Heard a lot of small partners have long wanted to dhcp relay contacts practical operation of the service content, let us work together to understand the next:
Step
into the GNS3 interface, drag take two routes to the topology operating area, and then drag to select HOST take three PC machine topology operating area; while two routers for configuration settings (for the switch to change the icon, change the device name sw2 / sw3, add Layer service blade, disk space is provided 100Mb)
a second step
to build the desired topology architecture : host host three: one called DHCP, change the icon to sever the server; the other two default configuration unchanged, were renamed: win10-1 and win10-2. Two routers: a renamed sw2, change the icon for the switch, modify the configuration to add Layer 2 services board, set the switch disk space 100Mb; a renamed sw3, change the icon for the three switches, modify the configuration to add Layer 2 services veneer, set the switch disk space 100Mb (optional space, can be enough).
The third step is
to connect the following manner, after the connection is completed, all display port number. Next we want to divide VLAN: win10-1 classified as vlan10, win10-2 classified as vlan20, DHCP classified as vlan100
fourth step
edit gateway settings: win10-1: 192.168.10.1/24,win10-2: 192.168.20.1 /24,DHCP:192.168.100.1/24.
Legend Sample: 
1, used in CentOS online 7 yum install dhcp service packages warehouse
enter: yum install dhcp -y (non-interactive installation mode)
to be installed after the end of the set the ens33 card
input: vim / etc / sysconfig / network-scripts / ifcfg-ens33
follows:
Then there is the process of configuring DHCP:
Enter: cp /usr/share/doc/dhcp-4.2.5/dhcpd.conf.example /etc/dhcp/dhcpd.conf
input: yes (to confirm coverage)
to enter the DHCP configuration file configuration : 
the TIP: address and the following gateways ";" number is not less sure
after editing wq save and exit.
Open dhcp service, check its status. FIG follows: 
the custom network adapter to the host only mode vmnet1,
2, win10-2: custom network adapter to the host only mode vmnet2
3, win10-1: custom network adapter to the host only mode vmnet8
back GNS3 double-click to open SW2 (switching Layer) configuration port:
set vlan three domains:
Trunk mode is set to exit route mode: 
double-click to open sw3 (three-tier exchange) and a port Trunk:
Reminder: Because this device is a three-tier exchange, the route used to make the conversion, so here routing function does not require closed
configuration Trunk: 
we have to do is to configure the DHCP relay in gateway interface sw3, the gateway role is to isolate and control the release of broadcasting. 
Finally, enter the verification system of the time, into win10-1:
first into the network adapter, the state set to automatically obtain IPv4 
Open cmd command
ipconfig / release: release the IP address back to the pool of command;
ipconfig / Renew: get a new IP address command;
to obtain a new IP address: 192.168.10.128. 
Then into win10-2
or first enters the network adapter, the state set to automatically obtain IPv4 
Open cmd command
ipconfig / release (release the IP address back to the pool)
ipconfig / Renew (get a new IP address)
get a new IP address is: 192.168 .20.128 
we can then ping command to test whether two hosts and 192.168.100.100 network communication
win10-1 system ping test: 
win10-2 system ping test: 
this time successfully sent four data packets to achieve a fully interoperable network .