Hello, I this is the topology of my network - http://www1.picturepush.com/photo/a/9706484/640/9706484...
On the router connected to the two switches, I've configured virtual interfaces, fa 0/0.1 , fa0/0.2 and so on. When the switches want to access the upper router all data goes through interface fa 0/0 on the lower router. The problems is that when the fa0/0 interfaces goes down there is no connection to the upper router.
My question is if there is a way to connect to the upper router even when int fa0/0 on the lower one is down?
Not sure how your diagram related to the description you have given with the sub interfaces.
My main difficultly here is figuring out what devices you have running as switches and which you have running as routers.
The diagram implies the first set of switches can run layer 3 routing. You would route between your router and these switches using 2 point to point subnet and then run the 2 edge switches using VRRP or something to protect the gateway.
If all your switches run layer 2 the only way you get it to fail over to the other port is to run the router in bridge mode. This is not a recommended configuration if you plan to run any significant traffic since this is all processor switched unlike a layer 3 switch which will do it with asic chips.
Yes, I see that the description I gave may be a little bit confusing.
The bottom two switches run layer 2.
The upper two run layer 3 and they are configured to route between vlans.
The Router connected to the L3 Switches have configured sub interfaces for each vlan, and is used as DHCP server and as default route for the vlan to the last router, that is the ISP router.
My sub interfaces are configures as fa 0/0.1, 0/0.2, on the FastEthernet 0/0 interfaces. As I said in the previous message if this interface fails, the switches cant ping the isp, the reason for which is the sub interfaces being configured at fa0/0.
My question is if I can by configuration or some hardware way to make it work, even though FA 0/0 int is shut down.
You would be much better off changing the design to let the layer 3 switches be the gateway and then route back to the router on both interfaces. You really don't need the subinterface. You can still use the router as your DHCP server by placing ip helpers in the vlans on the switch.
If I had to recommend a deign I would say
You want run the gateway for the PC lans on the layer 3 switch using VRRP to protect against a failure of the layer 3 switch. You would create a new vlan for each switch to router connection. You could then either use floating static routes or better a routing protocol to detect the failure of the connection between the switches and the router. So the router would know it could reach the same vlan via either interface and each switch would have a default route to the router.
The other method would require less changes but tends to be more complex to configure.
You would create the same 2 subinterfaces on fa 0/1
You create a bridge group for each vlan and move the ip to that.
you add each sub interface to its bridge group.
This is a layer 2 solution using spanning tree....which is pain to troubleshoot.
You can just use static routes by default they both run active so it will load balance the traffic. If you want then to run hot and standby you need to use what is called a floating static route which is simply a higher metric on the standby one.
If you want it automated something like OSPF is the easiest but EIGRP or RIP will work if you like those better.