Re: OT: Strange ARP Behavior

Thanks Nadeem. I tested this on an 1841 and a 2811 running different IOS
with the same results. Further research seems to indicate that whenever you
clear the ARP cache, the router will send a unicast ARP request to
everything in the ARP table at that moment. New to me! Learn something new
every day!!!

With 1 IP/MAC in my table not a big deal, but somewhat interesting to think
of that process happening on a router located on a segment where there are
many hosts. I wonder if it staggers those ARPs in that case.

On Fri, Jun 24, 2011 at 1:13 AM, Nadeem Rafi <nrafia_at_gmail.com> wrote:

> AFAIK, when you clear arp cache, it does not remove entries, it just
> refresh entries, and for refreshing entries it will query about already
> known entries via unicast.
>
> Please check this article, although specific scenario is not included, but
> it may help.
> http://nadeemrafi.com/arp-arpa-and-snap-wow-110/
>
> HTH
>
>
> On Fri, Jun 24, 2011 at 6:18 AM, Joe Astorino <joeastorino1982_at_gmail.com>wrote:
>
>> "When I initially ping 10.10.10.1 from Cat1" -- Whoops. I mean when I
>> initially ping 10.10.10.1 from R1. My bad.
>>
>> On Thu, Jun 23, 2011 at 11:10 PM, Joe Astorino <joeastorino1982_at_gmail.com
>> >wrote:
>>
>> > Has anybody else ever seen anything like this or know why this is
>> > happening? I have R1 connected to Cat1, a L3 switch. R1 has an IP
>> address
>> > 10.10.10.4 and Cat1 VLAN1 has an IP address of 10.10.10.1.
>> >
>> > When I initially ping 10.10.10.1 from Cat1 of course the ARP process
>> > happens and I end up with an ARP table entry on R1 for 10.10.10.1.
>> However,
>> > when I clear the arp cache via "clear arp" or "clear ip arp 10.10.10.1",
>> R1
>> > IMMEDIATELY sends a gratuitous ARP for it's own IP address (which is
>> normal
>> > and fine) but then the puzzle....it sends a UNICAST ARP request for
>> > 10.10.10.1 to the specific MAC address of Cat1 VLAN1 interface. How R1
>> > still knows this address is the mystery.
>> >
>> > Additional Information: R1 has no routes other than the directly
>> connected
>> > route. No default route, no static route, nothing. IP routing is
>> enabled.
>> > In my troubleshooting I disabled CEF and had the same issue. Debugs
>> below
>> >
>> > R1(config-if)#do sh ip arp
>> > Protocol Address Age (min) Hardware Addr Type Interface
>> > Internet 10.10.10.1 0 0018.1820.2740 ARPA
>> > FastEthernet0/0 <--- Cat1 VLAN1
>> > Internet 10.10.10.4 - 0019.e721.84da ARPA
>> > FastEthernet0/0 <--- R1 Fa0/0
>> >
>> > R1(config-if)#do clear arp
>> > R1(config-if)#
>> >
>> > /* GRATUITOUS ARP -- Normal */
>> > *Jun 24 02:27:00.663: ARP: flushing ARP entries for all interfaces
>> > *Jun 24 02:27:00.667: IP ARP: sent rep src 10.10.10.4 0019.e721.84da,
>> > dst 10.10.10.4 ffff.ffff.ffff FastEthernet0/0
>> >
>> >
>> > /* WTF ? /*
>> > *Jun 24 02:27:00.667: IP ARP: sent req src 10.10.10.4 0019.e721.84da,
>> > dst 10.10.10.1 0018.1820.2740 FastEthernet0/0
>> > *Jun 24 02:27:00.667: IP ARP: rcvd rep src 10.10.10.1 0018.1820.2740,
>> dst
>> > 10.10.10.4 FastEthernet0/0
>> >
>> > --
>> > Regards,
>> >
>> > Joe Astorino
>> > CCIE #24347
>> > Blog: http://astorinonetworks.com
>> >
>> > "He not busy being born is busy dying" - Dylan
>> >
>> >
>>
>>
>> --
>> Regards,
>>
>> Joe Astorino
>> CCIE #24347
>> Blog: http://astorinonetworks.com
>>
>> "He not busy being born is busy dying" - Dylan
>>
>>
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-- 
Regards,
Joe Astorino
CCIE #24347
Blog: http://astorinonetworks.com
"He not busy being born is busy dying" - Dylan
Blogs and organic groups at http://www.ccie.net