Configuring dynamic host configuration protocol (DHCP) - Metasys - LIT-12012458 - Field Device - 13.0

Metasys IP Networks for BACnet/IP Controllers Technical Bulletin

Brand
Metasys
Document type
Technical Bulletin
Document number
LIT-12012458
Version
13.0
Revision date
2023-10-23
Product status
Active
Language
English

DHCP is used to dynamically assign an IP address to a device, creating a binding of the IP address to the device’s MAC address. If IP addresses are to be statically configured on all the Metasys IP-enabled devices, DHCP is not required. If the Metasys devices are to be directly connected to the existing IT network, IT’s existing DHCP server will need to be updated to include the subnetwork (s) assigned to the BAS system. For recommendations on the partitioning of Metasys devices to subnetworks, see Planning your network.

To configure a DHCP server on a Cisco managed switch, complete the steps in the following table:

Table 1. Configuring a DHCP server on a Cisco managed switch

Configuration step

Cisco IOS CLI command

1

Enter global configuration mode.

Switch# configure terminal

2

Configure the DHCP server to save automatic bindings to a local file. In this example, the bindings are written to a file named DHCP text every 60 seconds.

Switch(config)# ip dhcp database flash: DHCP.text write-delay 60

3

Set the interval for cleaning up expired DHCP bindings. In this example, the binding are cleaned up every 30 seconds.

Switch(config)# ip dhcp binding clean up interval 30

4

Create a DHCP server address pool. For this example the pool is named "DHCP1".

Switch(config)# ip dhcp pool DHCP1

5

Use the subnetwork-specific broadcast address (172.16.10.255) instead of the default of 255.255.255.255 as the IP VAV box controllers and controllers do not respond to 255.255.255.255.

Switch(config)# ip dhcp limited-broadcast-address

6

Specify the subnetwork network number and mask of the DHCP pool.

Switch(dhcp-config)# network 172.16.10.0 /24

7

Specify the IP address of the device which should be used as the default router for the DHCP clients.

Switch(dhcp-config)# default-router 172.16.10.1

8

Specify the duration of the lease of the IP addresses. In this example the lease is set to 10 minutes.

Switch(dhcp-config)# lease 0 0 10

9

Create/associate a class with the pool. In this example the class is named “CLASS1”.

Switch(dhcp-config)# class CLASS1
10

Specify the range of addresses within the pool to be dynamically assigned. In this example, 210 addresses are allocated for assignment.

Switch(dhcp-pool-class)# address range 172.16.10.40 172.16.10.250
11

Exit configuration of the class.

Switch(dhcp-pool-class)# exit
12

Configure the pool to remember the IP bindings of a device so that it assigns the same address to a device after the switch or the device reloads.

Switch(dhcp-config)# remember
13

Exit global configuration mode.

Switch(dhcp-config)# end

DHCP can also be used to assign a predetermined IP address to a specific device, thereby preventing the need to configure a static IP address at each device. With this approach an IP address is statically mapped to a device’s MAC address in the managed switch, consolidating the static IP address mapping in one place. Some managed switches allow a text file containing the static bindings to be created externally and imported into the switch. For your managed switch regarding its static binding capabilities and the associated configuration commands, refer to the product documentation.

If an IP controller is configured for DHCP but the initial attempt to find a DHCP server fails, the device will temporarily assign itself an Automatic Private IP Addressing (APIPA) IP address. The 169.254.0.0 /16 subnetwork is reserved for APIPA. After assigning itself an APIPA address the IP controller will continue to look for a DHCP server. The current IP controller behavior is to make 3 attempts to find a DCHP server (at 0, 1, and 6 seconds) every 5 minutes until it is successful.

For a system that is not very large, there is an opportunity to reduce the cost of the system. You could configure a single DHCP server on the BAS aggregation switch, instead of individual DHCP servers on each of the BAS access switches. A cost savings can be realized because the BAS access switches no longer need a license for IP-routing functionality; a less expensive, light version of the license suffices. This approach can be taken when a single aggregation switch has enough ports and bandwidth to serve all the access switches, and the aggregation switch can be installed prior to, or at the same time as, the access switches so that the aggregation switch's DHCP server is available for devices connected to the access switches.