netdev - Re: recursive static routes

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Message-ID: <d99c7e0a-cb3b-6fef-aa39-51e4a2a41e7a@cumulusnetworks.com>
Date:   Mon, 19 Mar 2018 14:11:00 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Saku Ytti <saku@...i.fi>
Cc:     netdev@...r.kernel.org
Subject: Re: recursive static routes

On 3/19/18 12:58 PM, Saku Ytti wrote:
> Hey David,
> 
>> The Linux stack does not flatten routes when inserting into the FIB.
>> Recursion is expected to be done a routing daemon such as bgp which will
>> be able to handle updates as the network changes.
> 
> Are you saying that routing protocol would observe the next-hop
> change, then update the Linux kernel route to reflect that?

yes

> 
> Wouldn't that add another layer of state and the implied delays of
> maintaining and updating the state.
> 
> Is it not practical to do lookup per-packet to recurse until egress
> rewrite information is found? So literally no state in memory anywhere
> saying 0/0 next-hop is 192.0.2.42/22:22:22:22:22:22, it would always
> have to walk the FIB to find it.
> 

you want per-packet overhead instead of deferring the overhead event
based updates? network events tend to be much less frequent than
sending/forwarding packets.