Linksys doesn't actually specify whether the router is NAT or SPI based. From the trouble that I had running the QCheck UDP streaming tests, however, I'd say that the product is mostly NAT based, but with some SPI going on.
Although the admin screens for Port Forwarding and Filtering should be familiar to Linksys devotees, the functions that are actually implemented come up a little short vs. other members of Linksys' router product line. For example, Figure 5 shows the Port Forwarding interface.
Figure 5: Port Forwarding
If you look closely you'll note that the Port Triggering button is missing, even though it's described in the User Guide. When I asked Linksys about this, they said that the feature will be included in a future firmware update, but that they'll fix the User Guide for now.
I liked that all Port Forwarding entries are both editable and able to be disabled and left programmed, something I wish were standard on all routers. I also checked for and found that loopback is supported for servers on forwarded ports.
Tip: See this page of our Hardware Router NTK - Terminology Guide for an explanation of "loopback".
The router also supports putting one computer in DMZ via a setting on the Security tab of the interface.
On the negative side, port forwardings are not schedulable by day and time, and - although I don't really think it's a negative - UPnP is not supported.
Turning to the Filtering (Access Control) features shown in Figure 6, I found the interface somewhat confusing.
Figure 6: Filters screen
It wasn't obvious, at least to me, that the Internet Access Policy and Filtered Internet Port Ranges were entirely separate functions. The Access Policy is schedulable by day and time, and up to 10 different policies can be created. Each policy has its own schedule and group(s) of LAN computers that it applies to, but is an all-or-nothing proposition. The Access Policy either grants all or denies all Internet access, and can't be limited to specific ports.
Specific port filtering is controlled by the Filtered Internet Port Ranges settings, but these settings can't be scheduled and apply to all LAN machines.
It's worth noting that Linksys hasn't yet provided even simple keyword content filters even though many of its competitors have had them for awhile. They do plan to implement the Web Filters features described in the User Manual in a future firmware release, but for now, you'll have to do without the ability to block Web Proxies, ActiveX, Java, and Cookies.