trouble with IPv6-in-ipv4 tunnel configuration

Dear all, 

I had encountered some trouble when I configured my box to 6bone. 

The IPv6 service provide gives me a set of network parameters as following: 

======================================
Your IPv6 address block is:  3ffe:3216:2101:21ff::/64
The tunnel broker's Ipv4 address is : 202.38.64.185
Your registered tunnel end's IPv4 address is: 202.38.75.76
The tunnel broker's IPv6 address is : 3ffe:3216:2101:2100::5
======================================

I configured my box as follows:

#ifconfig gif0 tunnel 202.38.75.76 202.38.64.185
#ifconfig gif0 inet6 3ffe:3216:2101:21ff::1 -prefixlen 64
#route add -inet6 default 3ffe:3216:2101:2100::5 -ifp gif0

When I ran the command 'ping6 3ffe:3216:2101:2100::5', there was no reponse with ping6.
But I tried tcpdump to see what's going when I saw:

'tcpdump -i fxp1' showed that it had been receiving both ping icmp echo request and reply from the tunnel server, while 'tcpdump -i gif0' showed that there was no reply !  why?  I guess there must be something wrong  with my tunnel configurations.  ANy suggestion?

Best,
--
Hui Wang
InfoNet Lab,USTC
http://if.ustc.edu.cn/~hwang