08-17-2010 11:27 AM - edited 03-06-2019 12:31 PM
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:
08-17-2010 11:43 AM
No, this command is not enabled by default. here's a good link where you
can see the behavior of enabling this command depending upon the address pool ( local-
no local ).
http://www.cisco.com/en/US/docs/ios/ipaddr/command/reference/iad_arp.html#wp1013090
thanks
Manish
08-17-2010 11:54 AM
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?
08-17-2010 12:22 PM
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
08-17-2010 01:54 PM
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
08-17-2010 01:56 PM
Hello,
I didn't know about this. Wonderful! Thank you for sharing this knowledge with us!
Best regards,
Peter
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