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[AfriNIC-rpd] Anycast prefix's
aa at tenet.ac.za
Fri May 11 13:41:27 UTC 2007
Anycast can be run for a number of reasons, a classic example would be
Institution has a high traffic web server, that's accessed from all over
A.) they want to cut costs but having multiple cheapest point access to
the server (national versus international bandwidth etc)
B.) they want redundancy for the server
C.) they want a form of load balancing
There could be a number of additional reasons, but anyway...
Said server then gets 2 addresses, a unicast and an anycast address.
The web server will listen on the unicast address at each point, it will
also use the unicast interface to synchronize with the other servers or
a central point
On the anycast address, the web server will run a DNS Server.
Each DNS server will have a different A record for the web server dns
entry, that point to the servers unicast.
Server 1: Unicast 220.127.116.11
Server 2: Unicast 18.104.22.168
Server 3: Unicast 22.214.171.124
Server 4: Unicast 126.96.36.199
Anycast across all servers: 188.8.131.52
Server 1's DNS entry for www.anycastdemo.net responds with 184.108.40.206
Server 2's DNS entry for www.anycastdemo.net responds with 220.127.116.11
Server 3's DNS entry for www.anycastdemo.net responds with 18.104.22.168
Server 4's DNS entry for www.anycastdemo.net responds with 22.214.171.124
Client queries www.anycastdemo.net authorative nameserver (126.96.36.199)
and gets back the closest entry, being 188.8.131.52, and starts downloading a
large file from it.
Routing changes occur, suddenly server 2 is closest entry.
Due to the fact that the DNS is anycast, and the web server itself is
TCP based unicast he doesn't drop connection, UNTIL another query is
done (typically the A records here would have very low expiry times on
them), at which point he would get a new A record pointing to server 2.
Almost a... protected version of anycast for TCP.
This would be ONE of MANY possibilities for using Anycast. Would this
qualify as "critical infrastructure?" I'm not sure...
From: rpd-bounces at afrinic.net [mailto:rpd-bounces at afrinic.net] On Behalf
Of Michuki Mwangi
Sent: Friday, May 11, 2007 1:46 PM
To: AfriNIC Resource Policy Discussion List
Subject: Re: [AfriNIC-rpd] Anycast prefix's
Andrew Alston wrote:
> Very basic concept:
> Because you cannot announce anything smaller than a /24 into the
> tables, in order to provide for anycast you need to announce an entire
> /24, and in an anycast situation this is announced from multiple
> AfriNIC's current v4 policy states that the minimum allocation size on
> initial is a /22. To use a /22 for anycast when you potentially are
> only using 3 addresses in the block is a huge waste.
If the minimal allocation is a /22 there is no written rule that one
should aggregate and announce the entire /22 it can be de-aggregated to
announce a /24 from the allocation for anycast purposes.
> To my knowledge RIPE also has a policy that allows for this kind of
Question would be "who runs anycast and why?.If they form part of the
critical infrastructure there is room for them to apply additional /24
for anycasting as per the current AfriNIC IPv4 policy for critical
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