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Fast Convergence and areas

Subject: Fast Convergence and areas
From: Pierre Francois
Date: Fri, 11 Feb 2005 14:13:17 +0100
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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