nanog mailing list archives
Re: subnet prefix length > 64 breaks IPv6?
From: Joel jaeggli <joelja () bogus com>
Date: Wed, 04 Jan 2012 02:16:34 -0800
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
Current thread:
- Re: subnet prefix length > 64 breaks IPv6? Owen DeLong (Jan 03)
- Re: subnet prefix length > 64 breaks IPv6? Alexandru Petrescu (Jan 04)
- <Possible follow-ups>
- Re: subnet prefix length > 64 breaks IPv6? Owen DeLong (Jan 03)
- Re: subnet prefix length > 64 breaks IPv6? Karl Auer (Jan 03)
- Re: subnet prefix length > 64 breaks IPv6? Joel jaeggli (Jan 04)
- Re: subnet prefix length > 64 breaks IPv6? Bjørn Mork (Jan 07)
- Re: subnet prefix length > 64 breaks IPv6? sthaug (Jan 07)