Question: Okay, suppose that I have 2 computers: `Computer A` and `Computer B`, connected with this setup:

```
A->switch->router->internet
          /
B->router/

```

and the closest router to `computer B` is configured to block connections from `Computer A`s switch.

If `computer A` is desperately trying to attack `computer B`, sniffing packets, MiTM attacks, etc., how much bigger/smaller attack surface does it have than the usual

```
Computer A ->router->internet
           /
Computer B/

```

Answer: 
It depends on what type of MitM attack you want to do. 

`arp` spoofing or any type of attack which relies upon Layer 2 will not work across routing domains. 

However, you can perform other attacks such as modifying a host file, [DNS spoofing](http://www.windowsecurity.com/articles-tutorials/authentication_and_encryption/Understanding-Man-in-the-Middle-Attacks-ARP-Part2.html), or other attacks at the network level. You could attack the router itself so all traffic is directed to you, etc. You could trick the victim into installing malware which will redirect all requests to your server first.

If your specific host is being blocked at any point, you can use another machine or compromise another machine to try to find a valid path to your victim.
Computer A needs to control a device which handles Computer B's traffic on order to perform a MiTM attack. In your top scenario there's a bigger attack surface because there are 2 routers that computer A can attempt to compromise.