I have router (cisco 2811), 2 switches and client. I want that client has one default gateway to 2811 (for example gateway ip will be up on Loopback interface) and if one of the links from switch to gateway down, gateway ip will be available. I don't know how to do it, can u help me?
I think I get what you are trying to achieve. You want just one IP address as a default gateway which should be available from your client with redundancy between two fast-ethernet ports of your 2800 router. However it would be best to have two routers at your gateway and have HSRP between them.
As your current situation goes I think you could have one possible solution(or could be more which I could not think at this moment)...
1) You can configure a BVI interface and associate both the ethernet interfaces to that group. These two interfaces of the router would a part of the spanning-tree instance and one would be forwarding and another in blocking state. You need to have trunk between two 2960. Make sure both the router's fastethernet ports are in same VLAN on the 2960 switch.
bridge 10 protocol ieee
bridge 10 route ip
int bvi 10
ip address x.x.x.x x.x.x.x
int fas 0/0
no ip addr
int fas 0/1
no ip addr
Here if one port of the router goes down the other port would become forwarding. Note that convergence time from blocking to forwarding on your router could be 50 seconds.
You may check the following show command on your 2800
sh spanning-tree 10