IPv6: How to advertise a 6rd prefix from ISP in router advertisement messages without assigning address to router?

[Sam Brynes]

I'm receiving a /56 block from my ISP using 6rd.

How can I advertise an RA prefix from the /56 without actually assigning the router an IP address from that block?

I have:

int gi0/0

ipv6 enable

ipv6 address PREFIX-ISP 0:0:0:AB::/64 eui-64 <-------- router assigned an IPv6 address from the 6rd block

ipv6 address autoconfig

ipv6 nd autoconfig prefix

ipv6 nd autoconfig default-route

ipv6 nd prefix default 7200 3600

ipv6 nd ra lifetime 900

exit

ipv6 general-prefix PREFIX-ISP 6rd Tunnel1

--------------------> I'd like to do something like:

int gi0/0

ipv6 enable

ipv6 address PREFIX-ISP 0:0:0:AB::/64 eui-64

ipv6 address autoconfig

ipv6 nd autoconfig prefix

ipv6 nd prefix PREFIX-ISP 0:0:0:AB::/64 <-------- advertise this 6rd general prefix in RA messages without assigning the router an IPv6 address from this general prefix block (this doesn't work by the way; IOS won't let me specify a general prefix when defining an RA prefix to advertise).

ipv6 nd autoconfig default-route

ipv6 nd prefix default 7200 3600

ipv6 nd ra lifetime 900

exit

ipv6 general-prefix PREFIX-ISP 6rd Tunnel1

[Harold Ritter]

It is not possible. Why wouldn't you want to assign an address to the router from that block anyway? Why do you use SLAAC on the router interface (autoconfig) and who provide the RA for the SLAAC to take place? Is this interface facing the SP or the users?

Regards,

[Sam Brynes]

Hi Harold,

We have a layer 3 switch operating as the default gateway (IPv4 and IPv6). I wanted to force all the IPv4 and IPv6 traffic through the layer 3 switch before hitting the router, and to have the return traffic come through the router and back to the layer 3 switch before hitting the client.

If the router has an IPv6 address from the 6rd block, it installs a connected IPv6 route in the IPv6 routing table. This will cause the return traffic to go back to the clients over the same VLAN. I was thinking that maybe I could advertise the 6rd prefix without actually assigning an IPv6 address so that I could send the return traffic through the layer 3 switch on another VLAN, and have the layer 3 switch send it back to the client on their own VLAN.

Not sure why I had SLAAC configured on the router interface. This interface faces the user.

[Harold Ritter]

Thanks for the additional information. So if there is a L3 switch behind the ISP facing router and if both are used as L3 devices, you would need to configure routing on both devices, regardless if you assign addresses from the received block on the ISP facing devices or not.

Configuring the access on the L3 switch leads to another issue. How do you propagate the received block information to the L3 switch? You would be better off configuring the L3 switch as L2 and two use your ISP facing device as your L3 gateway.

Regards,