由于IPSEC只支持對單播流量的加密，所以我們使用GRE隧道可以將廣播、組播包封裝在一個單播包中，再用IPSEC進行加密。
在進行IPSEC配置前應首先配置好GRE隧道，下面是R1上的GRE隧道配置：
R1：
interface tunnel0
ip address 192.168.3.1 255.255.255.0
tunnel source s1/1
tunnel destination 192.1.1.20
exit
interface s1/1
ip address 192.1.1.40 255.255.255.0
ip access-group perimeter in
exit
interface lo0
ip address 192.168.1.1 255.255.255.0
exit
ip route 0.0.0.0 0.0.0.0 192.1.1.20
!在這里我將總公司內(nèi)部的骨干網(wǎng)絡設為Area0,隧道部分和分公司內(nèi)部網(wǎng)絡設為Area1
router ospf 1
network 192.168.1.0 0.0.0.255 area 0
network 192.168.3.0 0.0.0.255 area 1
exit
ip access-list extended perimeter
permit udp host 192.1.1.20 host 192.1.1.40 eq 500
permit esp host 193.1.1.20 host 192.1.1.40
permit gre host 193.1.1.20 host 192.1.1.40
deny ip any any
exit
R2：
interface tunnel0
ip address 192.168.3.2 255.255.255.0
tunnel source s1/0
tunnel destination 192.1.1.40
exit
interface s1/0
ip address 192.1.1.20 255.255.255.0
ip access-group perimeter in
exit
interface lo0
ip address 192.168.2.1 255.255.255.0
exit
ip route 0.0.0.0 0.0.0.0 192.1.1.40
router ospf 1
network 192.168.2.0 0.0.0.255 area 1
network 192.168.3.0 0.0.0.255 area 1
exit
ip access-list extended perimeter
permit udp host 192.1.1.40 host 192.1.1.20 eq 500
permit esp host 192.1.1.40 host 192.1.1.20
permit gre host 192.1.1.40 host 192.1.1.20
deny ip any any
exit
GRE隧道建立好后，就可以進行IPSEC配置了：
R1上的配置：
crypto isakmp enable
crypto isakmp identity address
crypto isakmp policy 10
encryption aes
authentication pre-share
group 2
hash sha
exit
crypto isakmp key cisco123 address 192.1.1.20 no-xauth
!IPSEC只對進入GRE隧道的流量進行加密
ip access-list extended ToR2
permit gre host 192.1.1.40 host 192.1.1.20
exit
!這里的GRE隧道是點對點模式的，所以傳輸集應使用傳輸模式
crypto ipsec transform-set trans esp-aes esp-sha-hmac
mode transport
exit
crypto map mymap 10 ipsec-isakmp
match address ToR2
set transform-set trans
set peer 192.1.1.20
exit
interface s1/1
crypto map mymap
exit
!最后別忘記刪除測試隧道時建立的流量：
ip access-list extended perimeter
no permit gre host 192.1.1.20 host 192.1.1.40
R2上的配置：
crypto isakmp enable
crypto isakmp identity address
crypto isakmp policy 10
encryption aes
authentication pre-share
group 2
hash sha
exit
crypto isakmp key cisco123 address 192.1.1.40 no-xauth
ip access-list extended ToR1
permit gre host 192.1.1.20 host 192.1.1.40
exit
crypto ipsec transform-set trans esp-aes esp-sha-hmac
mode transport
exit
crypto map mymap 10 ipsec-isakmp
match address ToR1
set transform-set trans
set peer 192.1.1.40
exit
interface s1/0
crypto map mymap
exit
ip access-list extended perimeter
no permit gre host 192.1.1.40 host 192.1.1.20
測試實驗結(jié)果：
r1#sh 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 192.1.1.20 to network 0.0.0.0
C 192.1.1.0/24 is directly connected, Serial1/1
C 192.168.1.0/24 is directly connected, Loopback0
192.168.2.0/32 is subnetted, 1 subnets
O 192.168.2.1 [110/11112] via 192.168.3.2, 00:00:17, Tunnel0
C 192.168.3.0/24 is directly connected, Tunnel0
S* 0.0.0.0/0 [1/0] via 192.1.1.20
R1上ping PC2:
r1#ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/56/84 ms
PC1上ping PC2:
r1#ping 192.168.2.1 source lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/55/104 ms
可以看到不管是從PC1到PC2的流量還是R1到PC2的流量，只要通過隧道，都會被IPSEC封裝加密，所以都能PING通PC2！