[6bone] Re: IPv6-only IXP's are absolutely wrong

[Pekka Savola]

On Mon, 21 Oct 2002, Bill Manning wrote:
> % On Mon, Oct 21, 2002 at 10:18:42AM -0400, Eric Gauthier wrote:
> % > I don't know about the various IX's, but the I2 gigapop that our University
> % > uses is concerned about this.  If I remember correctly (not that the exact 
> % > numbers are important), but IPv4 is in an ethernet frame with type 0x0800 and 
> % > IPv6 is in an ethernet frame with type 0x86dd, so layer 3/4 aware switches 
> % > will likely handle these frames differently.  In our case, the Cisco 12,000 
> % > and 6500's that we're using are great for IPv4 packets (they handle them
> % > in hardware), but IPv6 packets are handled in software so we don't expect 
> % > nearly the same type of performance.  I'd imagine that something like this 
> % > is what they're alluding to.  
> % 
> % This is definitely relevant for the individual participants - but for
> % the IXP switch (which is strictly layer 2 *only*, at least for all IXes
> % that I know), L3/4 forwarding performance should not be an issue.
> % 
> % One issue that I see is multicast (neighbor discovery etc) which isn't
> % seen on an IPv4 unicast exchange switch.
> % 
> % Gert Doering
> 
> 	reasons to split v4 and v6:
> 
> 	multicast - v4 and v6 treat this differently.
> 	fabric "optimizations" - framing support, buffer sizing, MTU, etc.
> 	a decent L2 fabric will be able to accomodate the larger MTUs
> 	of native v6 and won't complain about divergent framing.
> 	"Smart" fabrics tend to be tuned to IPv4.
> 
> 	then there are issues wrt RA/ND on an exchange... having all the
> 	participants trying to "stamp" the fabric with their version of
> 	which prefix to use is noisy at best and an effective DOS at worst.
> 
> 	then there are mgmt issues. most L2 fabrics do not have up to date
> 	v6 mib support, so the stats/traffic collection is not as accurate
> 	as it should be.
> 
> 	these things, in addition to the issues with the connecting gear
> 	(v4 in HW, v6 in SW), tend to argue that a mixed-mode exchange may
> 	be less stable / harder to troubleshoot than an single use 
> 	facility.  The offset is the capex/opex costs of connecting to two
> 	facilities, one for each protocol.

I believe most of your points would be satisfied by "run different vlan's
for v4 and v6'.  Seems sane to me, and works quite fine for L2 exchanges.  
If you need L3 for some reason the issue of HW/SW forwarding may become
important.

On smaller exchanges, building dedicated peer-to-peer VLAN's is IMO also
an option worth considering.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords