How long does an unused arp neighbour entry exist in arp cache before it get flushed?

Solution In Progress - Updated -

Issue

From the beginning, we found some kernel parameter to control the arp flush, for IPv4, they are:

 net.ipv4.neigh.default.gc_interval = 30  
 net.ipv4.neigh.default.gc_stale_time = 60

When refer to the man 7 arp, the explainsion such as below:

 gc_interval  
 How frequently the garbage collector for neighbour entries should attempt to run.Defaults to 30 seconds.  

gc_stale_time  
 Determines how often to check for stale neighbour entries. When a neighbour entry is considered stale it is resolved again before sending data to it. Defaults to 60 seconds.

But there is still another doc for describing the issue:

http://www.etpenguin.com/cgi-bin/index2file.cgi?/pub/Reference/DOC_arp.txt

It means, the time to live of arp neighbour is not only depended on the two parameters, but also depend on the "route cache", an arp entry will be flushed only when the same entry in "route cache" is removed.

and the customer raise a script to have a test, that's true, every time, only after the neighbour entry in "route cache" is flushed, then the neighbour arp entry entry can be flushed.

So now the problem is how long time will a neighbour entry in route cache be flushed?

I guess maybe maybe it's controled by any of the kernel parameter below.

# sysctl -a | grep route | grep gc | grep ipv4  

 net.ipv4.route.gc_elasticity = 8  
 net.ipv4.route.gc_interval = 60  
 net.ipv4.route.gc_timeout = 300  
 net.ipv4.route.gc\_min\_interval_ms = 500  
 net.ipv4.route.gc\_min\_interval = 0  
 net.ipv4.route.gc_thresh = 32768

So, My question is:

1. Do we have some official doc for describing how long an neighbour arp will be flushed ?

2. How long an neighbour entry will be flushed in route cache ?

3. if possible, can anyone explain the meaning of those kernel parameters above and find some relationship with the route cache flush?

The customer use AIX to have a test, that's true, but on Linux the time is different.

Environment

  • Red Hat Enterprise Linux 3
  • Red Hat Enterprise Linux 4
  • Red Hat Enterprise Linux 5
  • Red Hat Enterprise Linux 6

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