Connected route exists for a DUP IPV6 address.

[Noel]

Hey Guys,

I have 2 routers connected back to back. R1 and R2.

On R1, I've configured an IPv6 address 4444::2/64.

Now on R2, I've configured the same IPV6 address 4444::2/64.

When I see the show ipv6 interface on R2, it detects the address as duplicate and marks it DUP.

But when I see the routing table of R2, a connected route for that network exists. 

R2#show ipv6 interface

Ethernet0/0 is up, line protocol is up

  IPv6 is enabled, link-local address is FE80::CE02:1BFF:FE1C:0

  Global unicast address(es):

    4444::2, subnet is 4444::/64 [DUP]

  Joined group address(es):

    FF02::1

    FF02::2

    FF02::1:FF00:2

    FF02::1:FF1C:0

  MTU is 1500 bytes

  ICMP error messages limited to one every 100 milliseconds

  ICMP redirects are enabled

  ND DAD is enabled, number of DAD attempts: 1

  ND reachable time is 30000 milliseconds

R2#

R2#

R2#show ipv6 route

IPv6 Routing Table - 3 entries

Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP

       U - Per-user Static route

       I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary

       O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2

       ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2

C   4444::/64 [0/0]

     via ::, Ethernet0/0

L   FE80::/10 [0/0]

     via ::, Null0

L   FF00::/8 [0/0]

     via ::, Null0

Please let me know is this correct? 

I assume that when an interface which has a duplicate IPv6 address should not be processed and therefore corresponding route should be removed or marked as not forwarding.

Kindly help!

[Phillip Remaker · Accepted solution]

The address is a duplicate, but the network number is legitimate.

Traffic will not flow to the duplicate, invalid global address, but the network portion of that address establishes the network membership of that interface.

The next-hop addresses to other routers will always be the Link-Local addresses, so the fact that the global address of an interface is not valid will not affect routing. This is in contrast to IPv4 where there is no independent Link-Local address.

[Noel]

Hey Phillip,
Thanks for your input. I am not much familiar with IPv6 concepts, but wanted to understand more of it.

A video in Youtube says that, when a duplicate IPv6 address is detected on a node, a while later it is is deleted from the interface. I'm not sure about this behavior.
Can you shed some light on this?
Thanks for your time!

[Phillip Remaker]

You told the router two things when you set a static address: The network serviced by that port, and the address for packets destined to the supervisor (host address) of the router.

As a duplicate address, the address portion gets deactivated. However, until you manually remove that address, the network represented by the address will be considered directly attached to that interface,

I don't know about the YouTube video, you'd have to point me to it.

The duplicate address behavior for learned addresses (SLAAC, stateful DHCPv6) may vary from manually configured address. A host and a router also serve different roles.

[Noel]

Phillip, thanks for writing

Please refer to the below link which I've mentioned above.

https://www.youtube.com/watch?v=O1JMdjnn0ao&t=2122s