Devil's Advocate - Segment Routing, Why?

Mark Tinka mark.tinka at
Fri Jun 19 18:28:10 UTC 2020

On 19/Jun/20 17:13, Robert Raszuk wrote:

> So I think Ohta-san's point is about scalability services not flat
> underlay RIB and FIB sizes. Many years ago we had requests to support
> 5M L3VPN routes while underlay was just 500K IPv4.

Ah, if the context, then, was l3vpn scaling, yes, that is a known issue.

Apart from the global table vs. VRF parity concerns I've always had (one
of which was illustrated earlier this week, on this list, with RPKI in a
VRF), the other reason I don't do Internet in a VRF is because it was
always a trade-off:

    - More routes per VRF = fewer VRF's.
    - More VRF's  = fewer routes per VRF.

Going forward, I believe the l3vpn pressures (for pure VPN services, not
Internet in a VRF) should begin to subside as businesses move on-prem
workloads to the cloud, bite into the SD-WAN train, and generally, do
more stuff over the public Internet than via inter-branch WAN links
formerly driven by l3vpn.

Time will tell, but in Africa, bar South Africa, l3vpn's were never a
big thing, mostly because Internet connectivity was best served from one
or two major cities, where most businesses had a branch that warranted

But even in South Africa (as the rest of our African market), 98% of our
business is plain IP. The other 2% is mostly l2vpn. l3vpn's don't really
feature, except for some in-house enterprise VoIP carriage + some
high-speed in-band management.

Even with the older South African operators that made a killing off
l3vpn's, these are falling away as their customers either move to the
cloud and/or accept SD-WAN thingies.

> Last - when I originally discussed just plain MPLS with customers with
> single application of hierarchical routing (no BGP in the core)
> frankly no one was interested. Till L3VPN arrived which was game
> changer and run for new revenue streams ...

The BGP-free core has always sounded like a dark art. More so in the
days when hardware was precious, core routers doubled as inline route
reflectors and the size of the IPv4 DFZ wasn't rapidly exploding like it
is today, and no one was even talking about the IPv6 DFZ.

Might be useful speaking with them again, in 2020 :-).


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