IPv6 addressing for core network

Wed Feb 9 09:15:12 UTC 2011

On 9 feb 2011, at 5:24, Vikas Sharma wrote:

> I am looking for the recommendation for core interfaces IP addressing schema
> for Ipv6. Some different views are (PE- P - PE, point to point link) as
> below -

Is there a NANOG FAQ we can add this to?

> 1-  Use Public Ipv6 with /122 and do not advertise to Internet
> 2-  Use Public Ipv6 with /127 and do not advertise to Internet

The all zeros address is the all routers anycast address so on most non-Cisco routers you can't use it, ruling out /127. The top 128 addresses in any subnet are also reserved anycast addresses although they don't do much in practice. So the longest prefix length you should use is /120 and only use addresses 1 - 127.

I recommend /112 because that way the part of the address after the last colon is the subnet.

> 3-  Use Unique local ipv6 address

Not recommended, because now traceroute replies and, if applicable, much worse, PMTUD responses come from bogon space so some people will filter them. So absolutely do not do this if you have any non-1500-byte MTUs in your network.

> 4- Use Public Ipv6 with /64

/64 has the advantage that you can use EUI-64 addressing so you don't have to remember which exact address each router has, just let that be filled in from the MAC address. The IPv6 addressing architecture RFCs also say you must use /64 but no reason for this is given so I don't feel bound by that.

But having a global scope address on your router-to-router interfaces is such IPv4 thinking. You don't need that with IPv6 unless you want to be able to ping individual interfaces. Routing protocols only use the link local addresses and when ICMP messages have to be generated a global scope address is borrowed from another interface, such as the loopback interface.