Simplifying Network Traffic with SR-TE Policy


Segment Routing with Traffic Engineering (SR-TE) policies offer network operators a powerful tool to optimize traffic flows and enhance network performance.

In this blog post, we will walk through a simplified example of configuring an SR-TE policy using the Open Shortest Path First (OSPF) routing protocol. Traffic flows are directed along explicit paths defined by network operators. SR-TE provides granular control over the path that traffic takes through the network, allowing operators to optimize traffic flows based on specific requirements.

Here's a simplified explanation of how traffic flows using SR-TE:

1. Segment Routing (SR) Labels: SR-TE utilizes Segment Routing, which assigns unique labels to network segments or nodes. These labels are used to steer traffic along specific paths.

2. Path Computation: Network operators define the desired path for traffic flows by specifying a sequence of segment labels. This path can be based on various criteria, such as link utilization, latency, or policy requirements. The path computation can be done manually or with the help of a centralized controller.

3. Label Stacking: When a packet enters the network, it is assigned an initial segment label corresponding to the ingress node. As the packet traverses the network, additional segment labels are stacked on top of the existing label, representing the path it should follow.

4. Traffic Forwarding: Each node in the network examines the topmost label of the packet it receives and uses this label to determine the next hop towards the destination. The label also contains information about any service functions or modifications the packet should undergo at each hop.

5. Traffic Engineering: SR-TE enables traffic engineering capabilities by allowing operators to define specific policies and constraints. For example, operators can balance traffic across multiple paths, avoid congested links, or prioritize certain types of traffic based on defined policies.

6. Path Diversity: SR-TE supports path diversity by allowing operators to define backup paths or alternate paths for traffic in case of failures or congestion. This enhances network resilience and enables efficient utilization of network resources.


Step 1: Network Topology Analysis

Let's consider a network with four routers: R1, R2, R3, and R4, connected as follows, Each router is connected to adjacent routers through physical or logical links. The connections between routers R1, R2, R3 and R4 represent the underlying network infrastructure over which the SR-TE policy will be configured.

Step 2: Define Traffic Engineering Constraints

In this example, we aim to optimize for low latency. Therefore, we prioritize the shortest path between R1 and R4, while ensuring load balancing across available links.

Step 3: Path Calculation Algorithm

For simplicity, we will use the Shortest Path First (SPF) algorithm to calculate the paths based on OSPF link metrics. However, more advanced algorithms like Constrained Shortest Path First (CSPF) can be used for complex scenarios.

Step 4: Label Distribution Protocol (LDP) Configuration

Configure LDP on all routers to distribute labels and establish label-switched paths (LSPs).

On each router, enable MPLS and configure LDP as follows:

Step 5: SR-TE Policy Configuration

On each router, configure the SR-TE policy to define the desired path for traffic.

Router 1 (R1):

Router 2 (R2):

Router 3 (R3):

Router 4 (R4):

Step 6: Verify and Monitor

Use network monitoring tools or show commands to verify the SR-TE policy's effectiveness. For example, you can use the following command on each router to display the MPLS traffic engineering paths:

Output and Results:

Upon executing the "show mpls traffic-eng tunnels" command on each router, you should observe the following results

Router 1 (R1):

Router 2 (R2):

Router 3 (R3):

Router 4 (R4):

Packet Movement and Traffic Engineering Flows:

1. Traffic from R1 to R4: When a packet is sent from R1 to R4, it follows the SR-TE policy defined. The packet is forwarded from R1 to R3 and then to R4. The MPLS traffic engineering tunnel (Tunnel 1) established between R1 and R4 ensures the packet's path adheres to the desired traffic engineering constraints.

2. Traffic from R2 to R4: Similarly, when a packet is sent from R2 to R4, it follows the SR-TE policy defined. The packet is forwarded from R2 to R3 and then to R4. The MPLS traffic engineering tunnel (Tunnel 1) established between R2 and R4 ensures the packet's path adheres to the desired traffic engineering constraints.

3. Traffic from R4 to R1: When a packet is sent from R4 to R1, it follows the SR-TE policy defined. The packet is forwarded from R4 to R3 and then to R1. The MPLS traffic engineering tunnel (Tunnel 1) established between R4 and R1 ensures the packet's path adheres to the desired traffic engineering constraints.

Troubleshooting Tips:

1. Verify MPLS and LDP Configurations: Ensure that MPLS and LDP are properly configured on all routers participating in the SR-TE policy. Check the status of MPLS-related interfaces and verify LDP neighbor adjacencies.

2. Check OSPF Link Metrics: Confirm that OSPF link metrics are correctly configured to reflect the desired traffic engineering objectives. Adjust the metrics if necessary to influence the path calculation.

3. Verify SR-TE Policy Configuration: Double-check the SR-TE policy configuration on each router, including the interfaces involved, MPLS traffic-eng tunnel configurations, and explicit paths defined.

Design Considerations:

1. Scalability: Consider the scalability of the SR-TE policy when configuring it for larger networks. As the network grows, ensure that the policy can accommodate increased traffic demands without compromising performance.

2. Redundancy and Resiliency: Design SR-TE policies to incorporate redundancy and resiliency mechanisms, such as link or node protection, to ensure that traffic can be dynamically rerouted in the event of failures.

3. Traffic Engineering Constraints: Clearly define the traffic engineering constraints and objectives to align the SR-TE policy with specific network requirements, such as low latency, bandwidth optimization, or quality of service (QoS) prioritization.


By leveraging the power of SR-TE policy, you can simplify network traffic and optimize resource utilization with ease. This configuration example has showcased the effectiveness of SR-TE in achieving low latency and load balancing, ensuring a smooth and efficient network experience.

Embrace the possibilities of SR-TE policy in your network infrastructure and unlock a new level of performance and control. Simplify, optimize, and elevate your network traffic management with SR-TE policy today!

Stay tuned for our upcoming in-depth blogs on Segment Routing! We'll be diving deeper into advanced concepts, implementation strategies, and real-world use cases. Don't miss out on valuable insights and updates - happy reading!