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New Member

NAT CNBAR error on 857W


after 3 seperate attempts to gain help for why my internet keeps cutting out I think I am at the source of the problem!

The internet will go after an amount of time and nothing will be able to connect apart from the router requiring a restart.

Upon bootup of my router I get the two errors:

%NAT: Error activating CNBAR on the interface Dialer0


%NAT: Error activating CNBAR on the interface BVI1

I have done some background reading and desyfered that these errors are caused by the use of DNS and RTSP protocols.

As I have both DNS and RTSP servers whose ports are open on my NAT I feel that this is the prime reason for my workstations not being able to communicate with the WAN after a while.

I have read on this forum that it is an IOS bug and to fix it I need to upgrade my IOS version to 12.4(7.14)T although I don't know if this was for the 857 or if it's compatible with both wired and wireless models. Currently I am using Version 12.4(6)T7.

I hold a guest account with so I don't know if I will be able to download an upgrade?

Is there anything I can do?? This bug has meant that I needed to restart the router 4x in the last 2 or 3 days!! :-(

New Member

Re: NAT CNBAR error on 857W

New Member

Re: NAT CNBAR error on 857W

Ok I've just checked the links and it's what I found last night before my internet died yet again :-(

I don't have this function in my ios: ip nat service rtsp udp port 554??

There is no mention of rtsp anywhere even after runnning: ip nat service ? command

As I said before I'm using version 12.4(6)T7.

I also just removed any udp lines from my NAT config and took off any RTSP tcp ports, but still I get the same error??

As I also mentioned that my reading lead me to believe that it is a bug with my IOS.

I went to

and found a file called: rtsp.pdlm

I don't know if this is a fix for the problem as there is no documentation for it as to what it is or does or how to install it but I thuoght it might be something!

If I need to upgrade my IOS how would I do that as I don't have a Cisco service contract?

Thanks for the reply btw :-)

New Member

Re: NAT CNBAR error on 857W

New Member

Re: NAT CNBAR error on 857W

The link above suggests IOS version 12.4(7.14)T

is this compatible with the 857W??

Also where can I get it from?

I have checked the site and couldn't find it anywhere I found different versions though which I think were early deployment releases so not sure if they are stable either? But I think I could only browse as again no Cisco service contract :-(

Does this mean that I will have to live with a defective router untill I can get better Cisco support? As that's kinda not fair!

New Member

Re: NAT CNBAR error on 857W

you can only download the ios if your company has associated you with cisco. these are the privileges that your company gives. am not sure about the rules. you can also have a live chat with the cisco support team. they will definitely able to sort your problem.

New Member

Re: NAT CNBAR error on 857W

Ok I understand, but I don't currently work for a company with a Cisco contract I am doing a CCNA currently and the router is at home on a project network.

So is there any other way that I might be able to obtain the updated OS?

Maybe by going to the place I bought the router from in the first place all be it two years back now?

New Member

Re: NAT CNBAR error on 857W

well i think that would be the best idea (go where u have purchased ) hope the guarantee has not timed out.

New Member

Re: NAT CNBAR error on 857W

Thanks alot for your advise and help!

Hopefully they will be able to help even though it was 2 years ago but you never know.