Hi,
I'll descript this question in more details:
1. Two internet connections on the same router with pppoe both.
2. load balance default route:
default
nexthop via a.a.a.a dev ppp0 weight 1
nexthop via b.b.b.b. dev ppp1 weight 1
3. telnet to remote server through ppp1 first time, and maybe 10 minutes later, telent disconnected.
Something I found:
1. route cache will be flushed every 10 minutes by defualt(/proc/sys/net/ipv4/route/secret_interval)
2. after 10 minutes later, caches are flushed; then packets in telnet connection will be routed again;
at this time, multipath may choose another nexthop, e.g. ppp0
3. packets with ppp1 source ip transmited out from ppp0, and telnet disconneted
4. why does kernel flush the cache entry even if the entry is in use.
So, How do I fixed this situation?
Thank a lot!

You can change the value to a higher number I imagine.

sudo echo 20 > /proc/sys/net/ipv4/route/secret_interval

Thanks AutoBot.
It's a method, but after a longer time, connection will be still disconnected...
I still do not understand why does kernel flush the cache entry even if the entry is in use.
Maybe we should make kernel do not flush cache entry which is in use.
Any ideas?

Found it!
Here is the solution, I patched it on 2.6.20, it looks work fine.
http://www.ssi.bg/~ja/
Read http://www.ssi.bg/~ja/nano.txt first !
Thanks all!