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

Masataka Ohta mohta at necom830.hpcl.titech.ac.jp
Mon Jun 22 12:49:24 UTC 2020

Robert Raszuk wrote:

> 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.

For a single customer of an ISP with 1000s of end users. OK.

But, it should be noted that a single class B routing table entry
often serves for an organization with 10000s of users, which is
at least our case here at titech.ac.jp.

It should also be noted that, my concern is scalability in ISP side.

> 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 -
> RFC4684)

If you have 1000 PEs, you should be serving for somewhere around 1000

And, if I understand BGP-MP correctly, all the routing information of
all the customers is flooded by BGP-MP in the ISP.

Then, it should be a lot better to let customer edges encapsulate
L2 or L3 over IP, with which, routing information within customers
is exchanged by customer provided VPN without requiring extra
overhead of maintaining customer local routing information by the

If a customer want customer-specific SLA, it can be described
as SLA between customer edge routers, for which, intra-ISP MPLS
may or may not be used.

For the ISP, it can be as profitable as PE-based VRF solutions,
because customers so relying on ISPs will let the ISP provide
and maintain customer edges.

The only difference should be on profitability for router makers,
which want to make routing system as complex as possible or even
a lot more than that to make backbone routers a lot profitable

>> 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.

Label stack was there, because of, now recognized to be wrong,
statement of Yakov on day one and I can see no reason still to
keep it.

							Masataka Ohta

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