dhcp
DHCP:
先有client端发出discovery message
D:255.255.255.255
S:0.0.0.0 代表未知IP或默认路由
SERVER发送OFFER message 广播报文
client发送request message 该报文应用于有多个sever的情况下
该报文有两个作用:
1、当做ack
2、让另一个server回收地址
acknowledgment message是server端的确认报文
l2+ip+udp+bootstrap+dhcp+fcs dhcp只用在以太网环境
按上图做一个DHCP实验：
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获取地址是地址池里最小的开始分配
如果你想查看R1的dhcp池的分配情况，可以用这个命令: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
我们查看下抓包结果吧
1、就是discovery message
2、offer message
3、request message
4、ack message
ip dhcp excluded-address low_ip hight_ip 定义一个例外地址范围
R2上可以获取默认路由的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不在同一个子网的情况下，需要help address
过程如下：
当设备接收到一个广播报文，如果配置了help address 就把该报文的目的地址改为help address
变成单播报文，发送到指定IP地址
DHCP扩展应用helper-address
R1和R2之间链路的配置不变
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，我们发现没有IP地址被获取哦！
为什么呢？可能是路由问题吗？
在R1上show ip route 只有直连路由
手动写一条静态路由ip route 192.168.2.0 255.255.255.0 f0/0
过一会儿，我们发现R3获取IP地址了
我们抓包看看问题出在哪里
知道问题了吧，discovery offer request ack 报文都变成了单播报文了
而R1没有到192.168.2.0 的路由条目，所以报文送不到R3