ALIF: DHCP

Up until the release of AIM firmware version 4.1 on the second generation hardware platform, the AIM Manager issued static IP addresses to the endpoints (Transmitter and Receiver) in a pseudo DHCP like manner.

Out of the box, an endpoint was assigned an IP address using the following steps:-

1. Using its own link-local IP address, it contacted the AIM Manager's fixed link-local address of 169.254.1.2.
2. If using 3.3 firmware or higher, the AIM Manager and endpoint would exchange SSL Certificates to ensure that the communication between the two was secure.
3. The endpoint would be issued an IP address out of the user-defined IP address pool range.
4. The AIM's Administrator would then be given the opportunity to add and name the endpoint to its device list.

The IP address given to the endpoint was fixed and did not change unless the unit was either factory reset or the address was changed on the AIM Manager.

To accommodate the ever-increasing size of Infinity matrix installations and to make them easier to manage, from Infinity firmware version 4.1 onwards, a DHCP Server is required to allocate IP addresses to the endpoints and to supply Option 125 that instructs them where to find the AIM Managers on the network.

Subnet Mode

From 4.1 onwards, during the AIM's initial setup, you must specify whether Subnet mode should be on or off. Turning Subnet mode on allows the AIM to support multiple subnets. Once the setting is chosen and the network settings saved, this option can not be changed unless the AIM is fully factory reset.

With Subnet mode on, an external DHCP Server must be set up on all the subnets that contain endpoints. DHCP Option 125 must contain a list of AIM IP addresses for all the managers in the matrix.

With Subnet mode off, the AIM Manager will run an industry-standard DHCP Server to offer IP addresses to the endpoints on the same subnet which includes DHCP Option 125. The DHCP server running on the AIM should be thought of as a separate entity to the main firmware/graphical web interface. The main firmware only interacts with it in the following ways:-

• Configures the IP Pool range as per the user settings.
• Generates the DHCP Option 125 based upon the AIM Servers (Backups, Satellites) that it is aware of.
• Reserves IP addresses against the Mac Addresses of endpoints.

It is important to note that the DHCP Server on the AIM will lease an address for two hours.

Endpoint Boot Behaviour

When an endpoint is powered on, unless it has been manually configured, it will go through the following steps:-

• Automatically looks for a DHCP Server. In order to accept an IP address, the DHCP Server must offer DHCP Option 125.
• Using the AIM's IP addresses that are contained within DHCP Option 125, it will start to make contact with the Primary (or acting) AIM. NOTE: An unconfigured unit will not step through the list of AIM Servers, and will only try contacting the first AIM in the list.
• Upon contact, if unconfigured it will exchange SSL Certificates and allow the Administrator to configure the endpoint.
• The AIM will also update the IP address(s) that are shown for the endpoint under the appropriate Receivers or Transmitters tab. It is important to note, that all the endpoint IP addresses shown are updated in this manner and that this information is not gathered from the DHCP Server directly since this can be external to the AIM. This can lead to seeing duplicate IP address is in the list, where an endpoint maybe offline and the DHCP Server has allocated the same address to a different endpoint. There is no need to worry; when the offline endpoint comes back online, it will be allocated a different IP address.
• If no DHCP Server is found, the endpoint will revert to a link-local IP address. In this state, it can be used in a simply point to point setup.

DHCP Assigned

One common misunderstanding on the AIM's endpoint settings page is the behaviour of the checkbox called DHCP Assigned which is only available if Subnet mode is off. By default, the checkbox is ticked, and the two IP address boxes are greyed out.

Unticking the DHCP Assigned checkbox allows you to assign your own IP address to the endpoint. Provided that the IP address has not already been assigned to another endpoint when you save the change, the endpoint will automatically reboot and use the IP address that you have set.

However, this does not mean that endpoint has been given a static fixed address. What this setting does is reserve the IP address in the DHCP Servers configuration against the unique Mac Address of the endpoint, so that each time the endpoint is booted or requests an IP address when its lease has expired, it is always given the same IP address.

DHCP Lease

With Subnet mode off and the DHCP being supplied by the AIM Manager, the default IP lease time is two hours. However, as is default with most DHCP clients, the endpoint will start to renew its lease 50% of the lease time (e.g. after an hour). The main reason for choosing the short lease period is to ensure that the endpoints are updated with any change in a reasonable time. For example, if you add another AIM Server then option 125 will be updated to reflect the change and it will take up to an hour for all the endpoints to be updated when they renew their lease.

Duplicate IP Addresses that are shown on AIM

The best way to understand how the IP addresses are displayed on the AIM Manager is to think of the web interface and DHCP Server as being completely separate entities. The IP addresses shown on the AIM's web interface are updated when an endpoint comes Online. For those units that are Offline, the IP addresses displayed are their last known addresses. However, since the typical DHCP lease time is two hours, it is possible that when an endpoint renews its IP address it could be handed out an IP address that was previously used by another offline endpoint. This can give the impression that the IP address is duplicated and is assigned to both an online and offline device. However, when the offline device is brought back online again, it will be given a different IP address and the AIM's database will be updated. In summary, it is not possible for online endpoints that are under AIM's DHCP control to have duplicate IP addresses.