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

Robert Raszuk robert at raszuk.net
Sun Jun 21 13:35:20 UTC 2020

Let's clarify a few things ...

On Sun, Jun 21, 2020 at 2:39 PM Masataka Ohta <
mohta at necom830.hpcl.titech.ac.jp> wrote:

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?

Neither link wise nor host wise information is required to accomplish say
L3VPN services. Imagine you have three sites which would like to
interconnect each with 1000s of users.

So all you are exchanging as part of VPN overlay is three subnets.

Moreover if you have 1000 PEs and those three sites are attached only to 6
of them - only those 6 PEs will need to learn those routes (Hint: RTC -

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.

Same thing here.

> That is, with hierarchical routing, detailed information
> around destinations is actively hidden from sources.

  Same thing here.

That is why as described we use label stack. Top label is responsible to
get you to the egress PE. Service label sitting behind top label is
responsible to get you  through to the customer site (with or without IP
lookup at egress PE).

> 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.

Usually sources have no idea of MPLS. MPLS to the host never took off.

> 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

I do not think any one saying it here.

> 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.

Label stack is here from day one. Each layer of the stack has a completely
different role. That is your hierarchy.

Kind regards,
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