Devil's Advocate - Segment Routing, Why?

James Bensley jwbensley+nanog at
Tue Jun 30 18:30:46 UTC 2020

On Wed, 17 Jun 2020 at 22:09, <adamv0025 at> wrote:
> > From: NANOG <nanog-bounces at> On Behalf Of Mark Tinka
> > Sent: Wednesday, June 17, 2020 6:07 PM
> >
> >
> > I've heard a lot about "network programmability", e.t.c.,
> First of all the "SR = network programmability" is BS, SR = MPLS, any programmability we've had for MPLS since ever works the same way for SR.

It works because SR != MPLS.

SR is a protocol which describes many aspects, such as how traffic
forwarding decisions made at the ingress node to a PSN can be
guaranteed across the PSN, even though the nodes along the PSN path
use per-hop forwarding behaviour and different nodes along the path
have made different forwarding decisions.

When using SR MPLS segment IDs are used as an index into the label
range (SRGB) and so SIDs don't correlate 1:1 to MPLS labels, equally
with SRv6 the segment IDs are encoded as IPv6 addresses and don't
correlate 1:1 to an IPv6 address. There is a venn diagram with an
overlapping section in the middle which is "generic SR" with a bunch
of core features that are supported agnostic of the encoding


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