DHCP/Bootp operation

Overview. DHCP/Bootp is used to provide configuration data from a DHCP or Bootp server to the switch. This data can be the IP address, subnet mask, default gateway, Timep Server address, and TFTP server address. If a TFTP server address is provided, this allows the switch to TFTP a previously saved configuration file from the TFTP server to the switch. With either DHCP or Bootp, the servers must be configured prior to the switch being connected to the network.

NOTE:

The switches covered in this guide are compatible with both DHCP and Bootp servers.

The DHCP/Bootp process. Whenever the IP Config parameter in differences the switch or in an individual VLAN in the switch is configured to DHCP/Bootp (the default), or when the switch is rebooted with this configuration:

DHCP/Bootp requests are automatically broadcast on the local network. (The switch sends one type of request to which either a DHCP or Bootp server can respond.)
When a DHCP or Bootp server receives the request, it replies with a previously configured IP address and subnet mask for the switch. The switch also receives an IP Gateway address if the server has been configured to provide one. In the case of Bootp, the server must first be configured with an entry that has the switch’s MAC address. (To determine the switch’s MAC address, See Appendix D, “MAC Address Management”.) The switch properly handles replies from either type of server. If multiple replies are returned, the switch tries to use the first reply.)

NOTE:

If you manually configure default gateway, TTL, TimeP, and/or SNTP parameters on the switch, it ignores any values received for the same parameters via DHCP or Bootp.

If the switch is initially configured for DHCP/Bootp operation (the default), or if it reboots with this configuration, it begins sending request packets on the network. If the switch does not receive a reply to its DHCP/Bootp requests, it continues to periodically send request packets, but with decreasing frequency. Thus, if a DHCP or Bootp server is not available or accessible to the switch when DHCP/Bootp is first configured, the switch may not immediately receive the desired configuration. After verifying that the server has become accessible to the switch, reboot the switch to restart the process immediately.

DHCP operation. A significant difference between a DHCP configuration and a Bootp configuration is that an IP address assignment from a DHCP server is automatic. Depending on how the DHCP server is configured, the switch may receive an IP address that is temporarily leased. Periodically the switch may be required to renew its lease of the IP configuration. Thus, the IP addressing provided by the server may be different each time the switch reboots or renews its configuration from the server. However, you can fix the address assignment for the switch by doing either of the following:

Configure the server to issue an “infinite” lease.
Using the switch’s MAC address as an identifier, configure the server with a “Reservation” so that it will always assign the same IP address to the switch. (For MAC address information, See Appendix D, “MAC Address Management”.)

For more information on either of these procedures, See the documentation provided with the DHCP server.

Bootp operation. When a Bootp server receives a request, it searches its Bootp database for a record entry that matches the MAC address in the Bootp request from the switch. If a match is found, the configuration data in the associated database record is returned to the switch. For many Unix systems, the Bootp database is contained in the /etc/bootptab file. In contrast to DHCP operation, Bootp configurations are always the same for a specific receiving device. That is, the Bootp server replies to a request with a configuration previously stored in the server and designated for the requesting device.

Bootp Database Record Entries. A minimal entry in the Bootp table file /etc/bootptab to update an IP address and subnet mask to the switch or a VLAN configured in the switch would be similar to this entry:

8212switch:\
   ht=ether:\
   ha=0030c1123456:\
   ip=10.66.77.88:\
   sm=255.255.248.0:\
   gw=10.66.77.1:\
   hn:\
   vm=rfc1048

An entry in the Bootp table file /etc/bootptab to tell the switch or VLAN where to obtain a configuration file download would be similar to this entry:

8212switch:\
    ht=ether:\
    ha=0030c1123456:\
    ip=10.66.77.88:\
    sm=255.255.248.0:\
    gw=10.66.77.1:\
    lg=10.22.33.44:\
    T144="switch.cfg":\
    vm=rfc1048

where:

8212switch	is a user-defined symbolic name to help you find the correct section of the bootptab file. If you have multiple switches that will be using Bootp to get their IP configuration, you must use a unique symbolic name for each switch.
ht	is the "hardware type". For the switches covered in this guide, enter `ether` (for Ethernet). This tag must precede the ha tag.
ha	is the "hardware address". Use the switch's (or VLAN's) 12-digit MAC address.
ip	is the IP address to be assigned to the switch (or VLAN).
sm	is the subnet mask of the subnet in which the switch (or VLAN) is installed.
gw	is the IP address of the default gateway.
lg	TFTP server address (source of final configuration file)
T144	is the vendor-specific "tag" identifying the configuration file to download.
vm	is a required entry that specifies the Bootp report format. Use `rfc1048` for the switches covered in this guide.

NOTE:

The above Bootp table entry is a sample that will work for the switch when the appropriate addresses and file names are used.