Section Routing: SRv6 and Community Programming [Part 2]

Within the first weblog of this 2-part collection on section routing fundamentals, we checked out section routing’s extra summary definition and its implementation utilizing MPLS. On the finish, we famous that each Layer 3 and Layer 2 providers can profit from section routing and defined how. In case you’re solely simply becoming a member of us now, be at liberty to pause and compensate for Half 1 right here.

Able to dive into Half 2? Let’s get again to section routing itself!

To know how SRv6 operates, we should first take a look at its distinctive naming conventions and the logic that permits the community to perform as a programmable entity.

The logic of SRv6: community programming

SRv6 has its personal distinctive conference for naming segments.

The IGP prefix section was initially known as “Finish,” and the adjacency section was known as “Finish.X.” Different segments extra related (in apply) to particular conditions involving interworking and migration have been named “Finish.B6.Encaps.Purple” or “Finish.B6.BM” in SRv6.

The time period the router makes use of on the community ingress is “H.Encaps” or “H.Encaps.Purple.” Someplace in between, one thing like “H.Insert” or “H.Insert.Purple” may happen.

Does this dot-notation-based syntax appear acquainted? In case you’re reminded of object-oriented programming languages like Python or Java, you’re not flawed. So, how did this come to be a part of Section Routing?

From a Section Routing perspective, a section’s SID encodes an instruction the community should execute. The community thus behaves very like a program whose directions are processed by a pc.

Within the context of Section Routing, folks usually consult with the “programmable community” or “community programming.” The latter time period even made it into the title of an RFC that’s central to SRv6: “Section Routing over IPv6 (SRv6) Community Programming” (RFC 8986). This explains the naming conference for SRv6, which can appear unusual at first.

In SRv6, some segments use a SID with an argument that’s processed when the instruction runs. That is the case, for instance, when utilizing SRv6 as the info airplane for EVPN providers.

Anatomy of an SRv6 SID

To wrap up this dialogue of Section Routing, let’s look behind the scenes of SRv6.

In SR MPLS, it’s straightforward to duplicate the basic idea of section routing utilizing labels as SIDs and a label stack as a SID listing. However how does this work with IPv6 because the platform for Section Routing? Right here, specifically formatted IPv6 addresses function SIDs.

Usually, an IPv6 deal with house (referred to as an SR block) is reserved for this objective, often within the vary of /33, /40, or /48. That is adopted by a Node ID, sometimes 16 bits lengthy, then a 16-bit Operate discipline, an Argument, and a few padding.

From Locator to execution

Collectively, the SR block and Node ID kind the Locator. It doesn’t simply seem out of skinny air, and it could’t be routinely generated by good SRv6 routers (RFCs name this the “SRv6-enabled node”).

As a substitute, it should be configured by the administrator. The remainder of the SID is then routinely generated relying on the router’s process. The SRv6 router publishes the locator within the IGP (OSPF or IS-IS), so that each one SIDs fashioned from it are reachable all through the community through IPv6 routing.

The “Operate” portion, together with the “Argument,” encodes the instruction to be executed on the SRv6 router.

The directions should be executed on an SRv6 router primarily based on IPv6 routing. To do that, the SRv6 router creates an entry within the IPv6 routing desk for every SID generated from the locator, together with a notice specifying the instruction to be executed.

This kills two birds with one stone: transport to the SRv6 router and processing on that router.

In different phrases: what precisely must be accomplished with the site visitors on the section endpoint? Some concepts: nothing in any respect, simply forwarding to the subsequent section; forwarding to a selected hyperlink; handing it off to an L3 or L2 context for additional processing; forwarding through a site visitors engineering path; and so forth. The creativeness has practically no limits right here.

How the SID listing works

So how does the SID listing work? Since solely a single SID suits into the IPv6 header, the Section Routing header is included with the packets within the encapsulation. It comprises an listed listing of SIDs.

Sadly, this isn’t precisely intuitive. Since 0 is the index of the final section, the primary section is definitely on the finish of your complete listing.

The “Segments Left” discipline factors to the SID of the present section, which can be discovered because the IPv6 vacation spot deal with within the IPv6 header. Upon arrival on the section endpoint, “Segments Left” is shifted to the subsequent index, and this SID is then adopted because the IPv6 vacation spot deal with; a fast take a look at the IPv6 routing desk, and off it goes.

Overhead and the function of SDN

In fact, this provides plenty of overhead. Individuals have, naturally, complained vehemently. Later enhancements launched Micro SIDs, which take up considerably much less house. (I can talk about that together with Software program-Outlined Networking (SDN) in one other weblog.)

The excellent news is that you simply solely want SDN within the context of site visitors engineering. So in a section routing community with out site visitors engineering, there’s no want for SDN! Even in a community with site visitors engineering, it isn’t strictly obligatory. The community can deal with many site visitors engineering duties all by itself!

Later, I can talk about the place it falls brief in some duties, akin to:

• Interdomain site visitors engineering
• Disjoint path
• Bandwidth on demand
• Bidirectional co-routed path

Section routing within the Cisco ecosystem

Cisco has supported the event of Section Routing from the very starting with artistic vitality, having performed a number one function in standardization efforts. Section Routing is now totally mature at Cisco and a regular characteristic of the service supplier product line primarily based on IOS XR (Cisco 8000 Sequence Routers, NCS, and so forth.).

As a Cisco Platinum Studying Companion, we at ExperTeach are delighted to accompany Cisco on its Section Routing journey and information knowledge-hungry people by the internal workings of this thrilling, ever-evolving expertise by our coaching choices.