ipv6 next-hop link-local

George Bonser gbonser at seven.com
Mon Feb 21 00:17:01 CET 2011

> Specifically the topic at hand, using link-locals at an IXP, has some
> benefits - and at the same time, serious operational drawbacks, like
> "monitoring your eBGP peers in your NMS by IPv6 address" - now which
> the two IXPs I'm connected to is the fe80::ab:cd neighbour that just
> went down?

And that is really the problem.  Link locals can get very confusing when
a machine, even a host, has several interfaces.  If I have several vlan
interfaces on a host and I have a packet to send to fe80::ab:cd ... on
which interface will I find that neighbor?

Looking right now at a box in a data center.  Two different vlan
interfaces on the same box.  One is:

inet6 addr: fe80::221:28ff:fe57:3412/64

and the other is:

inet6 addr: fe80::221:28ff:fe57:3412/64

Same netmask, same IP address, two different networks.  On which one
will I find fe80:ab:cd?  I need to do ND on all of them until I find it,
I suppose.

I have some machines with a half-dozen interfaces on them.  If I tried
to use link-locals only, the kernel would probably be in Keystone Kops
mode trying to figure out where to send stuff.

If I were making the rules, link local would only mean "I don't have
enough information to build a 'real' IP address so I am using this
placeholder in the meantime.  Once a "real" address is configured, the
link-local would be dropped. 

Bottom line is that link local is fine where you have one interface.  It
gets to be a pain when you have many.

More information about the ipv6-ops mailing list