The Cisco Integrated Service Routers (ISRs) offer a scalable platform to address data and voice network requirements for a wide range of applications. Although the threat landscape of both private and Internet-connected networks is a very dynamic environment, Cisco IOS® Firewall offers stateful inspection and Application Inspection and Control (AIC) capabilities to define and enforce a secure network posture, while it enables business capability and continuity.
This document describes design and configuration considerations for firewall security aspects of specific Cisco ISR-based data and voice application scenarios. The configurations for voice services and the firewall are provided for each application scenario. Each scenario describes the VoIP and security configurations separately, followed by the entire router configuration. Your network possibly can require other configuration for services, such as QoS and VPN, to maintain voice quality and confidentiality.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware versions.
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The Cisco IOS Firewall is typically deployed in application scenarios that differ from the deployment models of appliance firewalls. Typical deployments include Teleworker applications, small- or branch-office sites, and retail applications, where low device count, integration of multiple services, and lower performance and security capability depth is desired.
While the application of firewall inspection, along with other integrated services in the ISR products, can appear attractive from cost and operational perspective, specific considerations must be evaluated to determine if a router-based firewall is appropriate. The application of each additional feature incurs memory and processing costs, and can likely contribute to reduced forwarding throughput rates, increased packet latency, and loss of feature capability within periods of peak load if an underpowered integrated router-based solution is deployed. Observe these guidelines when you decide between a router and an appliance:
Routers with multiple integrated features enabled are best suited for branch-office or telecommuter sites where fewer devices offer a better solution.
High-bandwidth, high-performance applications are typically better addressed with appliances; Cisco ASA and Cisco Unified Call Manager Server must be applied to handle NAT and security policy application and call processing, while routers address QoS policy application, WAN termination, and site-to-site VPN connectivity requirements.
Prior to the introduction of Cisco IOS Software version 12.4(20)T, Classic Firewall and Zone-Based Policy Firewall (ZFW) were unable to fully support capabilities required for VoIP traffic and router-based voice services, which required large gaps in otherwise secure firewall policies to accommodate voice traffic, and offered limited support for evolving VoIP signaling and media protocols.
Cisco IOS Zone-Based Policy Firewall, similar to other firewalls, can only offer a secure firewall if the security requirements of the network are identified and described by security policy. There are two fundamental approaches to arrive at a security policy: the trusting perspective, as opposed to the suspicious perspective.
The trusting perspective assumes that all traffic is trustworthy, except that which can be specifically identified as malicious or unwanted. A specific policy is implemented that denies only the unwanted traffic. This is typically accomplished through the use specific access-control entries or signature- or behavior-based tools. This approach tends to interfere less with existent applications, but requires a comprehensive knowledge of the threat and vulnerability landscape, and requires constant vigilance to address new threats and exploits as they appear. Additionally, the user community must play a large part in the maintenance of adequate security. An environment that allows broad freedom with little control for the occupants offers substantial opportunity for problems caused by careless or malicious individuals. An additional problem of this approach is that it relies much more on effective management tools and application controls that offer sufficient flexibility and performance to be able to monitor and control suspect data in all network traffic. While technology is presently available to accommodate this, the operational burden frequently exceeds the limits of most organizations.
The suspicious perspective assumes that all network traffic is undesired, except for specifically identified good traffic. It is a policy that is applied, which denies all application traffic, except that which is explicitly permitted. Additionally, application inspection and control (AIC) can be implemented to identify and deny malicious traffic that is specifically crafted to exploit good applications, as well as unwanted traffic that masquerades as good traffic. Again, application controls impose operational and performance burdens on the network, although most undesired traffic must be controlled by stateless filters, such as access-control lists (ACLs) or Zone-Based Policy Firewall (ZFW) policy, so there is substantially less traffic that must be handled by AIC, intrusion prevention system (IPS), or other signature-based controls, such as flexible packet matching (FPM) or network-based application recognition (NBAR). If only desired application ports (and dynamic media-specific traffic arising from known control connections or sessions) are specifically permitted, the only unwanted traffic that is present on the network must fall into a specific, more-easily-recognized subset, which reduces the engineering and operational burden imposed to maintain control over undesired traffic.
This document describes VoIP security configurations based on the suspicious perspective, so only traffic that is permissible in the voice-network segments is permitted. Data policies tend to be more permissive as described by notes in the configuration of each application scenario.
All security policy deployments must follow a closed-loop feedback cycle; security deployments typically affect the capability and functionality of existent applications and must be adjusted to minimize or resolve this impact.
If you need additional background to configure the Zone-Based Policy Firewall, review the Zone Firewall Design and Application Guide.
The Zone Firewall Design and Application Guide offers a brief discussion about router security with the use of security policies to and from the self zone of the router, as well as alternative capabilities that are provided through various Network Foundation Protection (NFP) features. Router-based VoIP capabilities are hosted within the self zone of the router, so security policies that protect the router must be aware of the requirements for voice traffic in order to accommodate the voice signaling and media originated by and destined to Cisco Unified CallManager Express, Survivable Remote-Site Telephony, and Voice Gateway resources. Prior to the Cisco IOS Software Version 12.4(20)T, Classic Firewall and Zone-Based Policy Firewall was unable to fully accommodate the requirements of VoIP traffic, so firewall policies were not optimized to fully protect resources. Self-zone security policies that protect router-based VoIP resources rely heavily on capabilities introduced in 12.4(20)T.
The Cisco IOS Software Release 12.4(20)T introduced several enhancements to enable co-resident Zone Firewall and voice capabilities. Three main features apply directly to secure voice applications:
SIP Enhancements: Application-Layer Gateway and Application Inspection and Control
Updates SIP version support to SIPv2, as described by RFC 3261
Broadens SIP signaling support to recognize a wider variety of call flows
Introduces SIP Application Inspection and Control (AIC) to apply granular controls to address specific application-level vulnerabilities and exploits
Expands self-zone inspection to be able to recognize secondary signaling and media channels that result from locally-destined/-originated SIP traffic
Support for Skinny Local Traffic and CME
Updates SCCP support to version 16 (previously supported version 9)
Introduces SCCP Application Inspection and Control (AIC) to apply granular controls to address specific application-level vulnerabilities and exploits
Expands self-zone inspection to be able to recognize secondary signaling and media channels that result from locally-destined/-originated SCCP traffic
H.323 Support for Versions 3 and 4
Updates H.323 support to versions 3 and 4 (previously supported versions 1 and 2)
Introduces H.323 Application Inspection and Control (AIC) to apply granular controls to address specific application-level vulnerabilities and exploits
The router security configurations described in this document include capabilities offered by these enhancements with explanations to describe the action applied by the policies. Hyperlinks to the individual feature documents are available in the Related Information