拓扑图如下：
设备配置如下： SW: SW#sh run Building configuration... Current configuration : 1538 bytes ! version 12.2 no service timestamps log datetime msec no service timestamps debug datetime msec no service password-encryption ! hostname Switch ! ! ! ! ! ip routing ! ! ! ! ! ! ! ! ! ! spanning-tree mode pvst ! ! ! ! interface FastEthernet0/1 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/2 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/3 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/4 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/5 ! interface FastEthernet0/6 ! interface FastEthernet0/7 ! interface FastEthernet0/8 ! interface FastEthernet0/9 ! interface FastEthernet0/10 ! interface FastEthernet0/11 ! interface FastEthernet0/12 ! interface FastEthernet0/13 ! interface FastEthernet0/14 ! interface FastEthernet0/15 ! interface FastEthernet0/16 ! interface FastEthernet0/17 ! interface FastEthernet0/18 ! interface FastEthernet0/19 ! interface FastEthernet0/20 ! interface FastEthernet0/21 ! interface FastEthernet0/22 ! interface FastEthernet0/23 ! interface FastEthernet0/24 ! interface GigabitEthernet0/1 ! interface GigabitEthernet0/2 ! interface Vlan1 ip address 192.168.0.254 255.255.255.0 ! interface Vlan10 ip address 192.168.10.254 255.255.255.0 ip helper-address 192.168.0.20 ! interface Vlan20 ip address 192.168.20.254 255.255.255.0 ip helper-address 192.168.0.20 ! ip classless ! ! ! ! ! ! ! line con 0 ! line aux 0 ! line vty 0 4 login ! ! ! end SW# ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ SW1： SW1#show run Building configuration... Current configuration : 1809 bytes ! version 12.1 no service timestamps log datetime msec no service timestamps debug datetime msec no service password-encryption ! hostname SW1 ! ! spanning-tree mode pvst ! interface FastEthernet0/1 switchport mode trunk ! interface FastEthernet0/2 switchport access vlan 10 ! interface FastEthernet0/3 switchport access vlan 10 ! interface FastEthernet0/4 switchport access vlan 10 ! interface FastEthernet0/5 switchport access vlan 10 ! interface FastEthernet0/6 switchport access vlan 10 ! interface FastEthernet0/7 switchport access vlan 10 ! interface FastEthernet0/8 switchport access vlan 10 ! interface FastEthernet0/9 switchport access vlan 10 ! interface FastEthernet0/10 switchport access vlan 10 ! interface FastEthernet0/11 switchport access vlan 10 ! interface FastEthernet0/12 switchport access vlan 10 ! interface FastEthernet0/13 switchport access vlan 20 ! interface FastEthernet0/14 switchport access vlan 20 ! interface FastEthernet0/15 switchport access vlan 20 ! interface FastEthernet0/16 switchport access vlan 20 ! interface FastEthernet0/17 switchport access vlan 20 ! interface FastEthernet0/18 switchport access vlan 20 ! interface FastEthernet0/19 switchport access vlan 20 ! interface FastEthernet0/20 switchport access vlan 20 ! interface FastEthernet0/21 switchport access vlan 20 ! interface FastEthernet0/22 switchport access vlan 20 ! interface FastEthernet0/23 switchport access vlan 20 ! interface FastEthernet0/24 switchport access vlan 20 ! interface GigabitEthernet1/1 ! interface GigabitEthernet1/2 ! interface Vlan1 no ip address ! interface Vlan10 no ip address ! interface Vlan20 no ip address ! ! line con 0 ! line vty 0 4 login line vty 5 15 login ! ! end SW1# ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- SW2： SW2#SH RUN Building configuration... Current configuration : 1742 bytes ! version 12.1 no service timestamps log datetime msec no service timestamps debug datetime msec no service password-encryption ! hostname SW2 ! ! spanning-tree mode pvst ! interface FastEthernet0/1 switchport mode trunk ! interface FastEthernet0/2 switchport access vlan 10 ! interface FastEthernet0/3 switchport access vlan 10 ! interface FastEthernet0/4 switchport access vlan 10 ! interface FastEthernet0/5 switchport access vlan 10 ! interface FastEthernet0/6 switchport access vlan 10 ! interface FastEthernet0/7 switchport access vlan 10 ! interface FastEthernet0/8 switchport access vlan 10 ! interface FastEthernet0/9 switchport access vlan 10 ! interface FastEthernet0/10 switchport access vlan 10 ! interface FastEthernet0/11 switchport access vlan 10 ! interface FastEthernet0/12 switchport access vlan 10 ! interface FastEthernet0/13 switchport access vlan 20 ! interface FastEthernet0/14 switchport access vlan 20 ! interface FastEthernet0/15 switchport access vlan 20 ! interface FastEthernet0/16 switchport access vlan 20 ! interface FastEthernet0/17 switchport access vlan 20 ! interface FastEthernet0/18 switchport access vlan 20 ! interface FastEthernet0/19 switchport access vlan 20 ! interface FastEthernet0/20 switchport access vlan 20 ! interface FastEthernet0/21 switchport access vlan 20 ! interface FastEthernet0/22 switchport access vlan 20 ! interface FastEthernet0/23 switchport access vlan 20 ! interface FastEthernet0/24 switchport access vlan 20 ! interface GigabitEthernet1/1 ! interface GigabitEthernet1/2 ! interface Vlan1 no ip address shutdown ! interface Vlan10 no ip address ! interface Vlan20 no ip address ! ! line con 0 ! line vty 0 4 login line vty 5 15 login ! ! end SW2# ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- 注意： DHCP DISCOVER 是以广播方式进行的，其情形只能在同一网段之内进行，因为路由器是不会将二层广播包转发出去的。需配置DHCP中继代理（ip helper-address）
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