problem with routing failover

Hi,
I try to configure router with connection to two internet providers. My
router works under Linux Ubuntu, kernel 2.6.10.
First connection is faster and should work as main internet connection.
Second provider should works as backup fail over connection.
My routing table looks following:
root@routerUbuntu:/boot # route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use
Iface
localnet * 255.255.255.248 U 0 0 0 eth0
217.8.184.72 * 255.255.255.248 U 0 0 0 eth4
217.8.184.80 * 255.255.255.240 U 0 0 0 eth3
213.186.87.224 * 255.255.255.224 U 0 0 0 eth1
192.168.1.0 * 255.255.255.0 U 0 0 0 eth2
default 217.8.184.65 0.0.0.0 UG 0 0 0 eth0
default 213.186.87.225 0.0.0.0 UG 0 0 0 eth1

I've switched timeout:
cat /proc/sys/net/ipv4/route/gc_timeout
10
According to founded description should it works.

I've tied test fail over situation. I've removed ethernet from eth0,
tried ping some host in internet and I've started tcpdump on backup
connection eth1. I had following effect:
- packages go out through eth1
- answer comes to eth1
- but ping shows no connection to host

When I make "ifdown eth0" ping works without problem through eth1.

Is it necessary to switch some special parameter in kernel?
Iptables is switched off (all chains are set Accept, without rules).

regards,
Marek