I can't believe I can't make it work!
I am using a Cisco 1721 router with two Ethernet ports and one FastEthernet ports. Here is my config:
ip address dhcp client-id Ethernet0 hostname E0
ip address dhcp client-id Ethernet1 hostname E1
no ip address
E0 and E1 are connecting with ISP's ADSL modem. However only one can get an IP address! ISP is fine because if I plug one cable to my laptop my laptop gets the second IP address without problems. Somebody please HELP!! Thanks a lot!
With this explanation the situation makes a lot more sense. And I believe that when you are actually connecting to 2 different ISPs that it will work ok. But, as you found out, trying to connect both router interfaces to the same provider does not work.
I am not sure that there is any good way to test when you are connected to only a single provider. I had thought some about configuring one interface with an address (not DHCP) and doing something like splitting the address range (create 2 small subnet rather than the original single subnet) which would allow both interfaces to have an IP address. But the problem is that there is only one next hop address for any routing that you would be doing. So you would not be able to really test redundancy or failover logic.
I believe that if you want to do any meaningful testing that you will need 2 separate providers active during the test.