DHCP to use last provided IP Address on Restart

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This feature supports the requiremens for telephony resilience at remote sites (branch offices), when the connection is lost between the remote site and the central Servers which provide DHCP. The request is that if the phones cannot complete the DHCP sequence to obtain or renew their IP addresses and other DHCP provided data they attempt to use the data they last received from DHCP/LLDP-MED).

The main scenarios are:

  • The DHCP lease for a working phone expires because the central DHCP server is unreachable.
  • A booting phone cannot obtain a DHCP lease due to the DHCP server being unreachable.

In either case, when configured to allow this enhancement, a phone will attempt to use the last net-work configuration it received, but first it must check:

  • Is the IP address in use by another device?
  • Does LLDP-MED indicate a possible change of network configuration or topology?

If the answer to either is yes then the phone will stay/go out of service and will wait for a valid DHCP lease.

Although initially requested in the context of a SIP customer, the changes will be generic – and will apply to all SIP and HFA OpenStage Phones.

The new feature will not change the phone’s default behaviour; it has to be enabled at administrator level.

Configuration Modes

When a phone connects to an IP network its network configuration can be established in a variety of ways:

  1. Manual:– the administrator manually configures the network settings (via the phone, WBM or even remotely via DLS).
  2. LLDP-MED with DHCP: The local network Infrastructure (Ethernet switches etc.) can auto-matically deliver some Network configuration data, including VLAN ID, prior to the IP layer be-ing established.
  3. DHCP: Network data such as IP address and VLAN ID can be established automatically from a DHCP server in the form of a DHCP lease.
  4. Manual VLAN with DHCP: The VLAN is set by administration but a DHCP lease is required for other network settings (IP address etc.).

Only Modes 2, 3 and 4 above apply in the context of this enhancement.

Whilst it is not the role of this document to describe the handling or support of individual network configuration items or protocols, the configuration modes will influence whether a phone which relies on a DHCP server can be allowed to join the network when a DHCP failure occurs.

User Interface Enhancements

There are no user specific User Interface enhancements required.

When a phone determines it cannot operate due to a network problem (in this case lack of an IP ad-dress due to DHCP failure) then it provides an indication of the problem to the user via an idle screen error message. This is normal behaviour and is unchanged.

However once the enhanced operation takes over and brings the phone into service by re-using an old address, then the failure warning will NOT be shown (even though at the network level the DHCP process is still technically failing). Notifying the user of a network problem on a phone that appears to be functioning perfectly will only cause confusion, or encourage the user to take some kind of action to recover the fault (e.g. reboot phone) and may incur costs associated with a unnecessary service calls.

There will be an addition to the network configuration to allow setting of the new feature.

The IP Configuration section of the Network Administration page (menu and WBM) will be enhanced to include an option to configure the new mode for DHCP.

The DHCP config options will be extended to include a new config item:

DHCP lease reuse
choice of: ON/OFF (Tick box on WBM) Yes/No (phone menu)

On the associated Web page a warning will be presented about the risk of address conflicts when set-ting DHCP lease reuse to on.

The new setting will be configurable independent of the current DHCP mode – however if set to Yes or ON the enhanced behaviour only applies when DHCP is actually enabled.

When switching from between manual and DHCP modes the DHCP re-use setting will not change automatically so, if On/Yes, it does not revert to its default of Off/No. However if the Phone is set to Manual configuration the Option for DHCP reuse should be made read-only ( greyed out).