route table is ALWAYS nonsense after PPTP connection from network-manager-pptp

Dear developers,

I tried to connect to my school with network-manager-pptp, but there was no traffic after the connection. The following is my attemption to fix the problem.
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hearson@hearson-laptop:/etc/ppp$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
192.168.0.0 0.0.0.0 255.255.255.0 U 2 0 0 ath0
0.0.0.0 0.0.0.0 0.0.0.0 U 0 0 0 ppp0
hearson@hearson-laptop:/etc/ppp$ sudo route del default
hearson@hearson-laptop:/etc/ppp$ ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
          inet addr:128.210.192.137 P-t-P:128.210.192.137 Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1496 Metric:1
          RX packets:421 errors:0 dropped:0 overruns:0 frame:0
          TX packets:521 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:105108 (105.1 KB) TX bytes:140946 (140.9 KB)

hearson@hearson-laptop:/etc/ppp$ sudo route add default gw 128.210.192.137 ppp0
hearson@hearson-laptop:/etc/ppp$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
192.168.0.0 0.0.0.0 255.255.255.0 U 2 0 0 ath0
0.0.0.0 128.210.192.137 0.0.0.0 UG 0 0 0 ppp0

hearson@hearson-laptop:/etc/ppp$ ping 208.67.222.222
PING 208.67.222.222 (208.67.222.222) 56(84) bytes of data.
64 bytes from 208.67.222.222: icmp_seq=1 ttl=54 time=263 ms
64 bytes from 208.67.222.222: icmp_seq=2 ttl=54 time=263 ms
^C
--- 208.67.222.222 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1006ms
rtt min/avg/max/mdev = 263.088/263.229/263.370/0.141 ms
------------------------------------------------

Meanwhile, before I start PPTP connection, I modified my /etc/ppp/ip-up like this
================================================
#!/bin/sh
#
# This script is run by the pppd after the link is established.
# It uses run-parts to run scripts in /etc/ppp/ip-up.d, so to add routes,
# set IP address, run the mailq etc. you should create script(s) there.
#
# Be aware that other packages may include /etc/ppp/ip-up.d scripts (named
# after that package), so choose local script names with that in mind.
#
# This script is called with the following arguments:
# Arg Name Example
# $1 Interface name ppp0
# $2 The tty ttyS1
# $3 The link speed 38400
# $4 Local IP number 12.34.56.78
# $5 Peer IP number 12.34.56.99
# $6 Optional ``ipparam'' value foo

# The environment is cleared before executing this script
# so the path must be reset
PATH=/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin
export PATH

# These variables are for the use of the scripts run by run-parts
PPP_IFACE="$1"
PPP_TTY="$2"
PPP_SPEED="$3"
PPP_LOCAL="$4"
PPP_REMOTE="$5"
PPP_IPPARAM="$6"
export PPP_IFACE PPP_TTY PPP_SPEED PPP_LOCAL PPP_REMOTE PPP_IPPARAM

# as an additional convenience, $PPP_TTYNAME is set to the tty name,
# stripped of /dev/ (if present) for easier matching.
PPP_TTYNAME=`/usr/bin/basename "$2"`
export PPP_TTYNAME

# If /var/log/ppp-ipupdown.log exists use it for logging.
if [ -e /var/log/ppp-ipupdown.log ]; then
  exec > /var/log/ppp-ipupdown.log 2>&1
  echo $0 $*
  echo
fi

# This script can be used to override the .d files supplied by other packages.
if [ -x /etc/ppp/ip-up.local ]; then
  exec /etc/ppp/ip-up.local "$*"
fi

run-parts /etc/ppp/ip-up.d \
  --arg="$1" --arg="$2" --arg="$3" --arg="$4" --arg="$5" --arg="$6"

# if pon was called with the "quick" argument, stop pppd
if [ -e /var/run/ppp-quick ]; then
  rm /var/run/ppp-quick
  wait
  kill $PPPD_PID
fi

env >/route.txt
route -n >>/route.txt
route del default
route add default gw $IPLOCAL dev $IFNAME
route -n >>/route.txt
================================================

Thus, I attempted to correct the route table after the PPTP connection was established. And the /route.txt is,
================================================
PPP_TTY=
DNS1=128.210.11.57
IFNAME=ppp0
DNS2=128.210.11.5
IPLOCAL=128.210.192.137
PPPLOGNAME=root
PPP_REMOTE=128.210.239.21
PPP_IFACE=ppp0
PPP_TTYNAME=
ORIG_UID=0
USEPEERDNS=1
SPEED=38400
PATH=/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin
IPREMOTE=128.210.239.21
PPP_LOCAL=128.210.192.137
PPP_IPPARAM=nm-pptp-service-8623
PPPD_PID=8626
PWD=/
PPP_SPEED=38400
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
192.168.0.0 0.0.0.0 255.255.255.0 U 2 0 0 ath0
0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0 0 ath0
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
128.210.239.21 192.168.0.1 255.255.255.255 UGH 0 0 0 ath0
192.168.0.0 0.0.0.0 255.255.255.0 U 2 0 0 ath0
0.0.0.0 128.210.192.137 0.0.0.0 UG 0 0 0 ppp0
===============================================

You can see the route table became correct for a while when it was still running the scripts. After that, the route table immediately became nonsense (no matter whether I modified the `ip-up' script or not).

My system is up-to-date. Please help me get rid of this situation. Thanks a lot!