On 10/14/2011 07:04 PM, Jeff Wheeler wrote:
On Fri, Oct 14, 2011 at 3:52 AM, Michele Bergonzoni<berg...@labs.it> wrote:
can only be done with TCAM. For those who want more info on this issue, this
is the very interesting reference that I received in a private email:
http://www.firstpr.com.au/ip/sram-ip-forwarding/
I wouldn't use that particular document as a "reference."
"Take 1st 24 bits of the IP, use as an index into RAM, profit. IPv6 and
longer than /24 left as an exercise for the reader"
Wow, that easy eh? Problem solved!
;o)
When your data set grows, like the DFZ, it is less expensive to have
an ordinary RAM and search algorithm. Which particular kind of RAM is
in use is not all that exciting, but in routers that use some kind of
tree search, you can find all manner of SRAM and DRAM.
Right. So, because traditional destination-based IP routing lookup is
generally fixed-length (i.e. at most 128 bits for IPv6) then performance
of a RAM-based forwarding algorithm is relatively predictable.
There has been some mention about TCAM being good at ACL matching. It
<snip>
The reason vendors do not want to do this is the TCAM can evaluate a
very large ACL in one step, while a ALU+RAM might be more
power-efficient, if you have a ACL with hundreds of entries, it will
take a long time to process packets, and you will not have wire speed
...whereas because ACLs are variable length, determined by customers and
possibly large, performance of a RAM-based ACL algorithm is hard to
predict, and people want predictable performance, and usually line-rate
performance.
Having said that - personally I might be willing to trade line-rate
performance for (say) an ACL mechanism with near line-rate for simple
ACLs, the option of a "jump" opcode, and some way of knowing what the
exact performance (range) of a given ACL/interface combo would be.
Do I take it that non-destination-based routing (policy routing, filter
based forwarding) are therefore implemented differently on boxes that
use RAM-based forwarding?
It is nice to understand how your routers work at a deeper level.
Indeed. I'm fairly familiar with our current generation of TCAM-based
Cisco kit, but we've found less room for "real routers" in the network
because of cost/performance considerations, so it's interesting to hear
that EX8200 switches are RAM-based.
More often than not, though, all it will do is make you wonder how a
given product ever shipped or make you angry that you can't just get
an MPLS P box for the same price as an Ethernet switch. :-)
Hehe. "Tag switching will make core routers really cheap, you'll have a
few really big PE routers only". Wasn't that the line we were sold with TDP?
It's particularly interesting to hear that RAM-based forwarding has
potentially far lower power usage; presumably this is why e.g. Cisco are
crowing about how "green" the ASR9k is. Higher UPS runtimes a side benefit!
Thanks for the informative replies everyone.
Cheers,
Phil
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