IP Phone Failover – Intra-site and Cross-site | Global CTI

IP Phone Failover – Intra-site and Cross-site

ShoreTel – IP Phone Failover – Intra-site and Cross-site (Spare Switch)


Preface: (Warning lengthy read)

The below information will help you better understand the IP phone “Failover” process.

I employ you to take few minutes to read the below information. Doing so, will help you better understand the ShoreTel “N+1” architecture and should clear up any misconceptions you may have.

By the end of your reading, you should have a full understanding of Intra-site and Cross-Site IP phone failover.



IP Phone Configuration Switches

When you add your first two ShoreGear switches to the system, they will automatically be assigned the roles of the “IP Phone Configuration Switches.”

More specifically, to be eligible to be assigned the role of an IP Phone Configuration Switch, the ShoreGear appliance must meet the following criteria:

• The ShoreGear switch must be managed by the HQ server

• The ShoreGear switch must be capable of supporting IP phones (i.e. The ShoreGear T1k, and 24a are not eligible)


The ShoreTel system uses these IP Phone Configuration Switches to assist with the assignment of IP Phones to specific ShoreGear switch resources for call control and dial tone.

The two selected switches are displayed in ShoreWare Director under “Administration > IP Phones > Options.” You can select any two ShoreGear switches that are managed by the HQ server and that support IP Phones.

It is the job of the IP Phone Configuration Switches to coordinate the assignment of each newly added IP Phones to the appropriate ShoreGear resource. A critical component of this process is the IP Phone Address Map List located in ShoreWare Director under “Administration > IP Phones > IP Phone Address Map”.


When a new ShoreTel IP phone is plugged into the data network, the IP Phone will contact the HQ (or DVS) server and download configuration files which contain, among other things, the current running ShoreTel software version and the IP addresses of the IP Phone Configuration Switches.

The IP Phone will upgrade to the current running firmware (if needed) and then contact one of the two IP Phone Configuration Switches. The new IP Phone will identify itself to the IP Phone Config Switch and ask to be assigned to a ShoreGear switch for dial tone and call control.

Note: Only those ShoreGear switches with assigned “IP Phone” resources are eligible to manage IP Phones


The IP Phone Configuration Switches act as “traffic cops” and coordinate the IP Phone requests for services by proxying the request from the IP Phone through to the IP Phone Configuration Service (IPCS) running on the HQ server. The HQ server is fully aware of all switches at all sites and the IP Phone resources that are available and in use by each and every ShoreGear switch. The IPCS service is also in possession of the IP Phone Address map.

With this knowledge, the requesting Phone’s IP address will be compared to the IP Phone Address Map, the proper site will be identified and an available IP Phone resource on a ShoreGear switch at that site will be assigned to serve as the “Managing” or “Resource” switch for that IP phone. This managing ShoreGear switch is sometimes called the “Call Manager” switch for that IP phone.


IP Phone Load Balancing

All switches with IP phone resources at a site will be actively load-balanced so that, on average, an equal number of IP phones will be assigned to each of the switches at a site.


Example: (3 – SG-90 at HQ, 3 – SG-50 – Branch Office)

HQ – IP Phone resources (90+90+90).

When the 150 IP phones at the HQ site register, they will be equally distributed across the three ShoreGear switches with available IP Phone resources. This results in 50 IP phones being assigned to each of the ShoreGear-90 switches at HQ.

Branch Office – IP Phone resources (50+50+50).

The 130 phones at the Branch Office will be assigned 44, 43 & 43 per ShoreGear switch, respectively.

The HQ site has lots of extra capacity. The Branch Office site has very little extra capacity.


Key points regarding IP Phone Configuration Switches:

• IP Phone Config Switches MUST be managed by the HQ server

• IP Phone Config Switches do NOT need to be assigned to, or located at, the HQ site. They can be located at any site (pursuant to the above).

• This allows for an HQ site to be located at a co-location facility with nothing but an HQ server and no ShoreGear switches at all. This is a valid deployment.

• For a spare switch to “kick in” the IP Phone Config Switches must be able to communicate with the HQ server to effect a database change.


N+1 Failover – Same Site (Intra-site)

ShoreGear Switch – “Hello” packets

Several important points should be noted:

“Hello” packets are generated by the ShoreGear switches approximately every 60 seconds

• If an IP Phone fails to get 4 “Hello” packets in a row (4 minutes) it will initiate a failover by communicating with the IP Phone Configuration Switches

• If any action is taken by the phone or the user prior to the 4 minute timer the failover process is begun immediately. “Any action” can be as simple as refreshing the display, lifting the handset, pressing any button, etc. This will immediately initiate failover to another ShoreGear switch

• Therefore, the MAXIMUM delay before failover occurs is 4 minutes. The minimum is “immediately,” if the phone’s state is in any way changed Failover, once initiated, generally takes less than 2 seconds to complete

• Any active calls will stay up and connected even if the ShoreGear switch failure occurs in the middle of a conversation. The IP Phone will actually failover to the new switch in the background while the conversation is still active

• Extra IP Phone resources located at another site will not be used by “Same-site IP Phone Failover.”  “Spare Switch” is required in order to  cross-site IP Phone Failover. This is known as Spare Switch failover or inter-site failover.

Common misperceptions about intra-site IP Phone Failover (N+1).

The first misconception is the term “N+1” itself. A common mistake is to assume that the “+1” switch is special or configured in a different way. This is not true. When adding ShoreGear switches to a site, all switches should be added as normal, or “primary”, switches. The IP Phone Config Switches will actively load balance newly added phones across all ShoreGear switches with IP Phone resources. The key is to do your math correctly. If you have 150 phones and you “need 150 resources” you could deploy three ShoreGear-50s to satisfy the need (N). Adding one more (+1) for a total of four ShoreGear-50 switches results in four actively used switches that are load balanced. If any of them goes offline there are ample remaining configured resources for all the “orphaned” IP Phones to be reassigned to the available IP Phone resources on the remaining three ShoreGear switches.

A second misconception of “N+1 Failover” is that extra IP Phone resources at one site will help another site. This is not true. IP Phone failover (N+1) is a site-specific solution. If you desire a cross-site failover solution see the next section on “Spare switches.”

A third misconception is the notion that N+1 switches will always be held in reserve. This is not true. In fact the “+1 switch” is just a normal, primary switch. If the customer adds phones, grows or changes configurations it is easy for that “+1 switch” (and all the resources that you intended to be used as backup in case of a failure) to become “used up” by normal, everyday growth and expansion. If a site starts with 200 ports and 150 IP phones and grows by adding ten or twenty+ more IP phones, before you know it they have 180 phones spread across their four ShoreGear-50 switches and, in case of a ShoreGear switch failure, there will not be enough extra/available IP Phone resources for ALL orphaned phones to be re-assigned. Be careful of this “growth creep” and ensure you plan for both capacity and failover. Frequent monitoring of IP Phone resource usage will identify the time when additional ShoreGear switch resources should be added to a site to maintain the availability of the necessary quantity of resources.

A fourth important clarification is regarding the registration of IP Phones. To be reassigned to a new ShoreGear switch to act as its controlling, or “managing”, resource switch requires that the change be written to the main configuration database. In ShoreTel 11 and 12 this main database located on, and can only be written to, by the HQ server. Therefore, it is imperative that the HQ server be reachable by the IP Phone Configuration Switches in order for any change in IP Phone assignment to occur. Both adding a new phone, and/or reassigning a phone to a new ShoreGear switch, require a change to the HQ server’s database. Without access to the HQ server no new IP Phone assignment or N+1 reassignment can occur.

IP Phone Failover—Cross-Site (aka: Spare Switch)

The concept of a “spare” switch was introduced in ShoreTel 9.

Spare switches are used to provide a reserve of extra IP Phone resources.

You define a ShoreGear switch as a “spare” switch by selecting “Switches > Spare” when added the appliance within ShoreWare Director.


Spare switches are added to a specific site and their use is dictated by the ShoreTel site tree hierarchy. A spare switch added to the HQ site will be available to be used by the HQ site and all sites below it. A spare switch assigned to a child site will ONLY be available for use by that child site and any children (grandchildren, etc.) of that site. The spare switch assigned to a child site will not be seen or used by any “parent” or “sibling” sites – only by the site it was added to and its children sites.


“Spare” switch configurations differ from same-site (N+1) configurations in several important ways:

• Same-site (N+1) failover uses all switches at a site in an active manner. “Spare” switches are dormant (i.e. held in reserve) and not used until brought into play by the ShoreTel system

• Same-site (N+1) failover provides resources for only the one site that has the switches (same-site failover). “Spare” switches can be used by the site to which it is added to as well as any child sites below it (referred to as “cross-site failover”)

• Same-site (N+1) failover allows active use of all switches for any purpose including analog trunks, analog phones, hunt groups, Ad-Hoc conference ports, Bridged Call Appearance, etc. “Spare” switches cannot be used for any purpose other than IP Phone resources.


The activation of a dormant spare switch follows the same rules as outlined above under the “N+1” section. If an IP Phone at a site loses connectivity to its managing, or “resource,” switch it will communicate to the IP Phone Configuration Switch that it needs to be reassigned to a new IP Phone resource on a different ShoreGear switch at its site. The IP Phone Config Switch will use up all available IP Phone resources on all existing switches at a site first (exactly as outlined above under “same-site N+1”).


After all available IP Phone resources on all ShoreGear switches at that site are consumed and in use, if another IP phone at that site is added or asks to be reassigned, the IP Phone Configuration Switch will look for a dormant “spare” switch at that site. If no “spare” switch is found at the local site it will continue hunting up the ShoreTel hierarchical tree until it finds an available (unused, dormant, idle, non-active) “spare” switch. If one is found, that dormant “spare” switch will be activated and assigned to the site in need.

When activated, the “spare” switch will reboot and come up, connected to the assigned site. Once that switch has rebooted and been “assigned” to the site in need, the IP phone that requested to be reassigned will be assigned to that new ShoreGear switch.


It is important to note that the ENTIRE “spare” ShoreGear switch is assigned to the site in need.

Not just enough IP Phone resources to meet the need—but the entire switch.


Important design considerations:

• It is a best practice to place your “spare” switches at the HQ site so they can be called into service by any site

• It is a best practice to have several small switches as your “spare” switches rather than one large switch since the entire switch is assigned to the site in need


When a spare switch is called into play by a site that needs additional IP Phone resources it is changed from a “dormant and unassigned” state to an “active and assigned” state.

Once a spare switch has been assigned to a site the ShoreTel administrator must take manual steps to reclaim the “spare” switch once additional IP Phone resources have been re-added to the site in need.

Note: Voicemail switches, or “V” switches, cannot be used as a spare switch