The subnet-router anycast address

Trevor Warwick (twarwick) twarwick at
Wed Oct 9 14:41:45 CEST 2013

I think that the RFC4291 2.6.1 definition of "required to support"  is not made clear enough.

Section 2.6 says that anycast addresses must be "explicitly configured".  2.6.1 mentions that one address per subnet is "pre-defined" as the subnet-router anycast address.   But the RFC doesn't say anything about the relationship between something being "explicitly configured" and "pre-defined", so you can argue for either behaviour being compliant to the wording.   The IOS (capital I!) implementation has always worked the way you note below, and requires explicit configuration. 

That apart, the HTC behaviour you mention is interesting, and I can't think what  it's for. Something related to tethering perhaps ??


-----Original Message-----
From: at [ at] On Behalf Of Harald Terkelsen
Sent: 09 October 2013 10:41
To: ipv6-ops at
Subject: The subnet-router anycast address


Is anyone actually using the subnet-router anycast address in your network?

RFC 4291 says in "2.6.1. Required Anycast Address":
"All routers are required to support the Subnet-Router anycast addresses for the subnets to which they have interfaces."

What does "required to support" mean here?

Should every device believing it is some kind of router always reply to neighbor solicitation requests for this address by default?

Should it be configurable and up to the administrator to decide which routers and interfaces should enable and use this address?

We have observed different behaviour from devices in our network. Our F5 
load balancer always responds to NS for the subnet-router anycast 
address with no obvious way to make it stop doing so. Our Cisco 6500 
only responds if the address is configured on the interface. Linux 
responds when IPv6 forwarding is enabled. On our wireless subnets, we 
see lots of DAD requests for the subnet-router anycast coming from MAC 
addresses registered to HTC. If the address is already in use, it looks 
like the HTC-device do not do anything about it. If the address is not 
in use, it will enable the subnet-router anycast address and starts 
responding to NS requests for this address.


Harald Terkelsen

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