Over-utilisation of v6 neighbour slots

Doug Barton dougb at dougbarton.us
Tue Oct 29 07:07:43 CET 2013

On 10/28/2013 10:49 PM, Lorenzo Colitti wrote:
> On Tue, Oct 29, 2013 at 6:53 AM, Phil Mayers <p.mayers at imperial.ac.uk
> <mailto:p.mayers at imperial.ac.uk>> wrote:
>     I wanted to follow up on this. Some folks from Cisco kindly
>     contacted me off-list, and correctly guessed that a large number of
>     the IPv6 neighbour entries were in state "STALE", and pointed me to
>     the relatively new:
>        ipv6 nd cache expire <seconds>
>     ...interface-level command. This wasn't in the IOS train we were
>     running until relatively recently, so I hadn't seen it before.
> I wonder what the designers were thinking when they did the original
> implementation. Without this option, a box with enough client churn
> could run out of neighbour cache entries even if all the clients are
> perfectly behaved.
> Perhaps they didn't think of it because it doesn't happen in IPv4 due to
> a) much fewer addresses on a given box due to scarcity and b) ARP has
> timeouts.

Probably not scarcity in 1918 world, but I think you hit the nail on the 
head with "arp has timeouts." :)


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