unified Serviceability > Tools > Control Center - Feature Services.
    Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting · Cisco Unified Communications Manager and TFTP Services Are Not Running; Troubleshooting Procedures > Start Service · Checked 2026-10-02
  47. 48
    On Catalyst 9300 switches running IOS XE 17.18.x, voice VLAN is supported only on access ports, not on trunk ports. The 9500X and 9600X models do not support the Voice VLAN feature.
    VLAN Configuration Guide, Cisco IOS XE 17.18.x (Catalyst 9300 Switches) - Configuring Voice VLANs · Prerequisites for Voice VLANs; Restrictions for Voice VLANs · Checked 2026-10-02
  48. 49
    Cisco documents that enabling IEEE 802.1X on a Catalyst 9300 access port that has a voice VLAN and a connected Cisco IP phone causes the phone to lose connectivity to the switch for up to 30 seconds.
    VLAN Configuration Guide, Cisco IOS XE 17.18.x (Catalyst 9300 Switches) - Configuring Voice VLANs · Voice VLAN configuration guidelines · Checked 2026-10-02
  49. 50
    On Catalyst 9300, 'switchport voice vlan' takes four options. A vlan-id (1-4094) makes the phone send voice traffic in that VLAN with 802.1Q priority 5. dot1p makes it use 802.1p frames tagged with VLAN 0. untagged makes it send untagged voice traffic. none lets the phone use its own configuration.
    VLAN Configuration Guide, Cisco IOS XE 17.18.x (Catalyst 9300 Switches) - Configuring Voice VLANs · Voice VLAN configuration (Cisco IP Phone Voice Traffic) · Checked 2026-10-02
  50. 51
    For Catalyst 9300 voice VLAN to configure a Cisco IP phone, CDP must be enabled on the switch port the phone connects to. CDP is enabled globally by default.
    VLAN Configuration Guide, Cisco IOS XE 17.18.x (Catalyst 9300 Switches) - Configuring Voice VLANs · Prerequisites for Voice VLANs · Checked 2026-10-02
  51. 52
    On Catalyst 9300 switches, 'show interfaces interface-id switchport' displays the voice VLAN configuration of a port.
    VLAN Configuration Guide, Cisco IOS XE 17.18.x (Catalyst 9300 Switches) - Configuring Voice VLANs · Verifying Voice VLAN · Checked 2026-10-02

Documents

tier 1 standards and regulators

RFC 2132: DHCP Options and BOOTP Vendor Extensions

RFC Editor (IETF) · 1997-03-01 · accessed 2026-10-02

tier 1 standards and regulators

RFC 5859: TFTP Server Address Option for DHCPv4

IETF / RFC Editor · 2010-06-01 · accessed 2026-09-24

tier 2 current vendor documentation

802.1X Authentication Services Configuration Guide, Cisco IOS Release 15M&T - IEEE 802.1X Multidomain Authentication

Cisco Systems · 2012-11-29 · accessed 2026-10-02

tier 2 current vendor documentation

About VLAN

Yealink · 2026-09-15 · accessed 2026-10-02

tier 2 current vendor documentation

Cisco IP Desk Phone with Multiplatform Firmware (MPP) - Administration Guide - Technical Details

Cisco Systems · 2025-11-19 · accessed 2026-09-24

tier 2 current vendor documentation

Cisco IP Desk Phone with Multiplatform Firmware (MPP) Administration Guide - Provisioning

Cisco Systems · 2025-11-19 · accessed 2026-10-02

tier 2 current vendor documentation

Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Technical Details

Cisco Systems · 2025-11-06 · accessed 2026-09-24

tier 2 current vendor documentation

Cisco IP Phone 8800 Series Administration Guide for Cisco Unified Communications Manager - Troubleshooting

Cisco Systems · 2025-11-06 · accessed 2026-09-24

tier 2 current vendor documentation

Interface and Hardware Components Configuration Guide, Cisco IOS XE Cupertino 17.7.x (Catalyst 9300 Switches) - Configuring LLDP, LLDP-MED, and Wired Location Service

Cisco Systems · 2021-12-07 · accessed 2026-10-02

tier 2 current vendor documentation

Interface and Hardware Components Configuration Guide, Cisco IOS XE Dublin 17.11.x (Catalyst 9300 Switches) - Configuring Power over Ethernet

Cisco Systems · 2023-03-28 · accessed 2026-10-02

tier 2 current vendor documentation

Obtain the Provisioning Server Address

Yealink · 2026-09-28 · accessed 2026-10-02

tier 2 current vendor documentation

Security Configuration Guide, Cisco IOS XE 17.15.x (Catalyst 9300 Switches) - Configuring IEEE 802.1x Port-Based Authentication

Cisco Systems · 2024-11-27 · accessed 2026-10-02

tier 2 current vendor documentation

System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure TFTP Servers

Cisco Systems · 2026-09-22 · accessed 2026-09-24

tier 2 current vendor documentation

VLAN Configuration Guide, Cisco IOS XE 17.18.x (Catalyst 9300 Switches) - Configuring Voice VLANs

Cisco Systems · 2025-07-31 · accessed 2026-10-02

Cite this page

APA

WarmTransfer. (2026, October 2). Troubleshooting IP phones that do not get a voice VLAN or DHCP options. WarmTransfer. https://warmtransfer.net/knowledge/ip-phone-dhcp-vlan-troubleshooting

BibTeX

@misc{warmtransfer-ip-phone-dhcp-vlan-troubleshooting,
  title  = {Troubleshooting IP phones that do not get a voice VLAN or DHCP options},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/ip-phone-dhcp-vlan-troubleshooting},
  note   = {Verified 2026-10-02}
}