Имеем:

1 Маршрутизатор Cisco 5510 ASA с реальным IP 1.1.1.1 (сеть XXX.XXX.0.0/24)
2. Маршрутизатор Linux CentOS 5.2 (ядро 2.6.18-92.el5) с установленным
ipsec-tools-0.6.5-13.el5_3.1 и реальным IP 2.2.2.2 (сеть XXX.XXX.0.0/16)

Конфигурация на маршрутизаторе Cisco:

   crypto isakmp policy 5
    encr aes
    authentication pre-share
    group 2
    lifetime 3600
    hash sha
   !
   crypto isakmp key SECRETKEY address 2.2.2.2
   crypto ipsec security-association lifetime seconds 3600
   crypto ipsec transform-set GK esp-aes esp-sha-hmac
   crypto map IPSec 7 ipsec-isakmp
   set peer 2.2.2.2
   set transform-set GK
   set pfs group2
   match address 666
  !
  interface GigabitEthernet0/0.1
   ip address 1.1.1.1 255.255.255.224
   crypto map IPSec
  !
  ip route XXX.XXX.0.0 255.255.255.0 2.2.2.2
  access-list 666 remark asGK
  access-list 666 permit ip  XXX.XXX.0.0 0.0.255.255  XXX.XXX.0.0 0.0.0.255
  access-list 666 deny   ip any any


Конфигурация на машине с Linux CentOS:

/etc/sysconfig/network-scripts/ifcfg-ipsec0

   TYPE=IPSEC
   ONBOOT=yes
   IKE_METHOD=PSK
   IKE_PSK=SECRETKEY
   IKE_DHGROUP=2
   ESP_PROTO=aes
   AH_PROTO=none
   AESP_PROTO=hmac-sha1
   SRC=2.2.2.2
   SRCGW=XXX.XXX.0.100
   DSTGW=1.1.1.1
   SRCNET=XXX.XXX.0.0/24
   DSTNET=XXX.XXX.0/16
   DST=1.1.1.1


/etc/racoon/racoon.conf

   path include "/etc/racoon";
   path pre_shared_key "/etc/racoon/psk.txt";
   log notify;

   listen
   {
      isakmp 2.2.2.2 [500];
   }

   sainfo address  XXX.XXX.0.0/24 any address XXX.XXX.0.0/16 any
   {
       pfs_group 2;
       lifetime time 6400 sec;
       encryption_algorithm aes;
       authentication_algorithm hmac_sha1;
       compression_algorithm deflate;
   }


и не забываем про iptables!!

после настройки

   #ifup ipsec0

после поднятия ipsec0 можно посмотреть есть ли туннель

   #setkey -D

   2.2.2.2 1.1.1.1
       esp mode=tunnel spi=3839224802(0xe4d5ebe2) reqid=0(0x00000000)
       E: aes-cbc  c98674dd c1cda3a8 36f39eb5 84fd56b4 192e4acd 7ad470d7 0176919b c955cc38
       A: hmac-sha1  d8e6305b 8b0352ab 249d125f 1515e6a8 136d8896
       seq=0x00000000 replay=4 flags=0x00000000 state=mature
       created: Jul  8 10:19:23 2010 current: Jul  8 10:44:57 2010
       diff: 1534(s)   hard: 86400(s)  soft: 69120(s)
       last: Jul  8 10:19:27 2010    hard: 0(s)        soft: 0(s)
       current: 2160(bytes)    hard: 0(bytes)  soft: 0(bytes)
       allocated: 18   hard: 0 soft: 0
       sadb_seq=1 pid=8863 refcnt=0
   1.1.1.1 2.2.2.2
       esp mode=tunnel spi=111533039(0x06a5dbef) reqid=0(0x00000000)
       E: aes-cbc  3e1f5040 cf6c15d2 8083dc28 aa6006ef df53337f 13b31da2 2782ef5c e46d3567
       A: hmac-sha1  a9553dd3 e9b431a5 534baef8 a2b1f34b cc2b8867
       seq=0x00000000 replay=4 flags=0x00000000 state=mature
       created: Jul  8 10:19:23 2010 current: Jul  8 10:44:57 2010
       diff: 1534(s)   hard: 86400(s)  soft: 69120(s)
       last: Jul  8 10:19:27 2010    hard: 0(s)        soft: 0(s)
       current: 833(bytes)     hard: 0(bytes)  soft: 0(bytes)
       allocated: 18   hard: 0 soft: 0
       sadb_seq=0 pid=8863 refcnt=0


Ссылки:
       https://www.opennet.ru/base/cisco/cisco_ipsec_freebsd.txt.html (очень помогла эта статья)
       http://netbsd.gw.com/cgi-bin/man-cgi?racoon++NetBSD-current