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".

  • 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.

    Correct.

    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

    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.

    you can configure it to ignore GARP's with global command "ip arp gratuitous none".

    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

     

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