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ARP Problems (Entires becoming Static)

Simon336697
Simon336697 asked
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Last Modified: 2010-03-19
Hi guys!

Have a weird one here that I hope you can help me understand how and why the following would happen.

In a remote site, on one subnet, there are 8 machines.

All are MEANT to have connectivity to one another, and ideally they do.

However, one day, one machine (machine B) can contact all the other machines on this subnet EXCEPT for machine A.
Now, all machines other than machine B, can contact machine A.
From machine A, I can ping all other machines other than machine B.
From machine B, I can ping all other machines other than machine A.

What I found:

I jumped on to machine A, and issued the following command:

arp -a
To display the arp table.
I found an entry in here for machine B that was type STATIC.
All other entries were dynamic.

I removed this static entry by:

arp -d <machine B ip address>

After this, I had connectivity back between machine A and machine B.

Question:

I know you can manually add a static entry in the arp cache for a system.
However, this has NOT been done in this case, and we dont have any need to do this.
So, my question is, under what circumstances, how, and why, would an arp entry for a machine become static if it was

NOT manually added to the arp cache as static. It really has got me stumped. Would a switch have something to do

with this?

Thanks in advance.

Simon
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Commented:
Does anybody know something??
Commented:
Not sure if this answers your question but it may help

From
http://www.microsoft.com/technet/pro...n/netdiag.mspx,
"Sometimes, an ARP cache entry can become stale and communication cannot
occur again until the bad ARP cache entry expires. It is also possible
that a bad static ARP cache entry could have been placed on the machine
and would never expire. The ARP cache is naturally flushed at 2 and 10
minute intervals, so this operation is considered safe. Note: If your
organization relies on static ARP cache entries, make sure there is a
mechanism for adding the entries back in once this tool is run." I
couldn't find the registry entry that defines the expiration intervals,
so maybe they are hardcoded. Looks like if the OP had waited a few
minutes that the problem might have gone away. However, entries added
with the -s parameter in the arp command are static and do not expire
out of the ARP cache.

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