Re: [irs-discuss] IRS comments

[Alia Atlas <[e-mail]>]

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

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