ip gratuitous-arps

Unanswered Question
Aug 17th, 2010

Is "ip gratuitous-arps" enabled on the switches/routers by default? (specifically Cat 6500's)

We're moving some SVI's from a switch to another one.

Since the servers will have the old ARP/MAC cached in their arp table, we'd like to figure out a way to make the servers learn the new ARP/MAC entry for their default gateway w/o manually clearing arp cache on each server.

Would the switches send gratuitous-arps out to inform the servers of the move of the new SVI's?

I found another thread about gratuitous-arp, but it has no mention of what I'm looking for:

https://supportforums.cisco.com/thread/2000695

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huangedmc Tue, 08/17/2010 - 11:54

thanks for the reply.

I looked at the page you provided; it's useful, but it's for router / WAN scenario...it mentions the address pool and clients that connect via PPP...

My scenario is a bit different - servers connected on the LAN.

Would the switch still send out gratuitous-arps to advertise the new SVI?

manish arora Tue, 08/17/2010 - 12:22

I think if you enable ip gratitious-arp on the  svi on the new switch, it will send the that arp packet to the connected hosts to update their Cache Entry for that IP address - basic gratitious arp defination. But if i was you , i would still test it in a test set up  before making changes in the production network.

Thanks

Manish

kyukim Tue, 08/17/2010 - 13:54

Hi,

If you are moving your SVI to different switch and want to update all servers/hosts's ARP cache, you can simply issue "clear arp" on switch.

This command will clear arp-chche on router/switch but also send out ARP Request as broadcast with its own mac address and IP address as source.

R2#clear arp-cache int f0/0  --> you can specify which interface you want to clear arp.

R2#

*Aug 17 21:01:10.754: ARP: flushing ARP entries for interface FastEthernet0/0

*Aug 17 21:01:10.754: IP ARP: sent rep src 10.1.34.4 0021.d8fd.d040,

                 dst 10.1.34.4 ffff.ffff.ffff FastEthernet0/0

This is very useful to refresh your router or switch MAC address to all hosts in same subnet.

KK

Peter Paluch Tue, 08/17/2010 - 13:56

Hello,

I didn't know about this. Wonderful! Thank you for sharing this knowledge with us!

Best regards,

Peter

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