Cisco WS-C4003 Software Guide - Page 49
Understanding How DHCP Works
View all Cisco WS-C4003 manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 49 highlights
Chapter 3 Configuring the Switch IP Address and Default Gateway Understanding Automatic IP Configuration Note If the CONFIG_FILE environment variable is set, all configuration files are processed before the switch determines whether to broadcast DHCP and RARP requests. For more information about the CONFIG_FILE environment variable, see Chapter 28, "Modifying the Switch Boot Configuration." If both the sc0 and me1 interfaces are unconfigured (IP address 0.0.0.0), the me1 interface is brought down to allow the switch to broadcast requests on the sc0 interface. If the me1 interface is configured and the sc0 interface is not, requests are not sent. Similarly, if the sc0 interface is not configured but the interface is configured down, requests are not sent. Understanding How DHCP Works In software release 5.2 and later, the switch can obtain an IP address and other IP configuration information using DHCP. There are three methods for obtaining an IP address from the DHCP server: • Manual allocation-The network administrator maps the switch MAC address to an IP address at the DHCP server. • Automatic allocation-The switch obtains an IP address when it first contacts the DHCP server. The address is permanently assigned to the switch. • Dynamic allocation-The switch obtains a "leased" IP address for a specified period of time. The IP address is revoked at the end of this period, and the switch surrenders the address. The switch must request another IP address. In addition to the sc0 interface IP address, the switch can obtain the subnet mask, broadcast address, default gateway address, as well as other information. DHCP-learned values are not used if user-configured values are present. The switch broadcasts a DHCPDISCOVER message one to ten seconds after all of the switch ports are online. The switch always requests an infinite lease time in the DHCPDISCOVER message. If a DHCP or Bootstrap Protocol (BOOTP) server responds to the request, the switch takes appropriate action. If a DHCPOFFER message is received from a DCHP server, the switch processes all supported options contained in the message. Table 3-1 shows the supported DHCP options. Other options specified in the DHCPOFFER message are ignored. Table 3-1 Supported DHCP Options Code 1 2 3 6 12 15 28 33 42 Option Subnet mask Time offset Router Domain name server Host name Domain name Broadcast address Static route NTP servers 78-12647-02 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 3-3