拓扑图如下：

配置交换机S1
Switch>en
Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#host S1
S1(config)#vlan 20
S1(config-vlan)#vlan 30
S1(config-vlan)#exit
S1(config)#int fa0/2
S1(config-if)#switchport mode access
S1(config-if)#switchport access vlan 20
S1(config-if)#exit
S1(config)#int fa0/3
S1(config-if)#switchport mode access
S1(config-if)#switchport access vlan 30
S1(config-if)#exit
S1(config)#int fa0/1
S1(config-if)#switchport mode access
S1(config-if)#switchport mode trunk
S1(config-if)#switchport trunk allowed vlan all !--- Trunk端口允许所有VLAN的发送和接口传输，这也是默认状态，该条指令可选。
S1(config-if)#end
S1#show vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/4, Fa0/5, Fa0/6, Fa0/7
                                                Fa0/8, Fa0/9, Fa0/10, Fa0/11
                                                Fa0/12, Fa0/13, Fa0/14, Fa0/15
                                                Fa0/16, Fa0/17, Fa0/18, Fa0/19
                                                Fa0/20, Fa0/21, Fa0/22, Fa0/23
                                                Fa0/24, Gig1/1, Gig1/2
20   VLAN0020                         active    Fa0/2
30   VLAN0030                         active    Fa0/3
1002 fddi-default                     act/unsup 
1003 token-ring-default               act/unsup 
1004 fddinet-default                  act/unsup 
1005 trnet-default                    act/unsup 


VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1    enet  100001     1500  -      -      -        -    -        0      0
20   enet  100020     1500  -      -      -        -    -        0      0
30   enet  100030     1500  -      -      -        -    -        0      0
1002 fddi  101002     1500  -      -      -        -    -        0      0   
1003 tr    101003     1500  -      -      -        -    -        0      0   
1004 fdnet 101004     1500  -      -      -        ieee -        0      0   
1005 trnet 101005     1500  -      -      -        ibm  -        0      0   


Remote SPAN VLANs
------------------------------------------------------------------------------




Primary Secondary Type              Ports
------- --------- ----------------- ------------------------------------------
S1#




配置路由器R1
Router>en
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#host R1
R1(config)#int fa0/0
R1(config-if)#no shut
R1(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
R1(config-if)#exit
R1(config)#int fa0/0.1   !--- 配置子接口是配置单臂路由的关键，它是一个逻辑接口，实际并不存在的物理接口，其功能却和物理接口一样。
R1(config-subif)#
%LINK-5-CHANGED: Interface FastEthernet0/0.1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0.1, changed state to up
R1(config-subif)#encapsulation dot1q 20   !--- 配置子接口使用的802.1Q协议，最后面的20是VLAN号
R1(config-subif)#ip add 192.168.1.1 255.255.255.0
R1(config-subif)#exit
R1(config)#int fa0/0.2
R1(config-subif)#
%LINK-5-CHANGED: Interface FastEthernet0/0.2, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0.2, changed state to up
R1(config-subif)#encapsulation dot1q 30
R1(config-subif)#ip add 192.168.2.1 255.255.255.0
R1(config-subif)#end
R1#
%SYS-5-CONFIG_I: Configured from console by console
R1#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
       i - IS-IS, 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 not set
C    192.168.1.0/24 is directly connected, FastEthernet0/0.1
C    192.168.2.0/24 is directly connected, FastEthernet0/0.2
R1#




测试单臂路由配置：
PC>ipconfig
FastEthernet0 Connection:(default port)
Link-local IPv6 Address.........: FE80::202:4AFF:FE14:89CB
IP Address......................: 192.168.1.2
Subnet Mask.....................: 255.255.255.0
Default Gateway.................: 192.168.1.1
PC>ping 192.168.2.2
Pinging 192.168.2.2 with 32 bytes of data:
Request timed out.
Reply from 192.168.2.2: bytes=32 time=0ms TTL=127
Reply from 192.168.2.2: bytes=32 time=0ms TTL=127
Reply from 192.168.2.2: bytes=32 time=0ms TTL=127
Ping statistics for 192.168.2.2:
    Packets: Sent = 4, Received = 3, Lost = 1 (25% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms
PC>
测试结构表明配置完全正确