Cisco routers ARP for themselves and send gratuitous ARP replies when they boot also.
Priscilla At 11:07 PM 10/3/01, John Neiberger wrote: >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 > > > > > >_______________________________________________________ >http://inbox.excite.com ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=22096&t=21984 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
