Best practice wise - should I let the router or the ASA handle NAT
(Overloading)?
In the most general of design best practices NAT is performed between an inside and outside network. NAT overloading is generally performed at the edge when there is limited public IP address space. You can learn more about NAT overloading, also known as Port Address Translation or PAT, in RFC 2663 (PAT is referred to as Network Address Port Translation (NAPT) in section 4.1.2).
In this particular scenario you can argue that you have two inside and outside networks and will need to perform some form of NAT on both the ASA (whether that is the NAT overloading you're using now, NAT exemption, static NAT, etc) and the Cisco Router.
I can ping the 172.16.2.2 interface but not 172.16.2.1 from a pc
connected to one of the layer 2 switches (proves intervlan routing is
working -- i have a 172.20.100.8 address on the PC). Why can't I ping
172.16.2.1 from a PC but I can from the Layer 3 Switch?
The ASA 172.16.2.2 is receiving the ICMP echo-request but does not have a route back to 172.20.100.0/27. The echo-reply is actually being forwarded to the Router 172.16.1.1 via the default route.
And most of all -- Why can't I get out to the Internet from the Layer 3 switch?
Currently your ASA and Cisco Router do not have routes to internal devices other than their connected routes.
Your ASA configuration:
route outside 0.0.0.0 0.0.0.0 172.16.1.1 1
This will provide a default route via the outside interface, but how will the ASA know how to reach subnets residing behind the Layer 3 Distribution Switch?
You'll need to add routes to the internal subnets via the inside interface using the Layer 3 Distribution Switch as the next-hop IP address.
ASA static routing example:
route inside 172.19.12.0 255.255.255.240 172.16.2.2
route inside 172.19.3.0 255.255.255.0 172.16.2.2
route inside 172.20.100.0 255.255.255.224 172.16.2.2
Further reading: ASA static routing
Your Cisco Router's configuration:
ip route 0.0.0.0 0.0.0.0 200.200.200.200
Additionally, how will your border router know how to reach subnets other than it's connected routes, and the catch all default route via the outside interface's next-hop address 200.200.200.200?
Router static routing example:
ip route 172.19.12.0 255.255.255.240 172.16.1.10
ip route 172.19.3.0 255.255.255.0 172.16.1.10
ip route 172.19.100.0 255.255.255.224 172.16.1.10
ip route 172.16.2.0 255.255.255.224 172.16.1.10
Further reading: ISR static routing
I cannot get an ip address right now from the DHCP server (Windows).
Any insight into why?
Ensure you have end-to-end IP reachability between the client(s) sending DHCP discover messages and the DHCP server.
From what I can gather from your topology and configuration, the subnets 172.19.3.0/24, 172.19.12.0/28 and 172.20.100.0/27 should have no issues connecting to each other (assuming they are configured to use their respective default gateways) from a networking perspective.
You can remove the ip helper-address syntax from the SVI 100 given that the DHCP server is on the same segment and that command is used for a DHCP server(s) that is on a different segment.
interface Vlan100
ip address 172.20.100.1 255.255.255.224
ip helper-address 172.20.100.27
I see you have an access-list :access-list outside_access_in
But idon't see it applied on the interface.
I think you should have an Access-list for inside interface too, and applied to it.
Permit icmp, http,https from inside and any other protocol you need.
Then do a traceroute.
You can also try this command: packet-tracer
see how to use it here:
https://supportforums.cisco.com/docs/DOC-5796
This command can show you were your packets fail.
EDIT
So you need 2 ACL's , one for inside interface and one for outside. I see you already have one for outside but it is not applied to the interface. Like this :access-group outside_access_in in interface outside
Create an ACL for inside too, and apply it.
It is useless to test packet-tracet from outside to inside. like this :"packet-tracer input outside icmp 75.75.75.75 0 0 10.1.10.11"
this is because it will be always dropped becuae you don't have a static NAT mapping and an ACL entry perminting the traffic.
SO the first packet-tracer test is what we needed and it looks ok.
We want to see that traffic coming form inside is forming a flow.The fireweall will permit the returning flow. (This is a statefull firewall)
So create an apply ACL for inside and outside and see if it works.
Best Answer
It won't drop the packets, because the ASA is a Stateful device.
First, you have to understand what I mean when I say the ASA is a stateful device. This means if the firewall allowed traffic in the direction outbound, it automatically allows the correlating return traffic back inbound -- even if the inbound ACL or Security-Level would have prevented all traffic inbound.
As for ICMP, by default, ICMP is not stateful. Which is to say, if you want to allow ICMP packets to leave your network, you must explicitly allow it (via ACL or Security-Level). And if you intend for ICMP packets to come back into your network, you must also explicitly allow it.
However, you can override that default behavior by enabling ICMP inspection. On the ASA inspecting ICMP makes the ASA consider ICMP stateful. Which means, if it allows an ICMP Echo Request leave your network, it will automatically allow the ICMP Echo Response come back into your network.