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