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Security Through Network Fundamentals

Chapter Description

In this sample chapter from Practical Cisco Unified Communications Security, you will explore the dependencies that Unified Communications (UC) systems have on a network while also highlighting the security features that can be implemented to provide additional security to the UC environment.


A best practice in today’s networks is to provide logical segmentation through the use of virtual LANs (VLANs). Within a UC environment, this approach is implemented on access switches at layer 2 in the form of voice VLANs and also at layer 3 to prevent attacks on an IP subnet by applying access control lists (ACLs) on VLAN interfaces. By segmenting UC and data traffic, organizations can reduce the attack surface to which attackers have access. Ideally, attackers would have access only to the segment they are connected to. Segmentation also helps with simplifying the network by allowing an administrator to view and distinguish the different types of traffic that traverse specific interfaces in and out of the logical network. By segmenting the network (virtually) into smaller networks (in this case, VLANs), administrators can easily control the types of traffic that are permitted across different layer 2 and layer 3 interfaces to prevent unwanted traffic.

Dedicated UC and data segments also help minimize the size of a broadcast domain, which reduces the amount of traffic flooding across a network. Within a single VLAN or broadcast domain, there is potential for devices to generate (and flood) the network with traffic, which can be problematic from a security perspective. We discuss this issue later in the section on security features. Large broadcast domains also have the potential to impact the performance of the network. When segmenting a network, an organization is better insulated from a broadcast storm or denial-of-service (DoS) attack. This way, only a portion of the network can be disrupted if an attack were to occur on a particular VLAN. Cisco best practices currently recommend limiting the size of a logical segment to 256 devices, if possible, and not to exceed 512 devices.

When an organization uses the latest IOS-XE platforms (e.g., 16.9), the data and UC networks can be logically segmented by applying configurations for data and voice VLANs on a single interface. The following example shows the syntax:

switch# conf t
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# int gig1/0/10
switch(config-if)# switchport mode access
switch(config-if)# switchport access vlan 10
switch(config-if)# switchport voice vlan 100

At this point, we have discussed segmentation at layer 2 of the OSI model. The next section provides additional detail about layer 3 segmentation using Virtual Route Forwarding (VRF).

VRF is a way to segment the network segmentation at layer 3 of the OSI model. VRF was originally designed for service providers to essentially provide existing and new customers with their own virtual private network (VPN) across the service provider’s physical infrastructure. This solution allowed service providers to take on new customers with minimal costs, without having to worry about a customer’s existing IP scheme or infrastructure. Within a VPN built with VRFs, each VRF instance has a separate layer 3 routing table. IP packets from one VRF are intentionally isolated from other VRFs.

If the flow of UC traffic isn’t taken into consideration, use of VRFs may unintentionally create user experience issues. These issues could be related ACLs inside a fusion router/firewall that break functions of a UC environment, such as screen sharing. Additionally, their use could cause issues with performance by introducing latency, jitter, and/or packet loss as traffic is routed between interfaces on a fusion router or firewall during a time in which the network has high utilization. As an example, an organization decided to segment the network with VRF. To minimize the attack surface, it decided to create VRFs for each line of business (sales and marketing, research and development, support) and also for each device type (e.g., printer, IP phone, Internet of Things [IoT] sensor). The belief was that creating specific groups would simplify how ACLs are applied on the network, with a fusion firewall that supports group-based security policy. During testing, engineers were able to print, make phone calls between IP phones, and access shared resources. Unfortunately, network administrators didn’t consider all of the network requirements for a UC client such as Jabber. Therefore, they unintentionally created firewall policies that prevent Jabber calls between IP phones.

Certainly, changes can be made to network policy, especially to help resolve problems as they arise. Some questions that a UC administrator can ask to proactively prevent issues with VRF include but are not limited to

  • ▪ Will a VRF disrupt the user workflow or calling patterns?

  • ▪ Where will the inter-VRF routing take place to ensure a quality user experience between UC endpoints (e.g., Jabber, IP phone, video device)?

It is important to realize that care should be taken when designing segmentation in the network to ensure that the network supports the desired features within a UC environment. It is also important to make sure that the network does not force UC traffic to flow across the network in ways that are less than optimal to help avoid the addition of latency, delay, and jitter.

At this point, we have discussed basic segmentation as one approach for providing security at different logical layers of the network so that organizations can apply security controls at whichever level of granularity is necessary to comply with security policies. Basic segmentation by itself, which logically separates different types of devices or business entities, can be referred to as segmentation at a “macro” level. As part of a defense-in-depth approach, another layer of segmentation is possible to further reduce the attack surface. This approach is known as micro segmentation, is discussed next.

3. Micro Segmentation | Next Section Previous Section

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