I presume this is some sort of lab exercise ? Otherwise this is not the sort of setup I would expect in a datacentre. If you have two ASA5520, then configure them as a failover pair, and use the management lan interfaces as the failover link, as this is much quicker, and keeps session information, than the dedicated failove interface. Do you want the ASA to connect to the 2800 using a VPN ? Otherwise it is just ethernet, so you need IP addresses in the same subnet, and a cross-over cat5 cable. Similarly, the 2800 to the 871 routers, is this VPN, or just ethernet again ? Same criteria. I think you need to supply some more information about what you are trying to achieve, then we can give a more deailed, and focused answer.

I presume this is some sort of lab exercise ? Otherwise this is not the sort of setup I would expect in a datacentre. If you have two ASA5520, then configure them as a failover pair, and use the management lan interfaces as the failover link, as this is much quicker, and keeps session information, than the dedicated failove interface. Do you want the ASA to connect to the 2800 using a VPN ? Otherwise it is just ethernet, so you need IP addresses in the same subnet, and a cross-over cat5 cable. Similarly, the 2800 to the 871 routers, is this VPN, or just ethernet again ? Same criteria. I think you need to supply some more information about what you are trying to achieve, then we can give a more deailed, and focused answer.