why am i in this handbasket? (was Devil's Advocate - Segment Routing, Why?)

Mark Tinka mark.tinka at seacom.mu
Sun Jun 21 15:50:45 UTC 2020

On 21/Jun/20 14:36, Masataka Ohta wrote:

> That is a tragedy.


> If all the link-wise (or, worse, host-wise) information of possible
> destinations is distributed in advance to all the possible sources,
> it is not hierarchical but flat (host) routing, which scales poorly.
> Right?

Host NLRI is summarized in iBGP within the domain, and eBGP outside the

It's no longer novel to distribute end-user NLRI in the IGP. If folk are
still doing that, I can't feel sympathy for the pain they may experience.

> Why, do you think, flat routing does not but hierarchical
> routing does scale?
> It is because detailed information to reach destinations
> below certain level is advertised not globally but only for
> small part of the network around the destinations.
> That is, with hierarchical routing, detailed information
> around destinations is actively hidden from sources.
> So, with hierarchical routing, routing protocols can
> carry only rough information around destinations, from
> which, source side can not construct detailed (often
> purposelessly nested) labels required for MPLS.

But hosts often point default to a clever router.

That clever router could also either point default to the provider, or
carry a full BGP table from the provider.

Neither the host nor their first-hop gateway need to be MPLS-aware.

There are use-cases where a customer CPE can be MPLS-aware, but I'd say
that in nearly 99.999% of all cases, CPE are never MPLS-aware.

> According to your theory to ignore routing traffic, we can be happy
> with global *host* routing table with 4G entries for IPv4 and a lot
> lot lot more than that for IPv6. CIDR should be unnecessary
> complication to the Internet

Not sure what Internet you're running, but I, generally, accept
aggregate IPv4 and IPv6 BGP routes from other AS's. I don't need to know
every /32 or /128 host that sits behind them.

> With nested labels, you don't need so much labels at certain nesting
> level, which was the point of Yakov, which does not mean you don't
> need so much information to create entire nested labels at or near
> the sources.

I don't know what Yakov advertised back in the day, but looking at what
I and a ton of others are running in practice, in the real world, today,
I don't see what you're talking about.

Again, if you can identify an actual scenario today, in a live, large
scale (or even small scale) network, I'd like to know.

I'm talking about what's in practice, not theory.

> The problem is that we can't afford traffic (and associated processing
> by all the related routers or things like those) and storage (at or
> near source) for routing (or MPLS, SR* or whatever) with such detailed
> routing at the destinations.

Again, I disagree as I mentioned earlier, because you won't be able to
buy a router today that does only IP any cheaper than it does both IP
and MPLS.

MPLS has become mainstream, that its economies of scale have made the
consideration between it and IP a non-starter. Heck, you can even do it
in Linux...


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