What are the key differences between MPLS segment routing and other routing protocols?

1 What is MPLS?

MPLS is a technique that uses labels to route packets across a network, instead of relying on the traditional IP routing. Labels are short identifiers that are attached to each packet at the ingress router and removed at the egress router. The labels indicate the path that the packet should follow, without requiring the intermediate routers to look up the destination IP address. This simplifies the routing process and improves the performance and scalability of the network.

2 What is segment routing?

Segment routing is a variant of MPLS that uses segments instead of labels. Segments are network instructions that define how a packet should be forwarded along a specific path. Segments can be either node segments, which identify a specific router, or adjacency segments, which identify a specific link between two routers. Segments are encoded as MPLS labels and stacked in a header that is added to each packet. The segment routing header can be inserted by the source node or by the ingress router, depending on the mode of operation.

3 What is LDP?

LDP is another variant of MPLS that uses a signaling protocol to distribute labels among the routers in the network. LDP establishes sessions between adjacent routers and exchanges label information for each destination prefix. LDP can operate in two modes: downstream unsolicited (DU) and downstream on demand (DoD). In DU mode, a router advertises labels to its neighbors without waiting for a request. In DoD mode, a router only sends labels when requested by a neighbor.

4 How do they differ?

Segment routing and LDP differ in several ways. Segment routing uses existing IP routing protocols like OSPF or IS-IS to distribute segment information, while LDP utilizes a separate protocol. Additionally, segment routing allows the source node or ingress router to specify the path a packet should follow, while LDP relies on a label switching path (LSP) determined by the label distribution process. Furthermore, segment routing reduces the number of labels that must be stored and processed by routers, while LDP requires each router to maintain a label forwarding information base (LFIB) for each destination prefix. Lastly, segment routing provides more granular and dynamic control over traffic flows than LDP's limited traffic engineering capabilities.

5 What are the pros and cons?

Both segment routing and LDP have their own advantages and disadvantages depending on the network requirements and scenarios. Segment routing offers simplicity, flexibility, scalability, and traffic engineering capabilities, eliminating the need for a separate signaling protocol and reducing the label overhead. However, it requires more configuration and coordination among the routers to ensure consistency and validity of the segments, as well as introducing more complexity in the packet header and forwarding plane. On the other hand, LDP provides compatibility and interoperability with existing MPLS networks and devices, as well as more reliability and stability in the label distribution process. Yet, it requires more resources and overhead to maintain the label distribution sessions and LFIB, while offering less control and optimization over the traffic flows than segment routing.