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??? wrote:
>
> ldirecterd seems to send arps when it takes over IP addresses.
> Is it mandatory ?
> What happens when we doesn't send arps and just change IP addresses ?
> ARP makes me always confusing.
the router which directs packets to the VIP is sending them to a MAC address
(the hardware address of the ethernet card that has the VIP configured
by ifconfig on it). The MAC address for a VIP is held in arp tables.
When a new box becomes director after director failover,
the VIP will now be on an ethernet card with a new MAC address.
However unless the router is told about this change its
arp table will still have the old MAC address for the VIP and
will continue to send packets for the VIP to the original MAC address.
There are 2 ways to fix this.
1. do nothing: the arp table for ethernet expires in about 90secs and
the router will broadcast the request "who has VIP tell router" and
the new director will reply. The problem will be that the packets
sent to the VIP during the changeover will get "host unreachable"
type errors.
2. have the new director send arp replies (without first
being asked). This will flush the arp tables of the MAC address
for the VIP for all machines on the network. This minimises the
time for which the router has the wrong MAC address for the VIP.
In both cases you have to first stop the original director from
sending arp replies for the VIP, eg by removing the VIP.
Joe
--
Joseph Mack PhD, Senior Systems Engineer, Lockheed Martin
contractor to the National Environmental Supercomputer Center,
mailto:mack.joseph@xxxxxxx ph# 919-541-0007, RTP, NC, USA
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