20.1.  使用ip Helper Addresses命令

提問 配置路由器對DHCP Request轉發的支持

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#interface Ethernet0

Router1(config-if)#ip helper-address 172.25.1.1

Router1(config-if)#ip helper-address 172.25.10.7

Router1(config-if)#exit

Router1(config)#end

Router1#

注釋 使用IP Helper Address命令把路由器配置成為一個DHCP代理服務器，轉發客戶端的DHCP Request至配置的ip helper address。

20.2.  限制IP Helper Addresses命令的影響

提問 配置IP Helper Address命令以后導致鏈路利用率增高或者DHCP服務器負荷增高

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#no ip forward-PRotocol udp tFTP

Router1(config)#no ip forward-protocol udp nameserver

Router1(config)#no ip forward-protocol udp domain

Router1(config)#no ip forward-protocol udp time

Router1(config)#no ip forward-protocol udp netbios-ns

Router1(config)#no ip forward-protocol udp netbios-dgm

Router1(config)#no ip forward-protocol udp tacacs

Router1(config)#end

Router1#

注釋 缺省情況下IP Helper命令會轉發很多UDP廣播數據包，不僅僅是DHCP數據包，并且不能針對不同的服務器轉發不同的廣播包

20.3.  使用DHCP來動態配置路由器IP地址

提問 配置路由器動態獲得IP地址

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#interface FastEthernet0/1

Router1(config-if)#ip address dhcp

Router1(config-if)#end

Router1#

Interface FastEthernet0/1 assigned DHCP address 172.25.1.57, mask 255.255.255.0

Router1#

注釋 在12.2(8)T之前此命令僅僅適用于以太網接口。從12.3(8)T以后可以對DHCP選項進行控制，下例配置為不獲得DNS服務器

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#interface FastEthernet0/1

Router1(config-if)#no ip dhcp client request dns-nameserver

Router1(config-if)#end

另外對于獲得的缺省路由，治理距離為254

S*   0.0.0.0/0 [254/0] via 172.25.1.1

從12.3(4)T開始增加了對獲得地址釋放和重新獲得的支持

Router1#release dhcp FastEthernet0/1

Router1#renew dhcp FastEthernet0/1

20.4.  通過DHCP來對客戶端進行動態IP地址分配

提問 配置路由器成為DHCP服務器

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#service dhcp

Router1(config)#ip dhcp pool 172.25.1.0/24

Router1(dhcp-config)#network 172.25.1.0 255.255.255.0

Router1(dhcp-config)#default-router 172.25.1.1

Router1(dhcp-config)#exit

Router1(config)#ip dhcp excluded-address 172.25.1.1 172.25.1.50

Router1(config)#ip dhcp excluded-address 172.25.1.200 172.25.1.255

Router1(config)#end

Router1#

注釋 注重的是要配置excluded命令來排除某些地址，防止出現地址沖突

20.5.  配置DHCP的配置選項

提問 配置更多的DHCP配置選項提供給客戶端

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp pool ORAserver

Router1(dhcp-config)#host 172.25.1.34 255.255.255.0

Router1(dhcp-config)#client-name bigserver

Router1(dhcp-config)#default-router 172.25.1.1 172.25.1.3

Router1(dhcp-config)#domain-name oreilly.com

Router1(dhcp-config)#dns-server 172.25.1.1 10.1.2.3

Router1(dhcp-config)#netbios-name-server 172.25.1.1

Router1(dhcp-config)#netbios-node-type h-node

Router1(dhcp-config)#option 66 ip 10.1.1.1

Router1(dhcp-config)#option 33 ip 192.0.2.1 172.25.1.3

Router1(dhcp-config)#option 31 hex 01

Router1(dhcp-config)#lease 2

Router1(dhcp-config)#exit

Router1(config)#end

Router1#

注釋 Option 66 定義TFTP服務器; Option 33定義靜態路由; Option 31定義客戶端使用IRDP.

20.6.  配置DHCP的分配時長

提問 修改缺省DHCP分配時長

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp pool 172.25.2.0/24

Router1(dhcp-config)#lease 2 12 30

Router1(dhcp-config)#exit

Router1(config)#end

Router1#

注釋 缺省分配為一天，配置選項為天，小時，分鐘

20.7.  分配靜態IP地址

提問 每次都分配給某個特定設備特定IP地址

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp pool IAN

Router1(dhcp-config)#host 172.25.1.33 255.255.255.0

Router1(dhcp-config)#client-identifier 0100.0103.85e9.87

Router1(dhcp-config)#client-name win2k

Router1(dhcp-config)#default-router 172.25.1.1

Router1(dhcp-config)#domain-name oreilly.com

Router1(dhcp-config)#dns-server 172.25.1.1

Router1(dhcp-config)#exit

Router1(config)#end

Router1#

注釋 這里通過MAC地址來綁定某個IP地址。Client-identifier后面跟的是MAC地址，不過比傳統MAC地址多了0100，代表是以太網，對于更多的媒介類型值參考RFC 3232中的Number Hardware Type部分

Router1#show ip dhcp binding

IP address       Hardware address        Lease eXPiration        Type

172.25.1.33      0100.0103.85e9.87       Infinite                Manual

172.25.1.52      0100.50da.2a5e.a2       Apr 11 2006 09:00 PM    Automatic

172.25.1.53      0100.0103.ea1b.ed       Apr 11 2006 08:58 PM    Automatic

20.8.  配置一個DHCP 數據庫客戶端

提問 在另一個設備上備份當前的DHCP數據庫

回答

FTP方式

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp database ftp://dhcp:bindsave@172.25.1.1/dhcp-leases

Router1(config)#end

Router1#

TFTP 方式

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp database tftp://172.25.1.1/dhcp-leases

Router1(config)#end

Router1#

RCP方式

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp database rcp://dhcp@172.25.1.1/dhcp-leases

Router1(config)#end

Router1#

注釋 通常DHCP數據庫保存于內存，假如重啟就會丟失，可以使用上述方式進行備份從而不會丟失，通過下述命令驗證

Router1#show ip dhcp database

URL      : ftp://dhcp:bindsave@172.25.1.1/dhcp-leases

Read     : Never

Written  : Apr 09 2006 10:24 PM

Status   : Last write sUCceeded. Agent information is up-to-date.

Delay    : 300 seconds

Timeout  : 300 seconds

Failures : 1

Successes: 30

20.9.  在同一子網配置多個DHCP服務器

提問 在同一子網配置多個DHCP服務器來增加可用性

回答

Router1:

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp pool 172.22.1.0/24

Router1(dhcp-config)#network 172.22.1.0 255.255.255.0

Router1(dhcp-config)#default-router 172.22.1.1

Router1(dhcp-config)#domain-name oreilly.com

Router1(dhcp-config)#dns-server 172.25.1.1 10.1.2.3

Router1(dhcp-config)#exit

Router1(config)#ip dhcp excluded-address 172.22.1.1 172.22.1.49

Router1(config)#ip dhcp excluded-address 172.22.1.150 172.22.1.254

Router1(config)#ip dhcp database ftp://dhcp:bindsave@172.25.1.1/dhcp-leases-rtr1

Router1(config)#end

Router1#

Router2:

Router2#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router2(config)#ip dhcp pool 172.22.1.0/24     

Router2(dhcp-config)#network 172.22.1.0 255.255.255.0

Router2(dhcp-config)#default-router 172.22.1.1      

Router2(dhcp-config)#domain-name oreilly.com        

Router2(dhcp-config)#dns-server 172.25.1.1 10.1.2.3 

Router2(dhcp-config)#exit

Router2(config)#ip dhcp excluded-address 172.22.1.1 172.22.1.149

Router2(config)#ip dhcp database ftp://dhcp:bindsave@172.25.1.1/dhcp-leases-rtr2

Router2(config)#end

Router2#

注釋 要確保配置的地址池不重復，Router1 分配地址為從172.25.1.50到172.25.1.149, Router2 分配地址為從 172.25.1.150 到172.25.1.254,

20.10.  DHCP靜態映射

提問 根據某個文本文件來進行IP地址的靜態指配

回答

先在TFTP服務器上創建此文本文件

Freebsd% cat /tftpboot/dhcp.static

*time* Aug 17 2006 03:52 PM

*version* 2

!IP address       Type      Hardware address               Lease expiration

10.1.1.16 /24    id        0100.104b.33da.74                Infinite

10.1.1.17 /24    id        0100.0dbc.eff6.38                Infinite

10.1.1.18 /24    id        0100.0a5e.4001.27                Infinite

10.1.1.19 /24    id        0100.0331.327e.41                Infinite

10.1.1.20 /24    id        0100.0d60.b21a.4c                Infinite

*end*

Freebsd%

路由器配置

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp pool OREILLY

Router1(dhcp-config)#origin file tftp://172.25.1.1/dhcp.static

Router1(dhcp-config)#default-router 10.1.1.1

Router1(dhcp-config)#dns-server 172.25.1.1 172.25.1.3

Router1(dhcp-config)#domain-name oreilly.com

Router1(dhcp-config)#lease 3

Router1(dhcp-config)#end

Router1#

注釋 20.7講到的靜態地址分配需要一個特定的DHCP Pool，擴展性不強，從12.3(11)T以后可以使用特定的文本文件來進行指配，不過必須遵照一定的格式。假如文本文件修改后需要生效，必須先no service dhcp 來停止DHCP服務然后service dhcp 命令重新啟用來生效

<!--[if !supportLists]-->20.11.       <!--[endif]-->安全DHCP IP地址指派

提問 同步ARP和DHCP地址綁定來防止出現IP地址欺騙

回答

Router1#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Router1(config)#ip dhcp pool OREILLY

Router1(dhcp-config)#update arp

Router1(dhcp-config)#end

Router1#

注釋 從12.2(15)T開始思科引入了安全DHCP IP地址指派（DHCP secured IP address assignment），啟用此特性后會針對每個DHCP綁定增加一個安全ARP條目，從而防止對此條目的修改，即使使用clear arp-cache命令也會保證此條目不被清除

20.12.  顯示DHCP狀態

提問 顯示DHCP服務器的狀態

回答

顯示綁定和相應的分配時長

Router1#show ip dhcp binding

顯示地址沖突

Router1#show ip dhcp conflict

顯示數據庫狀態

Router1#show ip dhcp database

顯示全局DHCP數據統計

Router1#show ip dhcp server statistics

注釋

Router1#show ip dhcp server statistics

Memory usage         17996

Address pools        4

Database agents      1

Automatic bindings   2

Manual bindings      1

Expired bindings     3

Malformed messages   0

Message              Received

BOOTREQUEST          0

DHCPDISCOVER         63

DHCPREQUEST          203

DHCPDECLINE          1

DHCPRELEASE          27

DHCPINFORM           19

Message              Sent

BOOTREPLY            0

DHCPOFFER            63

DHCPACK              139

DHCPNAK              2

Router1#

<!--[if !supportLists]-->20.13.       <!--[endif]-->DHCP排錯

提問 對DHCP出現的問題進行排錯

回答

Router1#debug ip dhcp server events

Router1#debug ip dhcp server packet

注釋 無