There's a lot of talk in the industry about getting away from VLAN segmentation and relying on stateful firewalls at our access-layer edge to govern control over what users have access to. This is a great idea, it solves issues with IPv6 and it simplifies network design. But there are some significant challenges that make it a no-go for today's enterprise networks. Most vendors are touting their "stateful" firewalls in the AP and edge switches solves those challenges. But I find the current generation of these solutions inadequate to solve this issue in enterprise networks.
Issue #1: I need your identity at more places that the access-layer edge
Web Content Filtering is a great example of needing your identity elsewhere in the network. In a restrictive corporate environment, there are Active Directory integration that help solve these problems, but what about non domain devices? What about organizations with the Internet of Everything?
I've seen solutions from Radius Accounting integration to agents on Domain Controllers, but these are usually point solutions and I personally have not had good luck with these nor are they widely supported. Also they are single device specific, so you can't send it to multiple devices for determining identity. Datacenter firewalls are another place where this falls apart. How do I write an ACL based off your identity when I may or may not have your identity. The solution inevitably leads to more identity verification: Captive Web Portals, VPN clients, etc.
Issue #2: Scalability of ACLs
Anyone who has tried to write complex ACLs to govern what a client can or can't get to, can tell you that ACLs get very unwieldy very quickly. You CANNOT effectively write ACLs for every resource and port that every potential client should be able to get to. It's also not effective as these ACLs take up precious TCAM space in our network equipment.
The solution to this problem is an identity exchange. Cisco has a pair of technologies called SGT and SXP with ISE or ACS (part of their TrustSec solution) that attempts to solve this problem. Instead of filtering traffic with ACLs on ingress to the network, they identify identity at the edge and pass that identity information to the rest of the network and filter packets on egress of the network. Both protocols are Cisco proprietary, but the idea is sound. While I'm not a fan of having to have special hardware to pass this info around the network, the idea of a central identity repository that all devices have access to solves the issue of having to filter all packets at the access-layer edge, we allow the rest of the network to share in this burden and create a solution that scales.
Personally, I don't think we will see single VLAN designs be successful for quite some time. The wide variety of firewalls, web content filtering, lack of network-wide identity and complex nature of BYOD policies really prevent us from completely abstracting out the devices IP addressing. My hope is that with the upcoming SDN-apocalypse that we will see SDN solutions providing ways to distribute identity throughout the network and get us closer than ever to the simplified access layer edge that so many vendors are suggesting today.