Devil's Advocate - Segment Routing, Why?

Mark Tinka mark.tinka at
Sat Jun 20 09:08:25 UTC 2020

On 19/Jun/20 18:00, Masataka Ohta wrote:
> There seems to be serious confusions between label switching
> with explicit flows and MPLS, which was believed to scale
> without detecting/configuring flows.
> At the time I proposed label switching, there already was RSVP
> but RSVP-TE was proposed long after MPLS was proposed.

RSVP failed to take off, for whatever reason (I can think of many).

I'm not sure any network operator, today, would allow an end-host to
make reservation requests in their core.

Even in the Transport world, this was the whole point of GMPLS. After
they saw how terrible that idea was, it shifted from customers to being
an internal fight between the IP teams and the Transport teams.
Ultimately, I don't think anybody really cared about routers
automatically using GMPLS to reserve and direct the DWDM network.

In our Transport network, we use GMPLS/ASON in the Transport network
only. When the IP team needs capacity, it's a telephone job :-).

> But, today, people are seems to be using, so called, MPLS, with
> explicitly configured flows, administration of which does not
> scale and is annoying.
> Remember that the original point of MPLS was that it should work
> scalably without a lot of configuration, which is not the reality
> recognized by people on this thread.

Well, you get the choice of LDP (low-touch) or RSVP-TE (high-touch).

Pick your poison.

We don't use RSVP-TE because of the issues you describe above.

We use LDP to avoid the issues you describe above.

In the end, SR-MPLS is meant to solve this issue for TE requirements. So
the signaling state-of-the-art improves with time.

> That is certainly a problem. However, worse problem is to know
> label values nested deeply in MPLS label chain.

Why, how, is that a problem? For load balancing?

> 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?

If by source you mean end-host, if the edge router they are connected to
only ran IP and they were single-homed, they'd still go down.

If the end-host were multi-homed to two edge routers, one of them
failing won't cause an outage for the host.

Unless I misunderstand.

> MPLS with hierarchical routing just does not scale.

With Internet in a VRF, I truly agree.

But if you run a simple global BGP table and no VRF's, I don't see an
issue. This is what we do, and our scaling concerns are exactly the same
whether we run plain IP or IP/MPLS.


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