- Cấu trúc gói tin của GRE căn bản như sau
- Để bảo vệ gói tin GRE ta gói tin này được bảo vệ bằng IPSec. Mà IPSec có hai mode là tunnel mode và transport mode. Thì GRE dùng Transport mode để tiết kiệm được 20 byte địa chỉ.
- Ta có mô hình bài Lab như sau:

các bước ta cần cấu hình

//router 1
enable
conf t
hostname R1
int s 2/0
ip add 120.0.0.1 255.255.255.252
no sh
exit
int fa 0/0
ip add 10.0.0.1 255.255.255.0
no sh
exit
ip route 0.0.0.0 0.0.0.0 120.0.0.2

interface tunnel 0
ip add 100.0.0.1 255.255.255.0
tunnel source 120.0.0.1
tunnel destination 123.0.0.2
tunnel mode gre ip
tunnel key 123456
exit

crypto isakmp policy 10
encrypt 3des
hash md5
authentication pre-share
group 2
exit
crypto isakmp key 6 cisco123 address 123.0.0.2
crypto ipsec transform-set vpn esp-3des esp-md5-hmac
mode transport
exit
access-list 116 permit gre host 120.0.0.1 host 123.0.0.2

crypto map gre 10 ipsec-isakmp
match address 116
set peer 123.0.0.2
set transform-set vpn
exit

int serial 2/0
crypto map gre
exit

router os 1
network 100.0.0.0 0.0.0.255 area 0
network 10.0.0.0 0.0.0.255 area 0

//router 2
enable
conf t
hostname R2
interface serial 2/0
ip add 120.0.0.2 255.255.255.252
no sh
clock rate 64000
exit
interface serial 2/1
ip add 123.0.0.1 255.255.255.252
no sh
clock rate 64000
exit


//router 3
enable
conf t
hostname R3
interface serial 2/0
ip add 123.0.0.2 255.255.255.252
no sh
int fa 0/0
ip add 20.0.0.1 255.255.255.0
no sh
exit
ip route 0.0.0.0 0.0.0.0 123.0.0.1

interface tunnel 0
ip add 100.0.0.2 255.255.255.0
tunnel source 123.0.0.2
tunnel destination 120.0.0.1
tunnel mode gre ip
tunnel key 123456
exit

crypto isakmp policy 10
encrypt 3des
hash md5
group 2
authentication pre-share
exit
crypto isakmp key 6 cisco123 address 120.0.0.1
crypto ipsec transform-set vpn esp-3des esp-md5-hmac
mode transport
exit
access-list 116 permit gre host 123.0.0.2 host 120.0.0.1

crypto map gre 10 ipsec-isakmp
match address 116
set peer 120.0.0.1
set transform-set vpn
exit

int serial 2/0
crypto map gre
exit

router os 1
network 100.0.0.0 0.0.0.255 area 0
network 20.0.0.0 0.0.0.255 area 0