Devil's Advocate - Segment Routing, Why?

Masataka Ohta mohta at
Fri Jun 19 14:45:23 UTC 2020

Mark Tinka wrote:

> MPLS has been around far too long, and if you post web site content
> still talking about it or show up at conferences still talking about it,
> you fear that you can't sell more boxes and line cards on the back of
> "just" broadening carriage pipes.

The problem of MPLS, or label switching in general, is that, though
it was advertised to be topology driven to scale better than flow
driven, it is actually flow driven with poor scalability.

Thus, it is impossible to deploy any technology scalably over MPLS.

MPLS was considered to scale, because it supports nested labels
corresponding to hierarchical, thus, scalable, routing table.

However, to assign nested labels at the source, the source
must know hierarchical routing table at the destination, even
though the source only knows hierarchical routing table at
the source itself.

So, the routing table must be flat, which dose not scale, or
the source must detect flows to somehow request hierarchical
destination routing table on demand, which means MPLS is flow

People, including some data center people, avoiding MPLS, know
network scalability better than those deploying MPLS.

It is true that some performance improvement is possible with
label switching by flow driven ways, if flows are manually
detected. But, it means extra label-switching-capable equipment
and administrative effort to detect flows, neither of which do
not scale and cost a lot.

It cost a lot less to have more plain IP routers than insisting
on having a little fewer MPLS routers.

					Masataka Ohta

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