Skip to main content

Managed Subnets

Background:

The most misunderstood topic of the configuration of NACA is Managed Subnets. Every time I get a call about a LAN deployment, which is not working, the first thing I say is "Managed Subnets!". Hopefully, by reading this you will start to understand the taboo term and know when/where to configure Managed Subnets.

Managed Subnets Theory:

"For all CAS modes in L2 deployments (Real-IP/Virtual Gateway) when configuring additional subnets, you must configure Managed Subnets in the CAS so that the CAS can send ARP queries with appropriate VLAN IDs for client machines on the untrusted interface."

The first question you must ask during deployment is "are there more than one VLAN on the untrusted side of the CAS?" If so, you need to give the CAS "logical interfaces" so that the CAS can "manage" those vlans/subnets. The best way to think about managed subnets is to think about a "router on a stick" deployment; A single interface has multiple sub-interfaces in order to reduce the quantity of physical interfaces on the router. This concept can be applied to the CAS. The CAS uses DOT1Q trunking to logically manage multiple subnets. Why does the CAS need to do this? The CAS needs to be able to communicate with the clients on each of the subnets connected to it untrusted interface. This includes things like Web Redirection, SWISS Protocol, etc. The first step in communication is being able to arp and without managed subnets the CAS cannot arp for the clients off of its UnTrusted interface.

When to use Managed Subnets:

"Managed Subnets are only for user subnets that are Layer 2 adjacent to the CAS. For all CAS modes in L3 deployment, Static Routes must be configured for the user subnets that are one or more hops away. Managed subnets should not be configured for these subnets. "

Layer 2 Deployment = Managed Subnets

Layer 3 Deployments = Static Routes

This logic can be used for In-Band/Out-of-Band, Real-IP/Virtual Gateway, Central/Edge Deployments. If you are a newbie to NACA please review the NACA ChalkTalks(CCO Login Required) before thinking too much into this.

How to configure Managed Subnets:

Managed Subnets are configured for each CAS at Device Management - Clean Access Server - manage X.X.X.X - Advanced - Managed Subnet

There are four configuration fields:

IP Address - This value varies based on the type of deployment:
  • Real-IP Gateway: Think of router on a stick. This ip address will be the Default Gateway for the clients on the UnTrusted VLAN.
  • Virtual Gateway: This needs to be an UNUSED IP address on the network.
Subnet Mask - Mask for the ip address used above.

VLAN ID - This is the VLAN ID of the UnTrusted VLAN. EVEN when using Virtual Gateway.

Description - Let remember that the next engineer might not understand managed subnets and needs to read this to get a better understand. Use best practice descriptions.

Figure 1 - Sample Managed Subnet

Summary:

Managed Subnets are something that are overlooked a lot, but after you take the time understand them, they really are just another check on the deployment checklist. Make sure that the next time you are practicing NACA, create a lab scenario that requires managed subnets! Cheers!

Source: CAS Admin Guide
Labels:

Comments

Post a Comment

Popular posts from this blog

Cisco Zero Trust Architecture

 As a follow up to the previous post around Zero Trust Architecture , Cisco has been delivering zero trust architectures for customers for many years. With the platform approach provided by Cisco Zero Trust organizations gain better visibility across users, devices, containers, networks, and applications, verifying their security states with every access request. Adopting this model provides a balance between security and usability. Security teams can make it harder for attackers to collect what they need (user credentials, network access, and the ability to move laterally), and users can get a consistent and more productive security experience, regardless of where they’re located, what endpoints they’re using, or whether their applications are on-premises or in the cloud. Cisco Zero Trust provides a comprehensive approach to securing all access across applications and environment, from any user, device and location. It protects the workforce , workloads and workplac...
6 comments
Read more

Why are Virtual Private Networks and Software Defined Perimeters mutually exclusive?

Increased remote work, vulnerabilities popping up and the #killthevpn movement has the cyber security industry laser focused on the transition from VPN to SDP. Let’s start with an acceptable definition of SDP from Wikipedia: “Software-defined perimeter (SDP) framework was developed by the Cloud Security Alliance (CSA) to control access to resources based on identity. Connectivity in a Software Defined Perimeter is based on a need-to-know model, in which device posture and identity are verified before access to application infrastructure is granted.” I hope we all can agree that the “ground truth" of SDP is valid and any organizations will benefit by adopting SDP architecture and principals(including Zero Trust). How is a Remote Access VPN any different than the “Client-to-gateway” deployment model defined for SDP? “In the client-to-gateway implementation, one or more servers are protected behind an Accepting SDP Host such that the Accepting SDP Host acts as a gateway between ...
5 comments
Read more

Cisco Releases Idenity Services Engine (AKA ISE)

Introduction After years of innovation around Network Access Control, Cisco has released its next generation NAC solution: Identity Services Engine. ISE is combines existing loosely coupled devices AAA, profiling, posture and guest management - in single, scalability appliance. As part of the Cisco TrustSec solution and Cisco’s SecureX architecture for Borderless Networks, the Cisco Identity Service Engine provides a centralized policy engine for business relevant policy definition and enforcement. This policy work horse enables centralized, coordinated policy creation and consistent policy enforcement across the entire corporate infrastructure, from head office to branch office. ISE Features & Benefits Visibility: Single Platform & Pane of Glass - Let IT see who and what is on the network for advanced discovery and troubleshooting Dynamically collects & consolidates endpoint information to make adaptive policy decisions based on ‘context’ Integrates functions previously d...
5 comments
Read more