Re: IPv6 router

[BERTRAND Joël <joel.bertrand%systella.fr@localhost>]

Le 11 mars 2026 13:06:40 GMT+01:00, Ignatios Souvatzis <ignatios%cs.uni-bonn.de@localhost> a écrit :
>Hello Joël,
>
>On Wed, Mar 11, 2026 at 12:32:23PM +0100, BERTRAND Joël wrote:
>
>> ISP gives me 2a0a:1c84:1000:a00::/56 adresses. 2a0a:1c84:1000:a00::1 is
>> IPv6 gateway. Between Cisco's routers and servers, I have configured
>> 2a0a:1c84:1000:a00::/64 network.
>
>> 	Thus, in a linux box, I have written :
>[...]
>
>>     pre-down /sbin/ip -6 route del unreachable 2a0a:1c84:1000:a00::/56
>>     post-up /sbin/ip -6 route add unreachable 2a0a:1c84:1000:a00::/56
>
>I think what you want to achieve with this is to avoid a packet ping-pong
>between your outside connection and your router machine for packets 
>addressed at your /56, but not served by your router itself or any
>connected network. They would travel back and forth at line speed
>until the hop limit is reached, thus amplifying any misaddressed 
>packet. The unreachable route will block this, more specific routes
>e.g. through additional interfaces would transfer only the explicitly
>configured address ranges.

Maybe... I have configured IPv6 a long time ago.

I suppose I don’t understand how IPv6 routes IP packets.

If my server is configured with x::1/64 and if my router sends to server a packet to a workstation on LAN side (thus, outside /64 but in /56 network), why does it receive and route this packet ? Cisco router doesn’t know gateway for this subnetwork.

   
>
>> 	In NetBSD side :
>> 
>> legendre# cat ifconfig.wm2
>> tcp6csum udp6csum
>> inet6 2a0a:1c84:1000:a00::3 prefixlen 64 alias
>> mtu 1500
>> up
>> !route add -inet6 default 2a0a:1c84:1000:a00::1
>
>To get the same effect, add the line
>!route add -inet6 2a0a:1c84:1000:100::/56 ::1 -reject
>
>(btw: you can write this:
>> inet6 2a0a:1c84:1000:a00::3 prefixlen 64 alias
>as follows:
>
>inet6 2a0a:1c84:1000:a00::3/64 alias
>
>)
>
>Regards,
>	-is

Hello Ignatios,