VPN的定义
虚拟专网（VPN-VIRTUAL PRIVATE NETWORK）指的是在公用网络上建立专用网络的技术。之所以称为虚拟网主要是因为整个VPN网络的任意两个节点之间的连接并没有传统专网所需的端到端的物理链路，而是架构在公用网络服务商所提供的网络平台（如INTERNET，ATM，FRAME RELAY等）之上的逻辑网络，用户数据在逻辑链路中传输。

VPN的功能
1、通过隧道（TUNNEL）或虚电路（VIRTUAL CIRCUIT）实现网络互联
2、支持用户安全管理
3、能够进行网络监控、故障诊断

VPN解决方案的优点
1、省钱：它可以节省长途电话费和长途专线电话费和长途专线网络费可以为用户节省30-25%的 网络应用的开销。
2、选择灵活、速度快：通过vpn网关，用户可以选择多种internet连通技术，而且对于 INTERNET的容量可以实现按需定制；
3、安全性好：VPN的认证机制将更好地保证用户的隐私权和收发数据的完整性；
4、实现投资的保护：VPN技术的应用可以建立在用户现有的防火墙的基础上，用户正在使用的 应用软件也不受影响。

VPN技术原理
1、 VPN系统使分布在不同地方的专用网络在不可信任的公共网络上安全的通信。
2、 VPN设备根据网管设置的规则，确定是否需要对数据进行加密或让数据直接通过。
3、 对需要加密的数据，VPN设备对整个数据包进行加密和附上数字签名。
4、 VPN设备加上新的收据包头，其中包括目的地VPN设备需要的安全信息和一些初始化参数。
5、 VPN设备对加密后的数据、鉴别包以及源IP地址、目标VPN设备IP地址进行重新封装，重新封装后的数据包通过虚拟通道在公网上传输。
6、 当数据包到达目标VPN设备时，数据包被解封装，数据包被解封装，数字签名，数字签名被 核对无误后，收据包被解密。

VPN配置实例

Intranet 内联网配置:
Figure 3-8: Intranet VPN Scenario Physical Elements
Headquarters Router 配置
hq-sanjose# show running-config
Building configuration...

Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname hq-sanjose
!
boot system flash bootflash:
boot bootldr bootflash:c7100-boot-mz.120-1.1.T
boot config slot0:hq-sanjose-cfg-small
no logging buffered
!
crypto isakmp policy 1
authentication pre-share
lifetime 84600
crypto isakmp key test12345 address 172.24.2.5
!
crypto ipsec transform-set proposal1 ah-sha-hmac esp-des esp-sha-hmac
mode transport
!
!
crypto map s1first local-address Serial1/0
crypto map s1first 1 ipsec-isakmp
set peer 172.24.2.5
set transform-set proposal1
match address 101
!
interface Tunnel0
bandwidth 180
ip address 172.17.3.3 255.255.255.0
no ip directed-broadcast
tunnel source 172.17.2.4
tunnel destination 172.24.2.5
crypto map s1first
!
interface FastEthernet0/0
ip address 10.1.3.3 255.255.255.0
no ip directed-broadcast
no keepalive
full-duplex
no cdp enable
!
interface FastEthernet0/1
ip address 10.1.6.4 255.255.255.0
no ip directed-broadcast
no keepalive
full-duplex
no cdp enable
!
interface Serial1/0
ip address 172.17.2.4 255.255.255.0
no ip directed-broadcast
no ip mroute-cache
no keepalive
fair-queue 64 256 0
framing c-bit
cablelength 10
dsu bandwidth 44210
clock source internal
no cdp enable
crypto map s1first
!
ip route 10.1.4.0 255.255.255.0 Tunnel0
!
access-list 101 permit gre host 172.17.2.4 host 172.24.2.5
!
line con 0
transport input none
line aux 0
line vty 0 4
login
!
end

Remote Office Router 配置:
ro-rtp# show running-config
Building configuration...

Current configuration:
!
version 12.0

Headquarters Router配置:
hq-sanjose# show running-config
Building configuration...

Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname hq-sanjose
!
boot system flash bootflash:
boot bootldr bootflash:c7100-boot-mz.120-1.1.T
boot config slot0:hq-sanjose-cfg-small
no logging buffered
!
crypto isakmp policy 1
authentication pre-share
lifetime 84600
crypto isakmp key test12345 address 172.24.2.5
crypto isakmp key test67890 address 172.23.2.7
!
crypto ipsec transform-set proposal1 ah-sha-hmac esp-des esp-sha-hmac
ode transport
!
crypto ipsec transform-set proposal4 ah-sha-hmac esp-des esp-sha-hmac
!
!
crypto map s1first local-address Serial1/0
crypto map s1first 1 ipsec-isakmp
set peer 172.24.2.5
set transform-set proposal1
match address 101
!
crypto map s4second local-address Serial2/0
crypto map s4second 2 ipsec-isakmp
set peer 172.23.2.7
set transform-set proposal4
match address 111
!
interface Tunnel0
bandwidth 180
ip address 172.17.3.3 255.255.255.0
no ip directed-broadcast
tunnel source 172.17.2.4
tunnel destination 172.2

Business Partner Router 配置:
bus-ptnr# show running-config
Building configuration...

Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname bus-ptnr
!
boot system flash bootflash:
boot bootldr bootflash:c7100-boot-mz.120-1.1.T
boot config slot0:bus-ptnr-cfg-small
no logging buffered
!
crypto isakmp policy 1
authentication pre-share
lifetime 84600
crypto isakmp key test67890 address 172.16.2.2
!
crypto ipsec transform-set proposal4 ah-sha-hmac esp-des esp-sha-hmac
!
!
crypto map s4second local-address Serial1/0
crypto map s4second 2 ipsec-isakmp
set peer 172.16.2.2
set transform-set proposal4
match address 111
!
interface FastEthernet0/0
ip address 10.1.5.2 255.255.255.0
no ip directed-broadcast
no keepalive
full-duplex
no cdp enable
!
interface Serial1/0
ip address 172.23.2.7 255.255.255.0
no ip directed-broadcast
no ip mroute-cache
no keepalive
fair-queue 64 256 0
framing c-bit
cablelength 10
dsu bandwidth 44210
clock source internal
no cdp enable
crypto map s4second
!
router bgp 10
network 10.1.5.0 mask 255.255.255.0
network 172.16.2.0 mask 255.255.255.0
!
access-list 111 permit ip host 10.1.5.3 host 10.2.2.2
!
line con 0
transport input none
line aux 0
line vty 0 4

详细配置说明请访问: http://www.cisco.com/univercd/cc/td.../core/7100/sw