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Date:   Tue, 10 Mar 2020 10:19:24 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     David Ahern <dsahern@...il.com>, Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        davem <davem@...emloft.net>, mmhatre@...hat.com,
        "alexander.h.duyck@...el.com" <alexander.h.duyck@...el.com>
Subject: Re: route: an issue caused by local and main table's merge

On Tue, Mar 10, 2020 at 9:01 AM Guillaume Nault <gnault@...hat.com> wrote:
>
> On Tue, Mar 10, 2020 at 04:56:32PM +0100, Guillaume Nault wrote:
> > On Mon, Mar 09, 2020 at 08:53:53AM -0700, Alexander Duyck wrote:
> > > Also, is it really a valid configuration to have the same address
> > > configured as both a broadcast and unicast address? I couldn't find
> > > anything that said it wasn't, but at the same time I haven't found
> > > anything saying it is an acceptable practice to configure an IP
> > > address as both a broadcast and unicast destination. Everything I saw
> > > seemed to imply that a subnet should be at least a /30 to guarantee a
> > > pair of IPs and support for broadcast addresses with all 1's and 0 for
> > > the host identifier. As such 192.168.122.1 would never really be a
> > > valid broadcast address since it implies a /31 subnet mask.
> > >
> > RFC 3031 explicitly allows /31 subnets for point to point links.
> That RFC 3021, sorry :/
>

So from what I can tell the configuration as provided doesn't apply to
RFC 3021. Specifically RFC 3021 calls out that you are not supposed to
use the { <network-prefix>, -1 } which is what is being done here. In
addition the prefix is technically a /24 as configured here since a
prefix length wasn't specified so it defaults to a class C.

Looking over the Linux kernel code it normally doesn't add such a
broadcast if using a /31 address:
https://elixir.bootlin.com/linux/v5.6-rc5/source/net/ipv4/fib_frontend.c#L1122

- Alex

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