Re: [Lsr] WG Adoption Call for "IGP Flexible Algorithms (Flex-Algorithm) In IP Networks" - draft-bonica-lsr-ip-flexalgo-01

Jeff Tantsura <> Mon, 07 December 2020 17:37 UTC

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Date: Mon, 7 Dec 2020 09:37:33 -0800
From: Jeff Tantsura <>
To: "=?utf-8?Q?" <>
Cc: lsr <>, "Acee Lindem (acee)" <>, Tony Li <>, Robert Raszuk <>
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Subject: Re: [Lsr] WG Adoption Call for "IGP Flexible Algorithms (Flex-Algorithm) In IP Networks" - draft-bonica-lsr-ip-flexalgo-01
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please see inline


4. I want to know the path for a specific IP Flex-Algorithm is calculated distributedly by each nodes paticipating this Flex-Algorithm or calculated centralized by an controller? I wonder we can guarantee the loop free  path with IP Flex-Algorithm especially when the path is calculated distributedly?

The valid topology must consist of a set of connected routers sharing a common Calc-Type, then loop-free calculation is done accordingly

Best Regards,
Zhenqiang Li

From: Jeff Tantsura
Date: 2020-12-04 09:18
To: Tony Li; Robert Raszuk
CC: lsr; Acee Lindem \(acee\)
Subject: Re: [Lsr] WG Adoption Call for "IGP Flexible Algorithms (Flex-Algorithm) In IP Networks" - draft-bonica-lsr-ip-flexalgo-01
Anything else than IGP metric based SPT is considered TE. Looking holistically - topology virtualization (or similar) could have been a better name.

On Dec 3, 2020, 4:25 PM -0800, Robert Raszuk <>et>, wrote:
	Hi Tony,

The moment I hit "Send" I knew that this response may be coming as it really depends what is one's definition of TE.

If indeed IGP TE is anything more then SPF - then sure we can call it a TE feature.

However, while a very useful and really cool proposal, my point is to make sure this is not oversold - that's all.


> On Fri, Dec 4, 2020 at 1:13 AM Tony Li <> wrote:
> >
> > Hi Robert,
> >
> >
> > > However I really do not think that what Flexible Algorithm offers can be compared or even called as Traffic Engineering (MPLS or SR).
> > >
> > > Sure Flex Algo can accomplish in a very elegant way with little cost multi topology routing but this is not full TE. It can also direct traffic based on static or dynamic network preferences (link colors, rtt drops etc ... ),  but again it is not taking into account load of the entire network and IMHO has no way of accomplish TE level traffic distribution.
> > >
> > > Just to make sure the message here is proper.
> >
> >
> > It’s absolutely true that FlexAlgo (IP or SR) has limitations. There’s no bandwidth reservation. There’s no dynamic load balancing. No, it’s not a drop in replacement for RSVP. No, it does not supplant SR-TE and a good controller. Etc., etc., etc….
> >
> > However I don’t feel that it’s fair to say that FlexAlgo can’t be called Traffic Engineering.  After all TE is a very broad topic. Everything that we’ve done that’s more sophisticated than simple SPF falls in the area of Traffic Engineering.  Link coloring and SRLG alone clearly fall into that bucket.
> >
> > I’ll grant you that it may not have the right TE features for your application, but that doesn’t mean that it’s not sufficient for some.  Please don’t mislead people by saying that it’s not Traffic Engineering.
> >
> > Regards,
> > Tony
> >
> >
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