Best practice for ptp/loopback numbering for "small" enterprise multihome setup

William Herrin bill at herrin.us
Fri Mar 26 19:01:36 UTC 2021


On Fri, Mar 26, 2021 at 11:07 AM vom513 <vom513 at gmail.com> wrote:
> As I said in the tl;dr - my main point of contention here is breaking up my /24 I.e. use the very top /30s / /31s for ptp/loop.  I would then have at most the bottom /25 to use contig. on my “lan” - and I would need to use the next /26, /27 and so on in some manner for the space to be useable...

If you feel like getting fancy...

Use /32 routes to reclaim the unused base and broadcast address in any /30s

Pick the next largest size block that has your /24 neither at the
start or end and assign that to your lan.

Use proxy arp and more specific routes to grab traffic that doesn't
fall in the /24 or is part of the loopback and point to point
numbering and move it off the lan and towards its destination
(including upstream).

This way, every address in the /24 that you don't specifically use
elsewhere is usable as a unicast address on your LAN segment,
including the .0 and .255 addresses.

e.g.

You have 10.0.1.0/24

Put 10.0.0.0/22 on your LAN
Add proxy arp and route 10.0.0.0/24 upstream
Add proxy arp and route 10.0.2.0/23 upstream
Add proxy arp and route 10.0.1.254/32 to your first router loopback
etc.

If you're really clever you can convince the stations that 10.0.0.1 is
the default gateway but convince the router that 10.0.0.1 is upstream
so that the router doesn't even need a dedicated IP address facing the
LAN.

Regards,
Bill Herrin


-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/


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