anycast address as deault router

[Benedikt Stockebrand]

Hello everybody,

"Michael K. Smith - Adhost" <mksmith at adhost.com> writes:

>> On 30/12/2009 21:25, The Dark One wrote:
>> > supposing that on a LAN there are 2 routers (default-gateway) and
> one
>> > host and that the routers don't support any
>> FirstHopRedundancyProtocol,
>> > does it make sense to configure a anycast address on both routers
> and
>> > configure that as the default-gateway of the host?

Yes, but... see below.

>> > If it does makes sense, is it also possible to have the routers
>> > advertise that anycast address in their RA?

Not as far as I know; there is no explicit router address field in a
Router Advertisement; instead the source address of the packet is
used, which must be a link-local address.  If you find a way to
configure your router with an extra anycast link-local address and
then make it use that address for its router advertisements, then it
might work, at least as far as I understand the standards.  I have
some doubts if this is actually works on existing implementations,
though.

Neither do I consider it a particularly good idea---see below for
alternatives.

>> Sounds like this would fall foul of duplicate address detection.  I
>> suspect
>> both your routers and your leaf nodes would probably get very excited
>> about
>> this, and not necessarily in a good way either.

There shouldn't be any problem: Configuring an address as anycast
disables Duplicate Address Detection, so it shouldn't be an issue on
the router side.  As of RFC 4862, 5.4 (p. 12): "Duplicate Address
Detection MUST NOT be performed on anycast addresses (note that
anycast addresses cannot syntactically be distinguished from unicast
addresses)."

And the hosts should play along simply because otherwise anycast as
such wouldn't work.

That doesn't mean that I consider the idea any better at this point...

>> By all means try it out, but when it refuses to work, take a look at
>> RFC2462 (section 5.4).

(RFC 2462 has been obsoleted by 4862 but this part seems to be
unchanged except for some clarifications.)

> I had to do exactly this on my GSR's because they don't support RA
> prioritization or HSRP v3 and most likely never will.  The anycasted
> default gateway from two routers does work, although the failover
> between routers is +/- 20 seconds.  If anyone would like a copy of the
> configuration snippets just let me know.

I think that Neighbor/Router Discovery is still the way to go.  It
just takes a bit of tuning:

- According to RFC4861 it is possible to update the Neighbor Discovery
  timeouts through the Reachable Time field in a Router Advertisement,
  speeding up timeouts significantly at the price of potentially more
  ND traffic.

- If that doesn't work: The timeouts may be configurable on the host,
  depending on the particular implementation.

- As of RFC 4191, Router Discovery has been extended to support router
  preferences, allowing for the definition if a "``default'' default
  router".

Using these features it should be possible to do away with HSRP and
such since hosts will switch to the surviving router if their "active"
one goes off-link.  See RFCs 4191, 4311 and 4861 for the protocol
details.

So far I haven't found the time to check existing implementations for
these features, so I suggest you do some thorough testing before you
go live with them.


Alternatively, if you are really desperate for sub-second failover,
consider reconfiguring your host as a router and running OSPF between
it and the routers.  (Sorry if you are running EIGRP in a "Cisco only"
setup...)


Cheers,

    Benedikt

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