Subject: RFC 4291: Questions about Anycast Addresses

From: "COMBES Jean-Michel RD-MAPS-ISS" <[email-masked]>
Date: Wed, 26 Apr 2006 19:06:46 +0200

Hi,

I read again the RFC 4291 and I have a question regarding the anycast
address format. The RFC says:

"The Subnet-Router anycast address is predefined.  Its format is as
follows:

|                         n bits                 |   128-n bits   |
+------------------------------------------------+----------------+
|                   subnet prefix                | 00000000000000 |
+------------------------------------------------+----------------+

The "subnet prefix" in an anycast address is the prefix that identifies
a specific link.  This anycast address is syntactically the same as a
unicast address for an interface on the link with the interface
identifier set to zero."

So my question is: is there only one possible anycast address per subnet
prefix?

If this is the case:
What is the technical reason?
What happens if there are 2 or more anycast based services on a same
subnet prefix?

Thanks for your help.

Best regards.

JMC.

France Telecom - R&D Division - MAPS/NSS   
Jean-Michel COMBES, Internet/Intranet Security
E-Mail: [email-masked]
Phone: +33 (0)1 45 29 45 94
Fax: +33 (0)1 45 29 65 19
Mobile: +33 (0)6 07 29 30 16 

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From: Bob Hinden <[email-masked]>
Date: Wed, 26 Apr 2006 10:43:29 -0700

Hi,

> "The Subnet-Router anycast address is predefined.  Its format is as

No, that is one that is required to be supported by routers on the  
subnet.  You can have as many anycast addresses as you want on a  
subnet.  Any unicast address can become an anycast address.   See  
section 2.6 and other places anycast is mention in RFC4291.  Also see  
RFC2526.

Bob

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From: Francis Dupont <[email-masked]>
Date: Fri, 28 Apr 2006 22:38:01 +0200

In your previous mail you wrote:

   The Subnet-Router anycast address is predefined.

=> note this is "the subnet-router", not anything.

=> no but there is only one subnet-router anycast address.

   If this is the case:

=> we need only one.

=> only the subnet-router service has to use the subnet-router address.

Regards
   
[email-masked]

PS: IMHO the subnet-router service is mainly the source routing, i.e.,
we can enforce a packet to go through a subnet by adding a routing
header with a subnet-router anycast in it.