On Tue, Feb 15, 2011 at 1:48 AM, Aaron Riemer <[email protected]> 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 -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. - Douglas Adams - _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
