However, I do have some routers with interfaces that need numbering,
and I'd rather avoid renumbering them when I change upstreams. Since
ULA-C is cheap and easy to get, I register myself a block of it, and
use it to number my router interfaces. Since I'd rather my customers
saw DNS names instead of IPv6 addresses in their traceroutes, I
delegate the reverse DNS for my ULA-C block to a nameserver on my
upstream's PA space, and set up proper PTR records for all my routers.
Now, whenever anyone does a traceroute into or out of my network,
they'll see ULA-C addresses in the traceroute. They don't need to
actually reach those addresses if they're not in my network, but they
will at least be able to resolve PTR records for them, so that the
traceroute cleanly shows whose network they're traversing.
Except that uRPF will prevent those traceroute packets from being
returned to people. This also breaks pMTU discovery and all the various
other things that rely on routers returning ICMP messages.
And whenever I decide to switch upstreams, all I have to do is update
my DHCP servers,
Your DHCP servers would presumably be using prefix delegation to request
a prefix to allocate and thus wouldn't require any configuration?
update my nameserver's A record to an IP out of my new upstream's PA
space, and we're done. I don't have to renumber a single router, I
don't have to run BGP, and I don't have to litter the DFZ with another
PI block.
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