Gratuitous ARP

[Charles Swiger]

On Sep 19, 2005, at 3:08 PM, Colin Farley wrote:
>    Thanks for your reply.   The model of the Cisco router is 2811.  Do
>    you think that lowering the timeout to 5 seconds would be ok? I  
> have
>    seen that Cisco does not recommend a timeout below 30 seconds but
>    after reading your reply and seeing as the re are only a couple  
> dozen
>    hosts on this subnet I would think that thi s would be fine.   
> Please
>    confirm. Thanks again.

Remember that the router is going to have to re-ARP for these hosts  
whenever something external sends traffic to them, unless the router  
already has another active connection going.  The thing is,  
ARPOP_REQUESTS use a broadcast MAC address which gets sent to all of  
the machines on the network, which adds processing overhead not just  
on the router itself but also on all of these machines.

Fortunately, you can see what this overhead is quite easily in order  
to tune things:

Run "tcpdump -nt arp" and see how often your Cisco is making requests  
with a 5-second ARP cache timeout.  So long as your network is only  
getting, say, a single-digit number of ARP requests per second, this  
amount of overhead is not going to matter significantly.  Adjust as  
needed.

-- 
-Chuck