So why is "IPv4 with longer addresses" a problem anyway?

Benedikt Stockebrand me at
Tue Jun 1 15:56:33 CEST 2010

Hi Elmar and list,

"Elmar K. Bins" <elmi at> writes:

> It gets more interesting if you try to really help your users with
> an anycast gateway address. How do you propagate this one, and not
> the routers', with RAs?

don't.  What you do is that you enable router advertisements on both
routers (and make sure they advertise the same prefixes).  If you
want, you can fiddle with their priority along the way.

Then you simply leave the rest to Neighbor Unreachability Detection
(NUD).  The hosts will maintain a list of default routers and switch
between them as soon as NUD indicates that the one used has become
unavailable.  Simple implementations maintain a single "active"
default router for all traffic, but the specs also allow
implementations to maintain individual first-hop routers per
destination.  NUD ensures that a dead router is detected at an average
30s, plusminus 15s to avoid synchronization effects.  RFC 4861 has all
the details.

In the end this gives you a failover in at worst 45s plus whatever a
new round of ND takes.  Not good enough for VoIP, but fast enough for
TCP and still much faster than switching over manually.  And all that
at minimal complexity.



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Benedikt Stockebrand, Dipl.-Inform.

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