Cisco PIX 525 Spezifikationen PDF herunterladen (Seite 109)

8-27

Cisco Security Appliance Command Line Configuration Guide

OL-6721-01

Chapter 8 Configuring IP Routing and DHCP Services

Configuring DHCP

Cisco IP Phones download their configuration from a TFTP server. When a Cisco IP Phone starts, if it

does not have both the IP address and TFTP server IP address preconfigured, it sends a request with

option 150 or 66 to the DHCP server to obtain this information.

• DHCP option 150 provides the IP addresses of a list of TFTP servers.

• DHCP option 66 gives the IP address or the hostname of a single TFTP server.

Cisco IP Phones might also include DHCP option 3 in their requests, which sets the default route.

Cisco IP Phones might include both option 150 and 66 in a single request. In this case, the security

appliance DHCP server provides values for both options in the response if they are configured on the

security appliance.

You can configure the security appliance to send information for most options listed in RFC 2132. The

following table shows the syntax for any option number, as well as the syntax for commonly-used

options 66,150, and 3:

• To provide information for DHCP requests that include an option number as specified in RFC-2132,

enter the following command:

hostname(config)# dhcpd option

number

value

• To provide the IP address or name of a TFTP server for option 66, enter the following command:

hostname(config)# dhcpd option 66 ascii

server_name

• To provide the IP address or names of one or two TFTP servers for option 150, enter the following

command:

hostname(config)# dhcpd option 150 ip

server_ip1

[

server_ip2

]

The server_ip1 is the IP address or name of the primary TFTP server while server_ip2 is the

IP address or name of the secondary TFTP server. A maximum of two TFTP servers can be

identified using option 150.

• To provide set the default route, enter the following command:

hostname(config)# dhcpd option 3 ip

router_ip1

Configuring DHCP Relay Services

A DHCP relay agent allows the security appliance to forward DHCP requests from clients to a router

connected to a different interface.

The following restrictions apply to the use of the DHCP relay agent:

• The relay agent cannot be enabled if the DHCP server feature is also enabled.

• Clients must be directly connected to the security appliance and cannot send requests through

another relay agent or a router.

• For multiple context mode, you cannot enable DHCP relay on an interface that is used by more than

one context.

Note DHCP Relay services are not available in transparent firewall mode. A security appliance in transparent

firewall mode only allows ARP traffic through; all other traffic requires an ACL. To allow DHCP

requests and replies through the security appliance in transparent mode, you need to configure two

ACLs, one that allows DCHP requests from the inside interface to the outside, and one that allows the

replies from the server in the other direction.

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