DHCP:
先有client端發出discovery message
D:255.255.255.255
S:0.0.0.0 代表未知IP或默認路由
SERVER發送OFFER message 廣播報文
client發送request message 該報文應用於有多個sever的情況下
該報文有兩個作用:
1、當做ack
2、讓另一個server回收地址
acknowledgment message是server端的確認報文
l2+ip+udp+bootstrap+dhcp+fcs dhcp只用在以太網環境
按上圖做一個DHCP實驗：
R1：
interface f0/0
ip address 192.168.1.1 255.255.255.0
no shut
!
service dhcp
ip dhcp pool ip_pool
network 192.168.1.0 /24
default-router 192.168.1.1
dns-server 61.147.37.1 61.177.7.1
lease 1
!
ip dhcp excluded-address 192.168.1.1
ip dhcp excluded-address 192.168.1.254
R2:
interface f0/1
no shut
no ip address
ip address dhcp
R2獲取地址是地址池裏最小的開始分配
如果你想查看R1的dhcp池的分配情況，可以用這個命令:show ip dhcp binding
R1#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.1.2 0063.6973.636f.2d63. Mar 02 2002 12:10 AM Automatic
3030.312e.3133.6663.
2e30.3030.312d.4661.
302f.31
我們查看下抓包結果吧
1、就是discovery message
2、offer message
3、request message
4、ack message
ip dhcp excluded-address low_ip hight_ip 定義一個例外地址範圍
R2上可以獲取默認路由的show ip route
R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 192.168.1.1 to network 0.0.0.0
C 192.168.1.0/24 is directly connected, FastEthernet0/1
S* 0.0.0.0/0 [254/0] via 192.168.1.1
helper-address client和server不在同一個子網的情況下，需要help address
過程如下：
當設備接收到一個廣播報文，如果配置了help address 就把該報文的目的地址改爲help address
變成單播報文，發送到指定IP地址
DHCP擴展應用helper-address
R1和R2之間鏈路的配置不變
R2：
interface f0/0
ip address 192.168.2.2 255.255.255.0
ip helper-address 192.168.1.1
no shut
R3:
interface f0/1
no shutdown
ip address dhcp
R1：
ip dhcp excluded-address 192.168.2.1 192.168.2.2
ip dhcp excluded-address 192.168.2.254
ip dhcp pool wolf
network 192.168.2.0 /24
default-router 192.168.2.1
dns-server 8.8.8.8
lease 1
在R3上show ip interface brief，我們發現沒有IP地址被獲取哦！
爲什麼呢？可能是路由問題嗎？
在R1上show ip route 只有直連路由
手動寫一條靜態路由ip route 192.168.2.0 255.255.255.0 f0/0
過一會兒，我們發現R3獲取IP地址了
我們抓包看看問題出在哪裏
知道問題了吧，discovery offer request ack 報文都變成了單播報文了
而R1沒有到192.168.2.0 的路由條目，所以報文送不到R3