subnet prefix length > 64 breaks IPv6?

Joel jaeggli joelja at bogus.com
Wed Jan 4 04:16:34 CST 2012


On 12/28/11 07:30 , Ryan Malayter wrote:

> Except nowhere in there is the prefix length for the test indicated,
> and the exact halving of forwarding rate for IPv6 leads one to believe
> that there are two TCAM lookups for IPv6 (hence 64-bit prefix lookups)
> versus one for IPv4.

A cam (assuming your router uses one) can easily be parititioned to
support 144 bit words, and you can look up the whole address in one go.

A router designer might well choose to fold the lookup and partion a cam
table in a different fashion, to reduce memory consumption, save power
etc. if they choose to split lookups (for example with the 72 most
significant bits in the first lookup and the last 56 in a second) it's
because they believe the tradeoff associated with two constant time
lookups  is acceptable. remember the cam table lookup is competing
against a prefix trie lookup with a variable stride pattern done in
really fast dram for mind/market share.

> For example, what is the forwarding rate for IPv6 when the tables are
> filled with /124 IPv6 routes that differ only in the last 60 bits?
> 
> Even then EANTC test results you reference make no mention of the
> prefix length for IPv4 or IPv6, or even the number of routes in the
> lookup table during the testing:
> http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd800c958a.pdf
> 
> 
> 




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