In a message written on Wed, Dec 28, 2011 at 10:19:54AM -0500, Ray Soucy wrote:
If every route is nicely split at the 64-bit boundary, then it saves a step in matching the prefix. Admittedly a very inexpensive step.
I expect that most hardware and software implementations store IPv6 as either a group of 4 32-bit integers or a pair of 64-bit integers, and a [ 7 or ] 8-bit prefix length field. I haven't read anything about a new 128-bit ASIC for IPv6, at least.
In this context, it is perfectly reasonable, and expected, that the use of longer prefixes will have a higher cost.
The routers are already having to do a 128-bit lookup under the hood. Consider you have a /48 routed in your IGP (to keep things simple). When you look up the /48 in a router you will see it has a next hop. A 128 bit next hop. This may be a link local, it may be a global unicast (depending on your implementation). This next hop has to be resolved, in the case of Ethernet as an example to a 48 bit MAC address. So a typical forwarding step is already a two step process: Look up variable length prefix to get next hop. Look up 128 bit next hop to get forwarding information. Once the vendor has built a 128-bit TCAM for step #2, there's no reason not to use it for step #1 as well. AFAIK, in all recent products this is how all vendors handle the problem (at a high level). Sadly, this is all a case where mind share is hobbled by a few early adopter problems. If you look at the first IPv6 images for platforms like the Cisco 7500 (in the VIP-2 days) that hardware was built to IPv4 criteria, and had 32 bit TCAM's. To make IPv6 work they did multiple TCAM lookups, some the simple 32 bits x 4, others fancy things trying to guess prefix lengths that might likley be used. All took a substantial line rate hit moving IPv6 as a result. Those problems simply don't exist in modern gear. Once products were designed to support native IPv6 rational design decisions were made. I don't know of any _current generation_ core router that has any performance difference based on prefix length. That's why prefix length isn't in the test criteria, it simply doesn't matter. I say this as a proud user of /128's, /126's, and /112's in a multi-vendor network, as well. -- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/