Questions on IPv6 deployment

William Herrin bill at
Tue Jan 17 23:53:04 UTC 2017

On Tue, Jan 17, 2017 at 6:06 PM, Sander Steffann <sander at> wrote:
> One thing that comes to mind is that it seems that some routers only have limited space in their routing tables for prefixes longer than a /64. If you would configure a /127 on the link but push the /64 to the routing table then you might both avoid ND Cache exhaustion and avoid the limitations on longer-than-/64 prefixes.

Hi Sander,

IIRC, the problem was that some routers could only fit the first 64
bits in the TCAM so routes longer than /64 fell out of the fast path.
That was about half a decade ago. No idea if anything modern still
suffers from this.

That would impact every route in Matthew's proposed solution: the
loopbacks from the infrastructure /64 and the /127's from otherwise
unpopulated /64's. Because programmatically those /64's don't have one
prefix, they have two: the /127 for the link and the implicit null
route for everything else. The router has to honor the implicit null
route so it can't aggregate the /127 to /64 and keep it in the fast

Bill Herrin

William Herrin ................ herrin at  bill at
Owner, Dirtside Systems ......... Web: <>

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