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NAT-PT configuration help


I'm playing with IPv6 and NAT-PT. I can't get it to work at all. I'm trying to translate from v4 to v6. If I address a loopback interface on the v4 router and ping a v6 address sourcing from the loopback, it works fine. If I ping from a host that's connected to the switch, it doesn't work. What I have is:

Host(><f0/1(>RouterA<f0/0(2001:3cad:2:3/64) --> <g0/0(2001:3cad:2:1/64)RouterB

On RouterA, I've got:

int fa0/0:

ipv6 address autoconfig

ipv6 nat

int fa0/1:

ip address

ipv6 nat

ipv6 nat v4v6 2001:3cad:2::15

ipv6 nat prefix 2001:3cad:2::/96

I can't ping from the host. Two things I've noticed from looking at other configs:

1.) There always seems to be a v6v4 config as well. (Do I really need this to get it to work?)

2.) The networks that are translated don't always match what their destination networks will be. ( translated to toward a subnet) <-- I know....makes no sense. :)



HTH, John *** Please rate all useful posts ***
Cisco Employee

Re: NAT-PT configuration help

Hi John,

You need a v6v4 entry otherwise:

- Router A doesn't know which IPv4 address to use as the source address when it receives the Echo reply. It only knows how to translate the destination address

- it will tell you which IPv4 address to ping from your host

ipv6 nat v6v4 2001:3cad:2:1 (any available IP address in this subnet)



Re: NAT-PT configuration help


Thanks for the reply. I still can't get it to work, but I think it's because of the other router.

RouterB has one interface using v6 addressing: 2001:3cad:2::1.

On RouterA (NAT-PT) I have one interface using v6 and one using v4. I have a laptop with only v4 addressing.

RouterA has something like:

ipv6 nat v4v6 2001:3cad:2::29

ipv6 nat v6v4 2001:3cad:2::1

I still can't ping anything, but I can see the translations happening with debug. I've got a default ipv6 route on RouterB like ::0/0 RouterA's v6 address, but that's not working either. I can't ping the from RouterB either.

How would I be able to add a router to a v4 subnet on a v6 router? I tried g1/0, but that didn't work either. (G1/0 is v6 addressed interface.)



HTH, John *** Please rate all useful posts ***
Cisco Employee

Re: NAT-PT configuration help

Could you try with a /96 prefix which does not overlap with your current /64 prefix.

Keep your default v6 route on router B

You can also activate deb ipv6 icmp and debug ipv6 packets on router B during the test in addition of checking NAT really occurs on router A.



Re: NAT-PT configuration help


Thanks for the replies :) I made the changes, but still no luck. Although, I now see that it's definitely getting to the other router and being translated outbound correctly:

*May 27 15:51:47.962: ICMPv6: Received echo request from 2001:3CAD:1A00:2::29

*May 27 15:51:47.962: ICMPv6: Sending echo reply to 2001:3CAD:1A00:2::29

*May 27 15:51:52.958: ICMPv6: Received ICMPv6 packet from FE80::207:EFF:FE63:8C29, type 135

Bolded is the local-link address for RouterA's v6 addressed interface. Bold and Italicized is the natted address of my host ( The return traffic is configured on RouterA like:

ipv6 v6v4 source 2001:3CAD:1A00:2::1

The interface on RouterB is addressed as 2001:3CAD:1A00:2::1.

My question is how will my return traffic reach my .2.15 host if the return traffic is natted back to the .2.254 address?

Here's the default route on RouterB:

S ::/0 [1/0]

via 2001:3CAD:1A00:2::23


HTH, John *** Please rate all useful posts ***