IPv6 link-local address for multiple VLAN networks over the same physical interface

Question

So I have a situation in which one physical interface (as a result, one MAC) hosts several VLANs in a bond. Seeing as the link-local address in IPv6 is generated from the MAC address all my VLANs end up with the same link-local address. This causes issues in the system.

Is this a known limitation of IPv6? Is there some accepted workaround beyond going into the kernel code and changing the way link-local addresses are assigned?

Answer 1

Is this a known limitation of IPv6? Is there some accepted workaround beyond going into the kernel code and changing the way link-local addresses are assigned?

No, this is not an issue, since one link-local address is affected to the physical interface and the other link-local addresses are affected to the vlan interfaces. Each link-local address can be distinguished by its associated network interface.

Here is an example:

• you have an eth0 physical interface
• your switch is configured to have several 802.1Q vlans on this interface:
  • the native vlan is, for instance, 200
  • the tagged vlan are 101 and 102

Therefore, you will have the following configuration for your interfaces:

% ifconfig eth0
eth0      Link encap:Ethernet  HWaddr C0:3F:D5:33:13:79
          adr inet6: fe80::c23f:d5ff:fe33:1379/64 Scope:Lien
% ifconfig vlan101
vlan101   Link encap:Ethernet  HWaddr C0:3F:D5:33:13:79
          adr inet6: fe80::c23f:d5ff:fe33:1379/64 Scope:Lien
% ifconfig vlan102
vlan102   Link encap:Ethernet  HWaddr C0:3F:D5:33:13:79
          adr inet6: fe80::c23f:d5ff:fe33:1379/64 Scope:Lien

Note: if you are on Linux, you should use ip address show instead of ifconfig, to use the state-of-the-art iproute2 Linux networking interface.

As you can see, the HWaddr and the link local addresses are the same. But since a link local address need to be scoped in a vlan, there is no problem for the kernel, nor for your switch and other hosts.

To ping the link-local on vlan101, you do:

ping6 fe80::c23f:d5ff:fe33:1379%vlan101

This way, the address is associated with the interface name and no confusion is possible.