Re: [v6ops] [EXTERNAL] Re: Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)

Gert Doering <> Tue, 16 February 2021 17:02 UTC

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Date: Tue, 16 Feb 2021 18:02:13 +0100
From: Gert Doering <>
To: Philip Homburg <>
Cc:, Gert Doering <>, "" <>
Subject: Re: [v6ops] [EXTERNAL] Re: Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)
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On Tue, Feb 16, 2021 at 05:34:38PM +0100, Philip Homburg wrote:
> >Linux, for example, will just pick one default gateway, usually "the one
> >from the RA received last".
> I think Linux also honours the priority of the default router. 

It does, but that does not help much in the scenario "Router A connects
to ISP A, announces prefix from A, Router B connects to ISP B, and
announces prefix from B, and packets sourced from A or B needs to go
to the respective router".

> Playing
> with default router priorities and the preference value of slaac prefixes
> allows routers to steer traffic.
> Of course that requires routers to coordinate... 

... and solves a different scenario ("primary ISP, backup ISP")...

The true greatness of dual-prefix multihoming, in my opinion, is 
"hosts get to decide which ISP to use", so a given host can decide
"I want to use ISP A for my web traffic, and ISP B for my voice traffic"
(by picking the corresponding source address).

Of course this is not there yet, because there is no guidance *how* to 
pick a proper source address in that scenario (and that leads to the
other problem of no guidance how to deal with "I picked GUA source A,
and can't reach my destination, so should I try GUA source B next?").

... and if we can't have the nice things, just using GUA + NPT66 on
the router will achieve the basic "I want to have two ISPs with failover
for my web traffic" just fine.

Gert Doering
        -- NetMaster
have you enabled IPv6 on something today...?

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