January 17, 2019
Simplifying MPLS Transport Networks with Segment Routing
By Said Jilani, Sr. Customer Solutions Architect
MPLS is a mature technology that has been deployed by service providers and private networks for many years. In a time of building pressure for organizations to modernize transport infrastructure to support new applications, MPLS enhances the performance, reliability, and scalability of IP based networks. Successful implementation has demonstrated a reduction of costs associated with maintaining complex expensive legacy TDM based transport network, improved scalability, and higher efficiency.
“Simplicity is prerequisite for reliability”, Edsger W. Dijkstra
MPLS introduces “circuit switched” behavior for IP traffic using the concept of Label Switch Paths (LSPs) that are established in advance to meet certain performance requirements. The primary applications of MPLS are:
- Traffic engineering – allowing for granular control over path selection, traffic priority and fast reroute protection.
- Network virtualization – using VPNs allowing layer 3, layer 2 and legacy TDM services to co-exist on a common network infrastructure.
IP based network rely on the destination IP addresses to forward packets based on the content of routing tables that are populated in advance by the routing protocols (e.g. OSPF, IS-IS or EIGRP). MPLS provided scalability and reliability to IP based transport network by introducing labels that define packet flow paths over pre-signaled LSPs (or tunnels). This brings additional protocols to create and exchange labels to setup LSPs and identify services, referred to as MPLS signaling protocols or control plane protocols. Protocols include:
- LDP and RSVP-TE are used to create and exchange transport labels between MPLS routers.
- Targeted LDP and M-BGP are used to exchange service labels between MPLS edge routers that provide MPLS VPN services to customers and applications.
Take note, MPLS signaling protocols require additional configuration and management effort for the setup, monitoring, and troubleshooting of transport tunnels and services. And these signaling protocols require extra computing resources and can become a limiting factor in sizing and scaling MPLS routers.
MPLS with Segment Routing Benefits
- Building on the success of MPLS technology, segment routing follows the same forwarding paradigm by using labels to direct traffic flows, but it achieves that with simplified control plane protocols. Segment Routing utilizes source-based routing scheme that encodes the traffic paths as MPLS labels into the packet header (called segments). Each segment can identify a node (transit or destination), link or service. Including these labels/segments in the packets removes the burden of establishing transport LSPs in advance. Also, by exchanging these segments via extensions of IGP protocols (which are already running on the IP network), segment routing eliminates the need for transport label signaling protocols (LDP and RSVP-TE). This simplifies the MPLS router configuration and enhances scalability due to the reduced burden on the routers.
- Enhancing MPLS Fast Reroute Capabilities utilizing an advanced Fast Reroute algorithm: Topology Independent Loop-Free Alternate (TI-LFA). Routers identify failures of adjacent links and nodes and perform quick local repair via pre-calculated backup routes. Traditional fast reroute schemes such as IP FRR (LFA or Remote LFA) or MPLS FRR may not provide optimal protection paths in certain topologies or scale due to a large number of RSVP-TE or LDP tunnels. Segment Routing TI-LFA calculates optimal post convergence backup routes that are readily available upon failure detection. By using segment labels to reach selected transit nodes along the backup routes, segment routing eliminates the need for LDP or RSVP-TE tunnels further simplifying MPLS configuration and enhancing network router scalability.
- Simplifying the Network Design and Operation of MPLS based network by consolidating signaling protocols and keeping the service and path setup at the edge of the network. Without the need to track transit tunnels, MPLS core routers can scale to support a larger number of paths and services. Additionally, keeping the network and services setup at the at the edge of the network allows for more efficient programming of traffic engineered paths. This can be accomplished via a centralized application (or network controller) using Path Computation Element (PCE) server, and this approach is aligned with the network centralization and automation objectives of Software Defined Networking (SDN).
Network transport infrastructure simplified leading to scalability, superior protection capabilities, coupled with compatibility with MPLS forwarding methods is a key tenant for the design of mission-critical networks. In conclusion, Segment Routing represents an evolution of IP/MPLS network architecture. The evolution embodying “Simplicity is prerequisite for reliability,” an adage by Edsger Dijkstra.
By Said Jilani, Sr. Customer Solutions Architect
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