[dnsop] Re: Anycast 101

[Dean Anderson <dean at av8.com>]

On Fri, 17 Dec 2004, Iljitsch van Beijnum wrote:
> Nope. Consider:
> 
>             +-------+   +-------+
>             |ISPrtr1+---+ACinstA|
> +------+---+---+---+   +-------+
> |source|       |
> +------+---+---+---+   +-------+
>             |ISPrtr2+---+ACinstB|
>             +-------+   +-------+


The setup above is not that uncommon. Further, the outbound links between
source and both ISPrtr1 and ISPrtr2 going out are probably OSPF or RIP or
static default routes. BGP is not really crucial to the problem statement.

Real life examples:
========================
Many customers that have redundant connections to their ISP will have for 
example, 2 t1's, which go into different POP sites for their ISP. These 
links have default routes and PPLB going out.  

Many ISPs themselves have internal default routes going out to different
peering routers at potentially different physical locations, and some of
those may want PPLB exits.

Basically, anywhere you find an opportunity for an assymetric path, you 
have a problem with anycast.  

Let me restate and re-emphasize the rule for determining if an application 
is suitable for anycast:  

	If you can't guarentee a single path to the anycast server,
	the application must make use of some other solution, as 
	described in RFC 1546.