Re: OT: Strange ARP Behavior

"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
Blogs and organic groups at http://www.ccie.net