{"id":17439,"date":"2026-10-15T17:54:32","date_gmt":"2026-10-15T17:54:32","guid":{"rendered":"https:\/\/dmsretail.com\/RetailNews\/segment-routing-fundamentals-from-mpls-to-srv6\/"},"modified":"2026-10-15T17:54:32","modified_gmt":"2026-10-15T17:54:32","slug":"segment-routing-fundamentals-from-mpls-to-srv6","status":"publish","type":"post","link":"https:\/\/dmsretail.com\/RetailNews\/segment-routing-fundamentals-from-mpls-to-srv6\/","title":{"rendered":"Segment Routing Fundamentals: From MPLS to SRv6"},"content":{"rendered":"

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Understanding the basics of segment routing<\/h2>\n

This blog aims to take you on a journey through some fundamental concepts of Segment Routing, highlighting the most important features and looking behind the scenes at the technical implementation.<\/p>\n

How segment routing works<\/h3>\n

You might ask, \u201cWhat on earth is a segment<\/em>?\u201d<\/p>\n

The answer is simple; a segment is a path in a network from one node to another. To distinguish between multiple segments, each segment is assigned a segment ID (SID).<\/p>\n

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If this sounds a bit abstract, remember that Segment Routing is a packet-switching concept<\/em> whose fundamental principle is to route packets through the network via a sequence of segments. The packets thus include a list of SIDs in the correct order, and once a packet has completed a segment, the SID is removed from the list.<\/p>\n

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If you\u2019re wondering what type of network we\u2019re talking about and where such a segment runs, don\u2019t worry! Industry standards provide precise definitions of segments that clarify this. Read on to learn more about the format of the segment ID and implementation of the SID list.<\/p>\n

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Comparing data planes: SR MPLS vs. SRv6<\/h2>\n

Historically, segment routing was implemented over MPLS networks, where label-based operation seemed ideal for meeting segment routing requirements. Now, these SR MPLS labels are used as SIDs. The SID list attaches to the packets as a label stack. It\u2019s that easy with MPLS!<\/p>\n

It was later clear that IPv6 networks could also implement Segment Routing. In this case, specially formatted IPv6 addresses serve as SIDs, and the SID list is included with packets in what was called the Routing Extension Header but was promptly renamed the Segment Routing Header. The packet\u2019s current SID serves as the IPv6 destination address.<\/p>\n

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Why SRv6 is gaining traction<\/h3>\n

Eventually, the IPv6-based implementation was simply dubbed \u201cSRv6,\u201d since a comparison between the two showed that SRv6 requirements were less demanding.<\/p>\n

You only need IPv6 routing, which even simple routers and switches can handle. SR MPLS, on the other hand, requires end-to-end label switching, supported only by carrier-grade routers, which are not cheap but offer high performance.<\/p>\n

The bottom line\u2014SRv6\u2019s lower requirements enable larger network construction than SR MPLS networks allow. So it\u2019s now easier to set up end-to-end services that use an SRv6 network as a data plane, resulting in a noticeable trend toward IPv6. Now we must face the unpopular topic of migration. But more on that later!<\/p>\n

Advanced traffic engineering with segment routing<\/h2>\n

From my experience teaching many Segment Routing courses, I<\/span>\u2019ve found that participants often initially see it as mysterious<\/span>\u2014even a little magical<\/span>\u2014how the network decides, at the entry point, which segments a packet should follow and which SID list to attach.<\/span><\/p>\n

Many people think some new kind of intelligence\u2014maybe even artificial intelligence\u2014has been added to the network.<\/p>\n

What is actually being used here is a tried-and-true method that\u2019s been available since the beginning of MPLS networking: traffic engineering, which is now used within the context of Segment Routing\u2019s data-forwarding concept.<\/p>\n

It works in two steps. First, the network calculates the best path to a destination based on limits and optimization goals. Second, it attaches a matching SID list to the packet to guide it precisely along this calculated path through the network.<\/p>\n

You\u2019ll rightly hear in many Cisco presentations, \u201cThe path is encoded in the packet!\u201d That path gives traffic engineering a major advantage over the traditional approach using RSVP-TE, which leaves a lot of state in the network (data that routers must store). So they scale poorly.<\/p>\n

Traffic engineering with Segment Routing requires no additional state in the network because the state is carried in the packet as a SID list. So are we witnessing a renaissance of traffic engineering with Segment Routing? Not necessarily; there\u2019s still administrative overhead to manage.<\/p>\n

When push comes to shove, such an SID list for traffic engineering can also be created manually. However, the administrator must really know their network (part of the administrative effort needed in traffic engineering)!<\/p>\n

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Implementing topology independent loop-free alternate (TI-LFA)<\/h3>\n

Amid all this enthusiasm for Segment Routing, we mustn\u2019t overlook another key feature: Topology Independent Loop-Free Alternate (TI-LFA). This feature also benefits from Segment Routing\u2019s stateless approach: it uses a list of SIDs to route packets along a loop-free alternate path that is calculated in advance.<\/p>\n

What makes it special is that, with Segment Routing, it works in any topology\u2014hence the term \u201ctopology independent.\u201d<\/p>\n

This capability existed even before Segment Routing, based on classic MPLS protocols or simply basic routing. However, those solutions were not topology-independent. In fact, TI-LFA has become best practice in production Segment Routing networks.<\/p>\n

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Both Traffic Engineering and TI-LFA attach a SID list to packets to route them through a specific sequence of segments.<\/p>\n

Key components of the SR architecture<\/h2>\n

This brings us back to a question that needs an answer: How exactly is a segment defined, and who or what determines its path through the network?<\/p>\n

You\u2019ll notice that the mentioned use cases for Segment Routing\u2014traffic engineering and TI-LFA\u2014were defined independently of the specific implementation (i.e., SR MPLS or SRv6), which means they are possible with both.<\/p>\n

The same applies to the two most important segment definitions:<\/p>\n