RE: IPv6 Anycast

AH! Thanks Bryan. I had not stopped to consider the implications of having 2
interfaces with the same Anycast address on the same layer 2 network. That
makes total sense now.

Thanks for not only taking the time to answer my question, but to also lab it
and provide the output.

Regards,

Nathan Richie

From: Bryan Bartik [mailto:bbartik_at_ipexpert.com]
Sent: Friday, October 09, 2009 1:41 PM
To: Nathan Richie
Cc: Ccie
Subject: Re: IPv6 Anycast

Nathan,

I think it turns of IPv6 DAD detection for that address. Run "debug ipv6 nd"
when you enable a unicast address and an anycast address. When you have an
anycast address, the router sends an NS instead of sendng an NA and waiting to
see if it unique.

R2(config-if)# ipv6 address 2002::2/64

*Mar 1 03:36:55.771: ICMPv6-ND: Adding prefix 2002::2/64 to Serial1/1
*Mar 1 03:36:55.775: ICMPv6-ND: Sending NS for 2002::2 on Serial1/1
*Mar 1 03:36:56.779: ICMPv6-ND: DAD: 2002::2 is unique.
*Mar 1 03:36:56.779: ICMPv6-ND: Sending NA for 2002::2 on Serial1/1
*Mar 1 03:36:56.783: ICMPv6-ND: Address 2002::2/64 is up on Serial1/1

R2(config-if)# ipv6 address 2003::2/64 any

*Mar 1 03:37:06.471: ICMPv6-ND: Adding prefix 2003::2/64 to Serial1/1
*Mar 1 03:37:06.471: ICMPv6-ND: Sending NA for 2003::2 on Serial1/1
*Mar 1 03:37:06.475: ICMPv6-ND: Address 2003::2/64 is up on Serial1/1

On Fri, Oct 9, 2009 at 9:34 AM, Nathan Richie
<nathanr_at_boice.net<mailto:nathanr_at_boice.net>> wrote:
So I understand the concept behind the Anycast feature of IPv6. But what I
am
failing to understand is the "anycast" option when configuring the ipv6
address.

I assume it is so that the router is aware that the assigned address is an
Anycast address, but I fail to understand why the router needs to know that.
I have read the documentation from Cisco Doc CD and all it says is:

"Assigning a unicast address to more than one interface makes a unicast
address an anycast address. Nodes to which the anycast address is assigned
must be explicitly configured to recognize that the address is an anycast
address."

Can anybody help me understand why the address must be recognized as an
Anycast?

Thanks,

Nathan Richie

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Received on Fri Oct 09 2009 - 16:00:29 ART