subnet prefix length > 64 breaks IPv6?

Bjørn Mork bjorn at mork.no
Sat Jan 7 12:00:43 UTC 2012


sthaug at nethelp.no writes:

> And yes, we know equipment that cannot *filter* on full IPv6 + port
> number headers exists (e.g. Cisco 6500/7600 with 144 bit TCAMs) - my
> original point was that I still haven't seen equipment with forwarding
> problems for prefixes > 64 bits. 

Depends on what you consider a problem and whether you consider a layer
3 switch a "router" at all, but there are certainly some switches which
will be more or less effective depending on prefix length.  Ref e.g.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_55_se/configuration/guide/swsdm.html#wp1257279

where you'll find this carefully worded hint:

 "Note: An IPv4 route requires only one TCAM entry. Because of the
    hardware compression scheme used for IPv6, an IPv6 route can take
    more than one TCAM entry, reducing the number of entries forwarded
    in hardware. For example, for IPv6 directly connected IP addresses,
    the desktop template might allow less than two thousand entries."


Translated: "The stated numbers for IPv6 routes are twice the real max.
However, prefix compression may give better utilisation under certain
conditions".


Bjørn




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