"Tactical" /24 announcements

Jon Lewis jlewis at lewis.org
Thu Aug 12 17:09:21 UTC 2021

On Thu, 12 Aug 2021, Tom Hill wrote:

> On 11/08/2021 14:09, Jon Lewis wrote:
>> What sort of hands-on experience is this opinion based on?
> Having an upstream provider that did it, in a very aggressive fashion.

Odds are, they did it wrong, and you had no control and limited, if any, 
visibility into what they did.  Obviously, if you're going to blindly 
filter routes based on prefix-length, you need to point default at 
something that doesn't...and if you're acting as a transit provider, 
you're likely no longer able to provide "full routes" to customers from 
devices doing this or fed the "not so full table" from devices doing it. 
I can work though, and on pretty much any platform.

> Limiting the pruning to cases with the same next-hop does indeed sound
> like it would be safer than what I've seen done in the past.
> Doing this with per-peer prefix-lists would not (certainly not in
> classic IOS) provide you with the ability to compare the next-hop before
> rejecting those more-specific prefixes, and likely why the attempts to
> do this caused the problems that I'm referring to.
> I'm glad to hear a vendor has implemented a useful knob. Which vendor?

Arista.  They call it FIB compression.  They mention it's a trade-off, 
more memory and CPU utilization (keeping track of things) in exchange for 
being able to keep hardware that might otherwise be out of FIB space able 
to cope with full tables.

  Jon Lewis, MCP :)           |  I route
  StackPath, Sr. Neteng       |  therefore you are
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