[c-nsp] Anycast Questions

[joshua sahala]

On Tue, Feb 15, 2011 at 1:48 AM, Aaron Riemer <ariemer at amnet.net.au> wrote:
> Hi Guys,
>
> Has anyone had experience with or knowledge of IP Anycast?

yes, it works quite well if the underlying mechanics are understood
and it is "designed" appropriately for the service(s) that you are
trying to offer via anycast.


> I am a little confused as to how the advertisement of the same Anycast
> address is possible at different routers in the network at possibly separate
> locations. Let's say I have a web service and I would like to Anycast the
> service to my national organisation with the help of my IGP. Am I right in
> thinking that each site location that has an instance of the Anycast service
> would need to advertise this Anycast address (typically a host route) into
> the routing table, and that routers within the organisation will simply use
> the mechanics of the routing protocol to direct client communication to the
> Anycast service via the best path or route?

you would have multiple hosts each advertising a host route into your
igp.  routers in your network would then pick the shortest route to
that destination address.


> Is the idea that the host route being advertised will have a longer match
> than any potential summarised network that may cover the range of Anycast IP
> addresses used?

longest-match+shortest distance, yes...anycast does not preclude the
use of aggregate/summary routes though.


> Is this why it is preferred to have a dedicated network that is not summarised at
> any point in the network to advertise Anycast services?

typically, you will have a subnet designated as your anycast network
(/24 or shorter for global reachability -- but any subnet will work if
it is internal-only).  the host/load balancer/proxy will announce the
host route to its upstream router via igp or bgp, the upstream router
then announces the aggregate route to the network via bgp (my
preference), or possibly your igp (not my preference).  once that
aggregate is in your bgp, it is easy to control propagation outside of
your network.  within your network, the igp distance to the bgp
next-hop (typically the aggregating router's loopback) will determine
which anycast cluster/host is used.


> I guess when it came to Anycast services over the Internet It would be
> fairly simple process to advertise your own Anycast addresses at any of your
> border routers around the world and AS-PATH would take care of the rest?

correct, once the network is in the bgp, you have the typical bgp
knobs available.

see the as112 project, or the f-root servers for examples of globally
announced anycast.

some other useful links:

tutorial with configurations and further explanation:
http://www.nanog.org/meetings/nanog29/abstracts.php?pt=NjcxJm5hbm9nMjk=&nm=nanog29

debunking tcp anycast myths:
http://www.nanog.org/meetings/nanog37/abstracts.php?pt=Mzg0Jm5hbm9nMzc=&nm=nanog37
   (notes on this preso:
http://www.merit.edu/mail.archives/nanog/2006-06/msg00139.html)



hth
/joshua
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