Friday's Random Comment - About: Arista and FIB/RIB's

Laszlo Hanyecz laszlo at
Fri Apr 29 13:05:56 UTC 2016

On 2016-04-29 12:48, Nick Hilliard wrote:
> Alain Hebert wrote:
>>      PS: "Superfluous" is a nice way to say that the best path of a
>> subnet is the same as his supernet.
> ... from the point of view of the paths that you see, which is to say
> two egress paths.  Someone else on the internet may have a different set
> of bgp views which will give a different set of results for the bgp
> decision process.  The more paths you receive from different sources,
> the more likely it is that this list of 120k "superfluous" prefixes will
> converge towards zero.
> You're right that it's often not necessary to accept all paths, and your
> fib view can optimised in a way that your rib shouldn't be.  All these
> things can be used to drop the forwarding lookup engine resource
> requirements, although it is important to understand that there is no
> such thing as a free lunch and if you do this, there might well be edge
> cases which could cause your optimisation to fail and things to blow up
> horribly in your face.  Still, it's an interesting thing to examine.
> Nick

What Nick said is basically what I was asking about in the Arista 
thread.  Are there new edge cases and new failure modes that are 
introduced by this strategy?  It seems like you'd have to recompute the 
minimal set of forwarding rules each time a prefix is added or removed, 
and a single update may cause you to have to do many adds/removes to 
bring your compressed rules into sync, like when a hole is punched in an 
aggregated prefix.

I'm curious about specific failure modes that can result from this, if 
anyone can share examples/experience with it.


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