I just noticed something that I've never seen before and thought I'd pass it
along for those of you who did not know this occurred.
I have two routers, R1 and R2. I have configured their ethernet interfaces
as 10.1.1.1 and .2 respectively. After reloading R1 and turning on
debugging I saw the following:
R1>sho arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.1 - 0000.0c8d.ce47 ARPA Ethernet0
R1>en
R1#deb all
This may severely impact network performance. Continue? [confirm]y
All possible debugging has been turned on
R1#
R1#clear arp
%IPFAST-6-INVALREQ: Cache invalidation request for all interfaces
IP arp mobility: aging arp mobility cache entries
IP ARP: sent req src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 0000.0c8d.ce47 Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
As you can see, when I cleared the ARP cache the router sent an ARP request
to itself and then sent four gratuitous ARP replies. Very interesting! It
doesn't stop there.
Next, I pinged 10.1.1.2 so that it would also show up in R1's ARP cache.
R1#sho arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 0 0000.0c8d.d283 ARPA Ethernet0
Internet 10.1.1.1 - 0000.0c8d.ce47 ARPA Ethernet0
Okay, now I clear the ARP cache again:
R1#clear arp
%IPFAST-6-INVALREQ: Cache invalidation request for all interfaces
IP arp mobility: aging arp mobility cache entries
IP ARP: sent req src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.2 0000.0c8d.d283 Ethernet0
IP ARP: sent req src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 0000.0c8d.ce47 Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
IP ARP: sent rep src 10.1.1.1 0000.0c8d.ce47,
dst 10.1.1.1 ffff.ffff.ffff Ethernet0
Now, the first ARP request is a unicast ARP request to verify the MAC
address of 10.1.1.2! I had *no* idea this would happen. Apparently, when
you clear the ARP cache the router wants to repopulate it as quickly as
possible. To do this, it checks to see if the devices it was previously
aware of still exist with the same MAC-to-IP address relationships.
Very cool, but it has some traffic implications that we should be aware of
in certain situations especially on routers with large ARP caches.
Very interesting. It's amazing what you can learn by tinkering at home
instead of playing it safe on production routers. ;-)
Regards,
John
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