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Re: Fast Convergence and areas

Subject: Re: Fast Convergence and areas
From: Alex Zinin
Date: Fri, 11 Feb 2005 16:24:20 -0800

  As you can see from my previous message, I do think we should leave the
  transit areas case for later.

  Areas in general, however, are much more real (though still used less often in
  ISP deployments than in enterprise ones), and I'd like us to have a good
  understanding of how IPFRR will work in those scenarios.


Friday, February 11, 2005, 5:13:17 AM, Pierre Francois wrote:
> Hi, 

> Areas in a link-state IGP turn it into a mix of distance-vector
> and link-state protocols. This makes a simple link event in one
> area look like a SRLG event in the others, as the rerouting routers
> inside an area will need to receive all the updated summary-LSAs of
> their Area Border Routers to be allowed to perform
> their final FIB Updates. Moreover, a single link event in an area A can
> lead to transient FIB updates in an area B to finally return to the same
> fowarding states in B as before the event. It's impossible to have those
> transient unecessary FIB updates/loops without area.

> Moreover, it puts some restrictions on the possible paths between a
> source and a destination, according to the network topology, as a S-D
> path cannot leave and re-enter a given area although this path could be
> the optimal one.

> Now that we are convinced that the SPF recomputation time is not 
> the critical time-component influencing the convergence of an IGP, why
> don't we reconsider the usage of mutiple areas inside a network where
> the convergence time is considered important ? 

> However, OSPF convergence with Stub areas shoudln't be a problem, as
> we only have to ensure that packets are consistently forwarded outside
> the stub and that forwarding inside area 0 is also consistent to reach
> the area Border Routers for the destinations that are in stubs. 

> Transit areas make the problem similar to the convergence of BGP between
> ASes, except that routers don't have complete paths but only distance
> information. Aren't the benefits of IGP areas historical ?

> Pierre.

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