PLANET XGS3-24042 - Manual (Page 252)

31-6
In the above example, layer 2 switches Switch1 and Switch2 are both connected to layer 3 switch Switch3,
Switch 3 will transmit the request message from DHCP client to DHCP serer as DHCP Relay Agent. It will also
transmit the reply message from the server to DHCP client to finish the DHCP protocol procedure. If the
DHCP option 82 is disabled, DHCP server cannot distinguish that whether the DHCP client is from the
network connected to Switch1 or Switch2. So, all the PC terminals connected to Switch1 and Switch2 will get
addresses from the public address pool of the DHCP server. After the DHCP option 82 function is enabled,
since the Switch3 appends the port information of accessing Switch3 to the request message from the client,
the server can tell that whether the client is from the network of Swich1 or Swich2, and thus can allocate
separate address spaces for the two networks, to simplify the management of networks.
The following is the configuration of Switch3(MAC address is 00:03:4f:02:33:01):
Switch3(Config)#service dhcp
Switch3(Config)#ip dhcp relay information option
Switch3(Config)#ip forward-protocol udp bootps
Switch3(Config)#interface vlan 3
Switch3(Config-if-vlan3)#ip address 192.168.10.222 255.255.255.0
Switch3(Config-if-vlan2)#ip address 192.168.102.2 255.255.255.0
Switch3(Config-if-vlan2)#ip helper 192.168.10.88
Linux ISC DHCP Server supports option 82, its configuration file /etc/dhcpd.con is ddns-update-style interim;
ignore client-updates;
class "Switch3Vlan2Class1" {
match if option agent.circuit-id = "Vlan2+Ethernet1/0/2" and option agent.remote-id=00:03:4f:02:33:01;
}
class "Switch3Vlan2Class2" {
match if option agent.circuit-id = "Vlan2+Ethernet1/0/3" and option agent.remote-id=00:03:4f:02:33:01;
}
subnet 192.168.102.0 netmask 255.255.255.0 {
option routers 192.168.102.2;
option subnet-mask 255.255.255.0;
option domain-name "example.com.cn";
option domain-name-servers 192.168.10.3;
authoritative;
pool {
range 192.168.102.21 192.168.102.50;
default-lease-time 86400; #24 Hours
max-lease-time 172800; #48 Hours
allow members of "Switch3Vlan2Class1";
}
pool {
range 192.168.102.51 192.168.102.80;
default-lease-time 43200; #12 Hours