[c-nsp] ARP strangeness

[Phil Mayers]

On 01/11/2011 08:06 PM, Keegan Holley wrote:
>
> That doesn't make sense though.  The cpe will need to broadcast for the
> initial request and after reboots regardless of what the provider router
> does.  The device that was blocking broadcast was a third party FTTH device.
>   I get the feeling I'm missing something here though.  Maybe it only allows
> broadcast for a specific interval after it detects a link down/link up.

As I understood it, the FTTH device permits broadcasts but they're only 
flooded on ports with FDB entries (!). So, once the FDB entry expires 
(as a result of a "quiet" CPE) it's impossible to refresh the ARP (and 
FDB) entry from the outside - only the CPE could do it, and it isn't 
doing it.

Therefore there is a need to tune ARP timers well below FDB timers on 
the FTTH device, to ensure that even for "quiet" hosts, ARP refresh 
traffic from the 7600 keeps the state.

This does seem like a broken "smart" layer2 to me, but I'm sure someone 
thought it was a good idea ;o)