10 Segment Routing Interview Questions and Answers

Segment Routing is a modern network routing paradigm that simplifies traffic engineering and network operations. By encoding the path that data packets should take through the network directly into the packet headers, Segment Routing eliminates the need for complex signaling protocols and allows for more flexible and scalable network designs. This approach is particularly beneficial in large-scale networks, where traditional routing methods can become cumbersome and inefficient.

This article offers a curated selection of Segment Routing interview questions designed to test your understanding and proficiency in this innovative technology. Reviewing these questions will help you gain confidence and demonstrate your expertise in network routing concepts during your interview.

Segment Routing Interview Questions and Answers

1. Describe the difference between SR-MPLS and SRv6.

Segment Routing (SR) is a modern approach to source routing, where the source node defines the path that the packet should take through the network. There are two main types of Segment Routing: SR-MPLS and SRv6.

SR-MPLS:

• SR-MPLS uses MPLS (Multiprotocol Label Switching) labels to encode the path.
• It leverages the existing MPLS infrastructure, making it easier to deploy in networks that already use MPLS.
• The path is defined by a stack of labels, where each label represents a segment of the path.
• SR-MPLS is widely used in service provider networks due to its compatibility with existing MPLS-based services.

SRv6:

• SRv6 uses IPv6 addresses to encode the path.
• It leverages the IPv6 protocol, which provides a larger address space and more flexibility.
• The path is defined by a list of IPv6 addresses, where each address represents a segment of the path.
• SRv6 is more suitable for networks that are transitioning to or already using IPv6, offering better scalability and programmability.

2. What is a Segment Identifier (SID) and what are its types?

A Segment Identifier (SID) is a unique identifier used in Segment Routing (SR) to represent a specific instruction or path in a network. Segment Routing is a source routing paradigm that simplifies traffic engineering and network operations by encoding paths as sequences of SIDs. These SIDs can be used to direct packets through specific paths or to apply specific network functions.

There are several types of SIDs in Segment Routing:

• Node SID: Represents a specific node in the network. It is typically used to direct packets to a particular router.
• Adjacency SID: Represents a specific link between two nodes. It is used to direct packets over a particular link.
• Prefix SID: Represents a specific IP prefix. It is used to direct packets to a particular destination prefix.
• Anycast SID: Represents a group of nodes that share the same SID. It is used to direct packets to the nearest node in the group.
• Binding SID: Represents a binding to a specific segment list or policy. It is used to simplify the encoding of complex paths.

3. Explain the concept of Traffic Engineering in the context of Segment Routing.

Traffic Engineering in Segment Routing allows network operators to control the path that data packets take through the network. Unlike traditional IP routing, which relies on shortest path algorithms, Segment Routing enables more flexible and efficient routing by using a list of segments. These segments can represent topological or service-based instructions, allowing for precise control over the packet’s journey.

In Segment Routing, each segment is identified by a Segment Identifier (SID). These SIDs can be stacked to form a Segment List, which dictates the path a packet should follow. This approach provides several advantages:

• Scalability: Segment Routing reduces the state information that needs to be maintained in the network, as the path information is carried within the packet itself.
• Flexibility: Operators can easily implement traffic engineering policies by defining specific segment lists for different types of traffic.
• Efficiency: By optimizing the path that packets take, Segment Routing can improve network resource utilization and reduce congestion.

Traffic Engineering in Segment Routing can be implemented using two main approaches:

• Explicit Path: The operator defines a specific path for the traffic by specifying a sequence of SIDs. This method provides fine-grained control over the routing path.
• Constraint-Based Path: The operator specifies certain constraints (e.g., bandwidth, latency) and the network dynamically calculates the optimal path that meets these constraints.

4. Describe the role of the Path Computation Element (PCE) in Segment Routing.

The Path Computation Element (PCE) is a network component responsible for determining the optimal path for data packets through a network. In the context of Segment Routing (SR), the PCE plays a role in computing efficient and scalable paths based on various constraints and policies.

Segment Routing simplifies the forwarding process by encoding the path information within the packet header as a sequence of segments. These segments can represent topological or service-based instructions. The PCE leverages its global view of the network to compute these paths, taking into account factors such as bandwidth requirements, latency, and administrative policies.

The PCE communicates with network devices using the Path Computation Element Communication Protocol (PCEP). This protocol allows the PCE to receive topology information and constraints from the network, compute the optimal path, and then communicate the computed path back to the network devices. This interaction ensures that the network can dynamically adapt to changing conditions and requirements, providing efficient and reliable data forwarding.

5. Explain the concept of Binding SID (BSID) and its use cases.

Binding SID (BSID) is a concept in Segment Routing (SR) that allows for the creation of a binding between a specific segment list and a single Segment Identifier (SID). In essence, BSID is used to represent a pre-defined path through the network, which can be referenced by a single SID. This simplifies the management and configuration of complex network paths.

In Segment Routing, a SID is a unique identifier that represents a specific instruction or a segment in the network. By using BSID, network operators can encapsulate a sequence of SIDs (a segment list) into a single SID. This encapsulation allows for more efficient and scalable network operations, as it reduces the complexity of managing multiple SIDs for a given path.

Use cases for BSID include:

• Traffic Engineering: BSID can be used to define and enforce specific traffic engineering policies. By binding a segment list to a BSID, network operators can ensure that traffic follows a pre-determined path, optimizing network resource utilization and improving performance.
• Service Chaining: In service chaining, traffic is directed through a series of network services (e.g., firewalls, load balancers). BSID can simplify the configuration of these service chains by representing the entire chain with a single SID.
• Fast Reroute (FRR): BSID can be used to quickly reroute traffic in case of a network failure. By pre-defining alternate paths and binding them to BSIDs, the network can rapidly switch to an alternate path, minimizing downtime and ensuring high availability.

6. Explain the concept of Segment Routing Global Block (SRGB).

Segment Routing Global Block (SRGB) is a range of Segment Identifiers (SIDs) that are reserved for global use within a segment routing domain. Segment routing is a network routing paradigm that simplifies traffic engineering and network operations by encoding the path that packets should take through the network directly into the packet headers.

The SRGB is a component of segment routing as it ensures that SIDs are consistent and unique across the entire network. This consistency allows for seamless interoperability and simplifies the management of SIDs. The SRGB is typically defined by a range of values, for example, 16000-23999, and these values are used to represent different network segments or functions.

In segment routing, there are two types of SIDs:

• Node SIDs: These represent specific nodes in the network.
• Adjacency SIDs: These represent specific links between nodes.

The SRGB ensures that these SIDs are globally unique, which is essential for the correct functioning of the segment routing protocol. When a packet is forwarded through the network, the SIDs in the packet header are used to determine the path the packet will take. By using a globally consistent SRGB, network operators can ensure that packets are routed correctly and efficiently.

7. What are the differences between Node SID, Adjacency SID, and Anycast SID?

In Segment Routing, SIDs (Segment Identifiers) are used to define the path that packets should take through the network. There are different types of SIDs, each serving a unique purpose:

• Node SID (Segment Identifier): This is a global identifier that represents a specific node in the network. It is unique within the Segment Routing domain and is used to route packets to a particular node. Node SIDs are typically used for shortest path routing to a node.
• Adjacency SID (Segment Identifier): This is a local identifier that represents a specific adjacency, or link, between two nodes. It is used to route packets over a specific link, regardless of the shortest path. Adjacency SIDs are useful for traffic engineering and ensuring that packets follow a particular path through the network.
• Anycast SID (Segment Identifier): This is a global identifier that represents a set of nodes, typically providing the same service. Packets routed to an Anycast SID will be delivered to the nearest node in the set, based on the network topology. Anycast SIDs are useful for load balancing and redundancy, as they allow traffic to be distributed across multiple nodes.

8. How does Segment Routing handle ECMP (Equal-Cost Multi-Path)?

Segment Routing handles ECMP by leveraging its inherent flexibility in defining paths through the network. When multiple equal-cost paths are available, Segment Routing can distribute traffic across these paths to achieve load balancing. This is done by encoding multiple segments in the packet header, allowing the packet to traverse different paths to the destination.

In Segment Routing, each segment represents a specific instruction, such as forwarding the packet to a particular node or through a specific interface. When ECMP is in play, the source node can encode multiple segments that correspond to different equal-cost paths. The network devices then use these segments to forward the packet along one of the available paths, effectively balancing the load.

The key advantage of Segment Routing in handling ECMP is its ability to provide fine-grained control over the path selection process. This allows for more efficient use of network resources and better traffic engineering capabilities.

9. Describe the process of SRv6 header insertion and processing.

Segment Routing over IPv6 (SRv6) is a network protocol that leverages the IPv6 address space to encode the path that packets should follow through the network. The SRv6 header, also known as the Segment Routing Header (SRH), is inserted into the IPv6 packet to define the sequence of segments (or waypoints) that the packet should traverse.

The process of SRv6 header insertion involves the following steps:

• Packet Creation: The source node creates an IPv6 packet and determines the path it should take through the network.
• SRH Construction: The source node constructs the SRH, which includes a list of segment identifiers (SIDs). Each SID represents a specific instruction or waypoint in the network.
• Header Insertion: The SRH is inserted into the IPv6 packet between the IPv6 header and the payload. The SRH contains the list of SIDs, the current active SID, and other relevant information.
• Packet Transmission: The packet is transmitted to the first segment (waypoint) in the list.

The process of SRv6 header processing involves the following steps:

• SID Processing: When a packet arrives at a node, the node examines the active SID in the SRH. The node performs the action specified by the SID, which could be forwarding the packet to the next segment, applying a specific policy, or any other instruction.
• Active SID Update: After processing the current SID, the node updates the SRH to point to the next active SID in the list.
• Packet Forwarding: The packet is then forwarded to the next segment in the list, and the process repeats until the packet reaches its final destination.

10. Explain the concept of SR Policy and its components.

Segment Routing (SR) Policy is a mechanism used in network engineering to define and enforce specific paths for traffic through a network. It is part of the broader Segment Routing architecture, which simplifies traffic engineering by encoding paths as sequences of segments. An SR Policy consists of several key components:

• Candidate Paths: These are the potential paths that can be used to route traffic. Each candidate path is a sequence of segments that define a specific route through the network.
• Segment List: This is an ordered list of segments that make up a candidate path. Each segment represents a specific instruction, such as forwarding the packet to a particular node or through a specific interface.
• Preference: This attribute is used to rank candidate paths. The path with the highest preference is selected for routing traffic.
• Binding SID (Segment Identifier): This is an identifier that binds the SR Policy to a specific segment. It allows the policy to be referenced and applied to traffic flows.
• Policy State: This indicates whether the SR Policy is active or inactive. An active policy is used for routing traffic, while an inactive policy is not.