Couple things regarding ARP
Suppose a situation PE-Cloud-CE, where something happens to CE and it needs to be replaced. I've noticed that more often than not, I do not need to clear arp on PE.
Is this because of Gratuitous ARP CE is sending once it brings its interface up?
If so, is it treated the same way as ARP Resulotion, in that it can update the PE's arp cache, regardless if there is already an entry?
Lastly, is there a GARP timer or is it event triggered, ie link up?
Comments
By default router will cache an ARP entry for 4 hours, this can be changed per interface using command "arp timeout xxxx". Different networking devices snd GARP messages in various situations; a Cisco router sends a GARP on link-up, if you change the MAC address of an interface (because interface flaps), when you enable IP addressing or change IP addressing on an interface. By default if a router receives a GARP it will put the entry in its ARP chache if it did not exist, or refresh/overwrite the entry if it existed already; you cannot configure a router to send GARP's, but you can configure it to ignore GARP's with global command "ip arp gratuitous none".
Correct.
Surprisingly, this is incorrect. I tested againstg IOS 12.4(15) and Cisco IOS will not create new entries based on Gratuitous ARP. It only updates existing entries. This is a sensible, table-space-conserving design.
It ignores gratuitous ARPs from IP addresses not already in the ARP table.This command doesn't affect gratuitous ARPs for addresses already in the table.
R1(config)#ip arp gratuitous none
R1(config)#end
R1#
*Mar 1 23:55:57.834: %SYS-5-CONFIG_I: Configured from console by console
R1#
*Mar 1 23:56:18.446: IP ARP: rcvd rep src 1.1.1.44 0000.2222.2222, dst 1.1.1.44 FastEthernet0/0
R1#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 1.1.1.1 - aaaa.bbbb.cccc ARPA
Internet 1.1.1.44 0 0000.2222.2222 ARPA FastEthernet0/0
Internet 1.2.1.1 - c200.0390.0001 ARPA FastEthernet0/1