IPv6 – Why Does the Router Need to Know if an Address is Anycast?

Recently a good question was posted on Groupstudy asking why a router needs to know that an IPv6 address is an anycast address. I often wondered this myself and it turns out, the router does not perform Duplicate Address Detection as it does for normal unicast addresses. A small lab scenario will suffice to give use the results!

First turn on debugging:

R2#debug ipv6 nd 
ICMP Neighbor Discovery events debugging is on
R2# 

Next assign one unicast address and one anycast address and see the difference:

R2(config)#int s1/1 
R2(config-if)# ipv6 address 2004::2/64 
*Mar  1 07:24:14.374: ICMPv6-ND: Adding prefix 2004::2/64 to Serial1/1
*Mar  1 07:24:14.378: ICMPv6-ND: Sending NS for 2004::2 on Serial1/1
*Mar  1 07:24:15.382: ICMPv6-ND: DAD: 2004::2 is unique.
*Mar  1 07:24:15.386: ICMPv6-ND: Sending NA for 2004::2 on Serial1/1
*Mar  1 07:24:15.390: ICMPv6-ND: Address 2004::2/64 is up on Serial1/1
 
R2(config-if)# ipv6 address 2005::2/64 anycast
*Mar  1 07:24:23.142: ICMPv6-ND: Adding prefix 2005::2/64 to Serial1/1
*Mar  1 07:24:23.142: ICMPv6-ND: Sending NA for 2005::2 on Serial1/1
*Mar  1 07:24:23.146: ICMPv6-ND: Address 2005::2/64 is up on Serial1/1
 

When you assign a normal IPv6 unicast address, the router sends a Neighbor Solicitation (NS) message to see if any neighbors may already be using that address. Once the timeout expires, it declares the address as unique and sends a Neighbor Advertisement for this address. When you define an anycast address, it is assumed that other routers may use the address. Whether or not they are on the same link doesn’t really matter, so DAD is not necessary. So, the router skips the NS message and goes right to the NA.

Good luck!

Bryan Bartik
CCIE #23707 (R&S), CCNP
Sr. Support Engineer – IPexpert, Inc.
URL: http://www.IPexpert.com

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