Re: [irs-discuss] IRS comments

[Alia Atlas <[e-mail]>]

Ed,

Sure - it provides the ability for centralized route computation -
whether SPF or other.
But that isn't all an IGP does - and the topological information may
still rely on link-state information from an IGP.   It also doesn't
address convergence concerns.

I think there is more to be considered than just whether a central box
can run an SPF and install the associated routes.

Alia


On Thu, Aug 30, 2012 at 2:58 PM, Edward Crabbe <[e-mail]> wrote:
> Alia;
>
> If there is
>
> a) a mechanism for installing routes, pbr or otherwise, which recurse to
> directly connected nexthops
> b) a mechanism for gathering topological information
>
> then you've inherently enabled centralized spf.
>
> On Thu, Aug 16, 2012 at 9:34 AM, Alia Atlas <[e-mail]> wrote:
>>
>> I haven't seen a good description of what is intended or desired by
>> moving the SPF functionality to a centralized location.  Clearly such
>> centralization can have a very bad impact on convergence - which is a
>> strong motivator for simultaneously computing fast-reroute
>> alternatives (with guaranteed coverage ala MRT) and installing both.
>>
>> I don't see IRS as having a way of "turning off" the SPF computation
>> in the IGP; a different lobotomized IGP protocol/process would be
>> needed.
>>
>> Alia
>>
>> On Thu, Aug 16, 2012 at 12:14 PM, Dutta, Pranjal K (Pranjal)
>> <[e-mail]> wrote:
>> > "LSDB (I saw an email which talks about reducing IGP to link
>> >> distribution protocol and running SPF in centralized network
>> >> controller)"
>> >
>> > I have seen discussions in the past on this and in fact I didn't get
>> > precisely what is meant. If anybody in the list could brief very
>> > precisely that would help a lot.
>> >
>> > Here is my understanding - the routers would do LSA/LSP flooding for
>> > OSPF/ISIS as it is done today. So routers would build neighboring
>> > relationship/adjacencies to participate in flooding and each router builds
>> > its LSDB.
>> >
>> > Then the IRS "application" would track LSDB changes and pull up the
>> > "diffs" from each router (thru "controller") whenever there is a change. The
>> > application would compute SPF on behalf of each router (LSDB). The result of
>> > the compute would be pushed by application to each Router (thru controller)
>> > and inject entries into RIB.
>> >
>> > Is that correct? How different this going to be from PCE?
>> >
>> > If this is correct then perhaps we would like to ask what are the
>> > scalability numbers in LSDB we are talking about?
>> >
>> > The "application" would be running in a high performance server and so
>> > SPF compute there is not an issue and perhaps it is good way to synchronize
>> > FIB update (to a certain extent) to avoid u-loops etc.
>> >
>> > But when we are managing all routers in the purview of the application,
>> > the computing power in each router is not uniform. To be realistic, I have
>> > some concerns on how much "real-timeness" we could achieve between
>> > application and controllers on such proposals, esp. when scaling numbers are
>> > high. This leads to higher time lag on inconsistency between application SPF
>> > compute and FIB update. It's almost like the classic "slow peering" issues
>> > with TCP like protocols - the high performance peer is slowed down by low
>> > performance peer.
>> >
>> > Static route interface is good thing because it is a state that persists
>> > longer.
>> >
>> > IGPs may be deployed in very dynamic environments where tight coupling
>> > is desirable between SPF compute and FIB update. In scaled environments the
>> > issue is less about SPF compute time and is more about synchronizing the
>> > FIB.
>> >
>> > Running on-demand CSPF by IRS application may be fine because of
>> > persistency of RSVP-TE tunnels in dynamic environments.
>> >
>> > I apologize if I misunderstood the intent.
>> >
>> > Thanks,
>> > Pranjal
>>
>> ---snip---
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