Cisco 800 Series Посібник з проектування - Сторінка 22

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LAN IP Addressing

Setup configures the IP stack of the PC and the IP address of the router for the end user. You can modify

the IP addressing parameters by using the CFG flags.

It is assumed that the end-user PC used to configure the router is the same PC as the one that will be

used to communicate over the WAN. Before running Cisco 800 Fast Step, the end-user PC can be in one

of three states:

•

If there is no IP stack in Windows 95, Setup asks Windows to install an IP stack. The new IP stack

defaults to DHCP without a leased IP address.

Windows 98 and Windows 2000 use a modified algorithm that gives the PC a temporary, unique static

address when no DHCP server is found. Setup assumes that no leased IP address exists and treats all

operating systems the same.

The router requires the Ethernet interface to have an IP address to communicate with the PC over Telnet,

yet it has only the factory-default configuration at this stage. This creates a state where neither the PC

nor the router has an IP address. To overcome this, an algorithm temporarily assigns an address to the

Ethernet interface. (The PC must have a functioning IP stack for the algorithm to work.)

A PC running Setup sends a DNS request to new-router.cisco.com, a request that the router is listening

for. The router takes the originating IP address and uses the Address Resolution Protocol (ARP) to find

a free address, either above or below the originating address.

Where there is no IP stack or no DHCP address, Setup assigns a temporary static address to the PC and

later resets the PC to DHCP.

Unless you are using Windows 2000, changing the PC addressing requires that the PC be rebooted

twice, once when Setup configures the PC IP stack and once after router setup and testing are complete.

(If you are using Windows 2000, it is not necessary to reboot the PC.) If the PC has no IP stack, the

LAN has never had an IP addressing scheme, so Setup imposes the most logical configuration:

•

Setup uses the same scheme for a PC that is set as a DHCP device with no IP address. We suggest that

you use this scheme for the majority of installations.

If the PC has DHCP with a valid address, Setup assumes there is a DHCP server on the LAN, and the

PC IP stack remains untouched. The router searches for a temporary IP address by using ARP, starting

at the PC IP address. The IP stack remains untouched. Because the DHCP server is local, it could assign

public IP addresses because the IP addressing scheme could be chosen at random. Setup uses (PAT) to

hide the DHCP server IP addresses from the ISDN interface.

Cisco 800 Fast Step Software Design Guide

2-8

Using dynamic host configuration protocol (DHCP) without a leased IP address

Using DHCP with a leased IP address

Statically addressed

PC is set to a DHCP client.

Router is set to a DHCP server and given a range of private address.

IP addresses in the DHCP pool are hidden by using Port Address Translation (PAT).

Chapter 2

CFG File Development

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