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Flat Network Strength Also A Security Weakness

Security is one of the biggest concerns and challenges for today’s interconnected systems, regardless of the network type deployed. However, the intricacies of flat networks bring new nuances to best security practices, which can be addressed by combining common sense with some industry-leading strategies.

Dreger identifies the key security elements for L2 as strong segmentation and filtering options between L2 entities; intuitive labeling and management of devices, because VLANs won’t necessarily provide context clues; consistent application of L2 access controls via usable tools; security controls designed for the unique requirements of virtual environments (for example, quick system builds and virtual machine migration); and an ability to clearly show how traffic is being controlled to meet audit requirements.

It is the above elements, combined with best practices, that will bring a stable security footing to flat networks. Naturally, it will also take an active management role to further improve security. That active management can take the form of defining rules on security appliances that support L2 firewalling. In most cases, those rules will take the form of VLAN access control lists (VACLs), private VLANs (PVLANs) and filtering controls.

Dreger reports that VACL technologies can be used in much the same way as traditional L3/L4 ACLs, with the added benefit that they are also applied at L2 on a physical switching/routing device. That means a VACL can filter traffic bridged between devices on the same VLAN and does not just need to apply to routed traffic going into or out of a VLAN. VACLs can be defined to block specific traffic types (such as UDP and TCP) and be applied directionally to and from various hosts. More specifically, VACLs can be tied to specific interfaces or be more generally applied to a whole VLAN.

Of course, VACLs are only the beginning when it comes to securing flat networks at the L2 level. Administrators will need to still take a layered approach to security, further fortifying virtual devices, virtual machines, applications and other members of a flat network architecture. Here, security begins to take a more familiar form, using the same security tools that tiered networks have come to rely on.

Dreger writes, "Fortunately, when you understand VACLs, PVLANs and L2 firewall control options in the physical realm, VM-centric controls will not appear radically different." In other words, if administrators can conquer the physical portion of L2 network security, the virtual elements should pose little or no problem.

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User Rank: Apprentice
7/14/2013 | 2:11:10 PM
re: Flat Network Strength Also A Security Weakness
This is a major problem. VACL's and the like will not provide stateful firewalling, IDS and other active features.

If a flat network is truly going to replace a tiered network we need switches with embedded security processing so we can keep the features of our layer-3 firewalls.

Also, the author didn't touch on the tendency to create a flat network simply by putting everything in a single vlan, without adding TRILL or SPB or VACL's. This removes the ability to firewall traffic and leaves the network at the mercy of spanning tree.
User Rank: Apprentice
4/4/2012 | 8:36:08 PM
re: Flat Network Strength Also A Security Weakness
Just because a network is 'flat' from the L2 perspective does not mean that any given subnet has to have more in it.

The flat l2 networks like Shortest Path Bridging simply give you the effect of more VLANs (2^24) that can be trivially accessed anywhere. These are called ISIDS and of course traffic in the same ISID flows on symmetric shortest paths. 16 of them in the case of SPB.

So your ISID corresponds to a subnet and if you want you can have many of them with small numbers of hosts in each, or a small number of ISIDs/subnets being large. So the security implications can remain unchanged if you so choose. I.e. smaller number of related devices per subnet, they communicate at L2, but the moment you need to go outside, .. well you cross a router/firewall etc.

Its not that different, its just more flexible and boy are they easy to use. Just tell a given {port/c-vlan/s-vlan} that faces a server what ISID its in and poof .. all done.
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