Devil's Advocate - Segment Routing, Why?

Robert Raszuk robert at
Fri Jun 19 18:53:36 UTC 2020

> But, today, people are seems to be using, so called, MPLS, with
explicitly configured flows, administration of which does not
> scale and is annoying.

I am actually not sure what you are talking about here.

The only per flow action in any MPLS deployments I have seen was mapping
flow groups to specific TE-LSPs. In all other TDP or LDP cases flow == IP
destination so it is exact based on the destination reachability. And such
mapping is based on the LDP FEC to IGP (or BGP) match.

Even worse, if route near the destination expected to pop the label
> chain goes down, how can the source knows that the router goes down
> and choose alternative router near the destination?

In normal MPLS the src does not pick the transit paths. Transit is 100%
driven by IGP and if you loose a node local connectivity restoration
techniques (FRR or IGP convergence applies). If egress signalled
implicit NULL it would signal it to any IGP peer.

That is also possible with SR-MPLS too. No change ... no per flow state at
all more then per IP destination routing. If you want to control your
transit hops you can - but this is an option not w requirement.

MPLS with hierarchical routing just does not scale.

While I am not defending MPLS here and 100% agree that IP as transit is a
much better option today and tomorrow I also would like to make sure we
communicate true points. So when you say it does not scale - it could be
good to list what exactly does not scale by providing a real network
operational example.

Many thx,
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