This document explains how to download a software image to a Cisco 2600/2800/3800 Series Router using Trivial File Transfer Protocol (TFTP) over the first LAN port using the ROMmon tftpdnldcommand.
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Basic Router Configuration ( NAT on HQ & Branch Office WAN Link )

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interface FastEthernet0/0
ip address 192.168.1.20 255.255.255.0
ip nat outside
ip virtual-reassembly
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 192.168.3.1 255.255.255.0
ip nat inside
ip virtual-reassembly
duplex auto
speed auto
!
ip classless
ip route 0.0.0.0 0.0.0.0 192.168.1.1
ip http server
no ip http secure-server
!
!
ip nat inside source list 1 interface FastEthernet0/0 overload
!
access-list 1 remark SDM_ACL Category=2
access-list 1 permit 192.168.3.0 0.0.0.255