Here’s an interesting question randomly appearing in my Twitter feed:
If you had a greenfield network, would you choose SR-MPLS, or SRv6? And why?
TL&DR: SR-MPLS, assuming you’re building a network providing end-to-end connectivity between hardware edge devices.
Now for the why part of the question:
One of my readers asked for my opinion about this question…
… and I promised something longer than 280 characters.
Jeff Tantsura left me tantalizing hint after reading the BGP Labeled Unicast on Cisco IOS blog post:
Read carefully “Relationship between SAFI-4 and SAFI-1 Routes” section in RFC 8277
The start of that section doesn’t look promising (and it gets worse):
Now for the details:
netlab release 1.2.0 adds full-blown MPLS and MPLS/VPN support:
- VRF definitions and layer-3 VRFs
- VRF-aware OSPF, IS-IS and BGP
- Traditional MPLS with LDP (SR-MPLS was already available)
- BGP Labeled Unicast
- MPLS/VPN: VPNv4 and VPNv6 address family support
You might want to start with the VRF tutorial to see how simple it is to define VRFs, and follow the installation guide to set up your lab – if you’re semi-fluent in Linux (and don’t care about data plane quirks), the easiest option would be to run Arista cEOS.
While researching the BGP RFCs for the Three Dimensions of BGP Address Family Nerd Knobs, I figured out that the BGP Labeled Unicast (BGP-LU, advertising MPLS labels together with BGP prefixes) uses a different address family. So far so good.
Now for the intricate bit: a BGP router might negotiate IPv4 and IPv4-LU address families with a neighbor. Does that mean that it’s advertising every IPv4 prefix twice, once without a label, and once with a label? Should that be the case, how are those prefixes originated and how are they stored in the BGP table?
As always, the correct answer is “it depends”, this time on the network operating system implementation. This blog post describes Cisco IOS behavior, a follow-up one will focus on Arista EOS.
In 2014 when I did the first prototype implementation of MPLS-SR node labels, I was stunned that just with an incremental add of 500 lines of code to the vanilla IPv4/IPv6 IS-IS codebase I got full any-to-any connectivity, no sync issues, no targeted sessions for R-LFA …. essentially labeled transport comes for free.
Based on that, one has to wonder “why did we take the LDP detour and all the complexity it brings?”. Here’s what Hannes found out:
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.
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…
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)?
I stumbled upon an article praising the beauties of SR-MPLS that claimed:
Yet MPLS, until recently, was deprived of anycast routing. This is because MPLS is not a pure packet switching technology, but has a control plane based on virtual circuit switching.
My first reaction was “that’s not how MPLS works,"1 followed by “that would be fun to test” a few seconds later.
I added two nodes to my lab setup, this time using IOSv as those nodes need nothing more than EBGP support (and IOSv is tiny compared to IOS XE on CSR):
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.
Continuing my archeological explorations, I found a dusty bag of old QoS content:
- Queuing Principles
- QoS Policing
- Traffic Shaping
- Impact of Transmit Ring Size (tx-ring-limit)
- FIFO Queuing
- Fair Queuing in Cisco IOS
I kept digging and turned out a few MPLS, BGP and ADSL nuggets worth saving:
More than a decade ago I published tons of materials on a web site that eventually disappeared into digital nirvana, leaving heaps of broken links on my blog. I decided to clean up those links, and managed to save some of the vanished content from the Internet Archive:
- OSPF Flooding Filters in Hub-and-Spoke Environments
- Implicit and Explicit Null Label in MPLS networks
- Default Routes in BGP
- Filter Excessively Prepended BGP Paths
I also updated dozens of blog posts while pretending to be Indiana Jones, including: