《这个张飞有点雷》（序言）

#ip dhcp pool global
(dhcp-config)#domain-name nhq.com
(dhcp-config)#network 192.168.1.0 255.255.255.0
(dhcp-config)#lease 7

ip dhcp pool global
network 192.168.1.0 255.255.255.0
domain-name nhq.com
default-router 192.168.1.254
lease 7

上面几步做好就能使用DHCP Server了，但是这做的并不全面，比如做绑定，给网段分不同地址等都还没做；我GNS3上面模拟的CISCO 3600路由器，没找到Vlan的地址分配配置；
再贴一个DHCP Server服务的请求过程：

VPCS[1]> dhcp -d
Opcode: 1 (REQUEST)
Client IP Address: 0.0.0.0
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Discover
Option 12: Host Name = VPCS1
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:
68:00
第一次请求，客户端并没有获取到IP地址

Opcode: 2 (REPLY)
Client IP Address: 0.0.0.0
Your IP Address: 192.168.1.1
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Offer
Option 54: DHCP Server = 192.168.1.254
Option 51: Lease Time = 604394
Option 58: Renewal Time = 302197
Option 59: Rebinding Time = 528844
Option 1: Subnet Mask = 255.255.255.0
Option 15: Domain = nhq.com
Option 3: Router = 192.168.1.254
第一次答复，DHCP服务器做的答复，分配给客户机一个192.168.1.1的地址，DHCP Server地址为192.168.1.254

Opcode: 1 (REQUEST)
Client IP Address: 192.168.1.1
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Request
Option 54: DHCP Server = 192.168.1.254
Option 50: Requested IP Address = 192.168.1.1
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:
68:00
Option 12: Host Name = VPCS1
客户端第二次请求，已经获取到IP地址

Opcode: 2 (REPLY)
Client IP Address: 192.168.1.1
Your IP Address: 192.168.1.1
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Ack
Option 54: DHCP Server = 192.168.1.254
Option 51: Lease Time = 604800
Option 58: Renewal Time = 302400
Option 59: Rebinding Time = 529200
Option 1: Subnet Mask = 255.255.255.0
Option 15: Domain = nhq.com
Option 3: Router = 192.168.1.254

IP 192.168.1.1/24 GW 192.168.1.254
DHCP服务器第二次答复，确认IP地址已经被分配掉

这里我们可以从Option 53这一行看到数据包的状态，从Discover到Offer再到Request最后到Ack；
一次DHCP请求就这样结束了
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