Configuration Considerations for DHCP Servers

The following configuration considerations apply to DHCP servers, the DHCP binding database, and DHCP address pools:
  • The DHCP server is supported for the default VRF only.
  • Physical ports, LAG ports, Virtual Ethernet (VE) ports, and management ports are supported.
  • Stacking ports, PE ports, and loopback ports are not supported.
  • In the event of a controlled or forced switchover, a DHCP client requests from the DHCP server the same IP address and lease assignment that it had before the switchover. After the switchover, the DHCP server is automatically re-initialized on the new Active Controller or management module.
  • For DHCP client hitless support in a stack, the stack mac command must be used to configure the MAC address, so that the MAC address does not change in the event of a switchover or failover. If stack mac is not configured, the MAC address or IP address pair assigned to a DHCP client will not match after a switchover or failover. Furthermore, if the stack mac configuration is changed or removed and the management port has a dynamic IP address, when a DHCP client tries to renew its lease from the DHCP server, the DHCP server will assign a different IP address.
  • If any address from the configured DHCP pool is used, for example, by the DHCP server or TFTP server, you must exclude the address from the network pool.
  • Ensure that DHCP clients do not send DHCP request packets with a Maximum Transmission Unit (MTU) larger than 1500 bytes. RUCKUS devices do not support DHCP packets with an MTU larger than 1500 bytes.
  • A network cannot be configured for a DHCP server pool if that network is already part of a network in a different DHCP server pool.
  • A network cannot be configured for a DHCP server pool if that network is already a superset of a network in a different DHCP server pool. For example, if network 10.10.10.0/24 is configured in DHCP server pool 1, then then network 10.10.0.0/16 and 10.10.10.0/26 cannot be configured for other DHCP server pools.
  • When there is a burst of DHCP requests from clients (scaled up to 3000), any existing SSH sessions may be disconnected if the Keep Alive option in not enabled in the client. RUCKUS recommends a Keep Alive value of 3 to 4 seconds. If disconnected, the SSH client can reconnect once the client requests are addressed.

DHCP Binding Database

  • The IP addresses that have been automatically mapped to the MAC addresses of hosts are found in the DHCP binding database in the DHCP server.
  • An address conflict occurs when two hosts use the same IP address. During address assignment, the DHCP server checks for conflicts. If a conflict is detected, the address is removed from the pool. The address will not be assigned until the administrator resolves the conflict.
  • The following table shows IP DHCP binding scalability for RUCKUS ICX devices for a stand-alone switch or a stack:

    IP DHCP Binding Scalability

    Device Maximum Number of IP DHCP Clients Supported
    RUCKUS ICX 7150 3000
    RUCKUS ICX 7550
    Please confirm
    		3000
    RUCKUS ICX 7650 3000
    RUCKUS ICX 7850 3000
    RUCKUS ICX 8100 500
    RUCKUS ICX 8200 3000
    Note: ICX 7150-ES and ICX 8100 models do not support stacking.

DHCP Address Pools

  • A DHCP address pool can be configured with a name that is a symbolic string (such as "cabo") or an integer (such as 0).
  • Configuring a DHCP address pool also puts the device into DHCP pool configuration mode, where the pool parameters can be configured.
  • If the DHCP server address is part of a configured DHCP address pool, you must exclude the DHCP server address from the network pool.
  • While in DHCP server pool configuration mode, the system will place the DHCP server pool in pending mode and the DHCP server will not use the address pool to distribute information to clients.
  • The options in the DHCP server pool cannot be configured without first configuring the network option.
  • DHCP options are supported on a per-pool basis as required by the DHCP clients to be serviced in the sub-network.
  • DHCP defines a process by which the DHCP server knows the IP subnet in which the DHCP client resides, and the DHCP server can assign an IP address from a pool of valid IP addresses in that subnet.

    If the client is directly connected (the giaddr field is zero), the DHCP server matches the DHCP DISCOVER message with DHCP pools that contain the subnets configured on the receiving interface. If the client is not directly connected (the giaddr field of the DHCP DISCOVER message is not zero), the DHCP server matches the DHCP DISCOVER message with a DHCP pool that has the subnet that contains the IP address in the giaddr field.

Parent topic: DHCP Servers