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Re: IPv6 dual stacking and route tables
From: Owen DeLong <owen () delong com>
Date: Fri, 3 Feb 2012 15:02:23 -0800

On Feb 3, 2012, at 12:10 PM, -Hammer- wrote:

So, we are preparing to add IPv6 to our multi-homed (separate routers and carriers with IBGP) multi-site business. 
Starting off with a lab of course. Circuits and hardware are a few months away. I'm doing the initial designs and 
having some delivery questions with the carrier(s). One interesting question came up. There was a thread I found (and 
have since lost) regarding what routes to accept. Currently, in IPv4, we accept a default route only from both 
carriers at both sites. Works fine. Optimal? No. Significantly negative impact? No. In IPv6, I have heard some folks 
say that in a multi-homed environment it is better to get the full IPv6 table fed into both of your edge routers. Ok. 
Fine. Then, The thread I was referring to said that it is also advisable to have the entire IPv4 table fed in 
parallel. Ok. I understand we are talking about completely separate protocols. So it's not a layer 3 issue. The 
reasoning was that DNS could potentially introduce some latency.

"If you have a specific route to a AAAA record but a less specific route to an A record the potential is for the trip 
to take longer."

That was the premise of the thread. I swear I googled it for 20 minutes to link before giving up. Anyway, can anyone 
who's been thru this provide any opinions on why or why not it is important to accept the full IPv6 table AND the 
full IPv4 table? I have the hardware to handle it I'm just not sure long term what the reasoning would be for or 
against. Again, I'm an end customer. Not a carrier. So my concern is (A) my Internet facing applications and (B) my 
users who eventually will surf IPv6.

Any guidance would be appreciated. Thanks.


"I was a normal American nerd"
-Jack Herer

On a purely theoretical level, mores specific routes are always better than default.

So, on a purely theoretical level:

IDEAL:                  Full routes, both protocols
        Advantage:  Most information available, theoretically best decisions possible.
        Disadvantage:   Router cost rivals national debt of third-world country.

Second best:            Full routes IPv6, default or partial routes IPv4
        Advantage:  Lots of information available, theoretically best IPv6 decisions.
        Disadvantage:  IPv6 might outperform IPv4 (not really a problem in most cases)
        Bigger disadvantage: Some IPv4 destinations might get blackholed from time to time.

Third choice:           Default both protocols
        Advantage:  At least you're still dual-stacked.
        Disadvantage: All the disadvantages applied to IPv4 above now apply to IPv6, too.

Worst choice:   Don't implement IPv6
        Advantage:  Absolutely none.
        Disadvantage: You can reach progressively less and less of the internet over time.

However, the real answer is more complex than that. Sometimes the route that looks the best in BGP might not actually 
be the best and so in some cases with full tables you might send to the provider with the less effective path even 
though default would have had you going via the more effective path. These circumstances are rare, however, but, BGP 
has lots of knobs and depending on how well you and your upstreams set those knobs, your experience can be radically 
better or worse as a result.

If your trip to the A destination via default takes longer than your trip to the AAAA destination via specifics, I'm 
not seeing a problem. Clients that have IPv6 capability will get a better user experience and clients that don't have 
IPv6 will get the same experience they got with default-based IPv4 prior to you implementing IPv6.


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