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Category: segment routing

Scalability Aspects of SR-MPLS

Henk Smit left a wonderful comment discussing various scalability aspects of SR-MPLS. Let’s go through the points he made:

When you have a thousand routers in your networks, you can put all of them in one (IS-IS) area. Maybe with 2k routers as well. But when you have several thousand routers, you want to use areas, if only to limit the blast-radius.

Absolutely agree, and as RFC 3439 explained in more eloquent terms than I ever could:

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On Applicability of MPLS Segment Routing (SR-MPLS)

Whenever I compare MPLS-based Segment Routing (SR-MPLS) with it’s distant IPv6-based cousin (SRv6), someone invariably mentions the specter of large label stacks that some hardware cannot handle, for example:

Do you think vendors current supported label max stack might be an issue when trying to route a packet from source using Adj-SIDs on relatively big sized (and meshed) cores? Many seem to be proposing to use SRv6 to overcome this.

I’d dare to guess that more hardware supports MPLS with decent label stacks than SRv6, and if I’ve learned anything from my chats with Laurent Vanbever, it’s that it sometimes takes surprisingly little to push the traffic into the right direction. You do need a controller that can figure out what that little push is and where to apply it though.

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Hub-and-Spoke VPLS: Revenge of LDP

In the Segment Routing vs LDP in Hub-and-Spoke Networks blog post I explained why you could get into interesting scaling issues when running MPLS with LDP in a large hub-and-spoke network, and how you can use Segment Routing (MPLS edition) to simplify your design.

Sample hub-and-spoke network

Sample hub-and-spoke network

Now imagine you’d like to offer VPLS services between hubs and spokes, and happen to be using equipment that uses targeted LDP sessions to signal pseudowires. Guess what happens next…

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Segment Routing vs LDP in Hub-and-Spoke Networks

I got an interesting question that nicely illustrates why Segment Routing (the MPLS variant) is so much better than LDP. Imagine a redundant hub-and-spoke network with hundreds of spokes. Let’s settle on 500 spokes – IS-IS supposedly has no problem dealing with a link-state topology of that size.

Let’s further assume that all routers advertise only their loopbacks1 and that we’re using unnumbered hub-to-spoke links to minimize the routing table size. The global routing table thus contains ~500 entries. MPLS forwarding tables (LFIB) contain approximately as many entries as each router assigns a label to every prefix in the routing table2. What about the LDP table (LIB – Label Information Base)?

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Worth Reading: Do We Need Segment Routing?

Etienne-Victor Depasquale sent me a pointer to an interesting NANOG discussion: why would we need Segment Routing. It’s well worth reading the whole thread (until it devolves into “that is not how MPLS works” arguments), which happens to be somewhat aligned with my thinking:

  • SR-MPLS makes perfect sense (excluding the migration-from-LDP fun)
  • SRv6 (in whatever incantation) is mostly a vendor ploy to sell new chipsets.

Enjoy!

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Segment Routing Segment IDs and MPLS Labels

In one of my introductory Segment Routing videos, I made claims along the lines of “Segment Routing totally simplifies the MPLS control plane, replacing LDP and local labels allocated to various prefixes with globally managed labels advertised in IGP

It took two years for someone to realize the stupidity over-simplification of what I described. Matjaž Strauss sent me this kind summary of my errors:

You’re effectively claiming that SRGB has to be the same across all devices in the network. That’s not true; routers advertise SIDs and must configure label swap operations in case SRGBs don’t match.

Wait, what? What is SRGB and why could it be different across devices in the same network? Also, trust IETF to take a simple idea and complicate it to support vendor whims.

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netsim-tools Release 0.6: BGP, IS-IS, SR-MPLS, FRR

TL&DR: If you want to test BGP, OSPF, IS-IS, or SR-MPLS in a virtual lab, you might build the lab faster with netsim-tools release 0.6.

In the netsim-tools release 0.6 I focused on adding routing protocol functionality:

  • IS-IS on Cisco IOS/IOS XE, Cisco NX-OS, Arista EOS, FRR, and Junos.
  • BGP on the same set of platforms, including support for multiple autonomous systems, EBGP, IBGP full mesh, IBGP with route reflectors, next-hop-self control, and BGP/IGP interaction.
  • Segment Routing with MPLS on Cisco IOS XE and Arista EOS.

You’ll also get:

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Why Is MPLS Segment Routing Better than LDP?

A while ago I made a statement along the lines of “MPLS segment routing is the best thing that happened to MPLS control plane in a decade”. Obviously some MPLS-focused engineers disagree with that and a few years ago I decided to write a lengthy blog post explaining the differences between using MPLS SR with IGP (or BGP) versus more traditional IGP+LDP approach.

Obviously, I wasn’t making any progress on that front, so the only way forward was to record a short video on the topic which didn’t work well either because the end-result was a set of three videos (available with free or paid ipSpace.net subscription).

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Segment Routing Anyone?

One of my readers listened to a podcast where a $vendor described how they found another use case for source routing IPv6 segment routing (SR): 5G networks… and wondered whether SR made a comeback or is about to.

To figure out what segment routing is, watch the webinar we did with Jeff Tantsura a while ago.

I don’t know nearly enough about mobile networks to have an opinion, however…

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