求救!!!关于cisco ipsec加密延时问题

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网络用IPSEC加密后，业务终端在几分钟不办理业务后重办业务时，会出现30秒左右的时间延时，请各位大哥帮助解决延时问题。
    现将基本情况介绍如下：
    业务网络为电信提供的64K带宽的DDN，中心机房路由器为CISCO3745，（IOS：C3745-JK9S-M，VER:12.3(6)，RAM：192M，flash：32M，CPU：350MHZ）；营业点路由器为CISCO1721，(IOS：C1700-K903SY7-M，VER:12.2(13)T5，RAM：64M，flash：32M)，营业点所用终端为实达的网络终端，业务前置机在中心机房。
    网络配置加密部份基本情况如下：
    中心机房端cisco3745路由器加密部份配置
rypto isakmp policy 1
hash md5
authentication pre-share
crypto isakmp key cisco123 address 10.0.10.1
crypto isakmp keepalive 10
!
crypto ipsec security-association lifetime seconds 86400
!
crypto ipsec transform-set lzsh-des ah-md5-hmac esp-des esp-md5-hmac
!
crypto map abcdef 1 ipsec-isakmp
set peer 10.0.10.1
set transform-set lzsh-des
match address 120
!
interface FastEthernet0
ip address 192.168.10.254 255.255.255.0
speed auto
!

!
interface Serial1/0:0
bandwidth 64
ip address 10.0.10.2 255.255.255.252
crypto map abcdef
!
access-list 120 permit ip 192.168.10.0 0.0.0.255 192.168.200.0 0.0.0.255

营业点路由器cisco1721加密部基本配置：
crypto isakmp policy 1
hash md5
authentication pre-share
crypto isakmp key cisco123 address 10.0.10.2
crypto isakmp keepalive 10
!
crypto ipsec security-association lifetime seconds 86400
!
crypto ipsec transform-set lzsh-des ah-md5-hmac esp-des esp-md5-hmac
!
crypto map lzsh-map 1 ipsec-isakmp
set peer 10.0.10.2
set transform-set lzsh-des
match address 100
!
interface FastEthernet0
ip address 192.168.200.254 255.255.255.0
speed auto
!
interface Serial0
description time 11
ip address 10.0.10.1 255.255.255.252
backup delay 1 10
backup interface Async5
no fair-queue
crypto map lzsh-map
!
access-list 100 permit ip 192.168.200.0 0.0.0.255 192.168.10.0 0.0.0.255