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
Best Answer
I've run into this problem before and there's a couple things happening in this scenario.
First, The Management interface does not play by the same rules as other interfaces on the FW. By default it will not pass or receive traffic from any other interface on the device due to the "Management-Only" setting.
Second, the way that Cisco implements the Management interface causes a routing loop with the ASA. You would like to route all traffic to the management network through the L3 switch on the Inside, but the ASA sees the Management network as directly connected via the Management interface
You would like the traffic to take the following path:
IPS > L3 Switch > ASA Inside > Internet > ASA Outside > L3 Switch > IPS
Unfortunately, the path it is actually taking looks like this:
IPS > L3 Switch > ASA Inside > Internet > ASA Outside > Bit Bucket
Any packet sent from the IPS to the internet is returned to the ASA Outside interface, at which point the routing table is checked and it sees that the management network is directly connected via the Management interface. Since the Management interface will not receive traffic from another interface by default, the bits hit the floor.
Unfortunately, the best way to resolve this issue is to abandon using the Management interface to manage the firewall and instead use the Inside interface. If you remove the IP address of the Management interface (but still keep the port enabled for the IPS module), that will remove the Management network from the ASA routing table. This will allow the traffic to take the correct path back to the L3 switch on the Inside when it returns from the Internet.
I hope this helps