What is Asymmetric Routing?

Asymmetric Routing

Asymmetric routing occurs when the forward and reverse paths of network traffic between a source and destination are not the same. In complex network setups, traffic may take different routes due to load balancing, redundant links, or routing protocols. This can lead to asymmetric routing situations.


Firewalls are network security devices or software that monitor and control network traffic based on predefined rules. They act as barriers between different network segments, protecting against unauthorized access and threats. Firewalls inspect packets, enforce security policies, and make decisions on whether to allow or block traffic.

Issues with SIP

SIP is a signaling protocol used for initiating and managing real-time communication sessions, such as voice and video calls, over IP networks. Asymmetric routing and firewalls can cause problems in SIP communications due to the following reasons:

  1. NAT traversal: SIP often requires Network Address Translation (NAT) traversal to establish successful communication between devices located behind NAT devices. Asymmetric routing can disrupt the expected NAT translations performed by firewalls, leading to communication issues. For example, the SIP packets may traverse different paths, resulting in inconsistent NAT mappings and making it challenging for devices to establish direct communication.

  2. Session state tracking: Firewalls typically employ stateful inspection to track the state of network connections. They maintain information about established sessions and use it to allow or block subsequent packets. However, when asymmetric routing is present, firewalls may struggle to accurately track session states because the forward and reverse paths of SIP packets do not align. This can lead to dropped or blocked SIP packets, resulting in failed call setups or call interruptions.

  3. Media flow and quality: SIP sessions involve the transmission of media streams (e.g., voice or video). Asymmetric routing can disrupt the expected path for media traffic, potentially causing issues with one-way or degraded audio or video quality. Firewalls may need to be configured to handle media traffic correctly when it arrives through different paths.

Mitigating Issues

  1. Symmetric routing: Ensuring symmetric routing, where the forward and reverse paths of traffic align, can help minimize issues caused by asymmetric routing. Network configuration changes or the use of routing protocols that support symmetric routing can be implemented.

  2. Firewall configuration: Configuring firewalls to handle asymmetric routing is crucial. This includes adjusting stateful inspection and connection tracking mechanisms to accommodate SIP traffic that arrives through different paths. Path-specific rules or configurations may be needed to allow SIP packets to pass through the firewall correctly.

  3. SIP-aware firewalls: Consideration can be given to deploying firewalls specifically designed for SIP traffic. These firewalls are SIP-aware and equipped with features to address NAT traversal, session tracking, and media flow challenges common in SIP communications.

  4. Session Border Controllers (SBCs): SBCs are specialized devices that sit at the edge of SIP networks and handle various tasks, including NAT traversal, session management, and media handling. Deploying SBCs can help mitigate issues caused by asymmetric routing, as they are designed to handle the complexities of SIP communications.

In summary, asymmetric routing and firewalls can introduce complications in SIP communications, impacting NAT traversal, session tracking, and media flow. Proper configuration of firewalls, consideration of symmetric routing, and utilizing SIP-aware firewalls or SBCs can help overcome these challenges and ensure smooth and reliable SIP-based communication.

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