Gratituous ARP and the em driver

Robert Watson rwatson at
Tue Jan 13 20:38:39 PST 2004

On -1 xxx -1, Nielsen wrote:

> When I change IP addresses on my 'em' gigabit NIC, ARP isn't sent
> properly. This appears to be the problem in the following bug report,
> however i'm using the 'fixed' version of the em driver (in FreeBSD 4.9). 
> Does anyone have any tips on how to get around this? 
> I'm building new systems with gigabit ethernet support and this problem
> keeps cropping up. I have a failover system, and when moving an IP alias
> between machines, the em NIC driver doesn't properly send out gratituous
> ARP, resulting in the IP being inaccessible. 
> - The problem does not occur when plugged into a 100BaseTX switch -
> FreeBSD 4.9p1 / em version 1.7.16 - Tried various gigabit switches.  -
> One other odd thing is that when configuring the NIC (ifconfig) the
> machine locks up for several seconds. 

If you run tcpdump on the machine to sniff the interface in question
looking for arp packets, does tcpdump see the gratuitous arp?  I'm
guessing that it does, and the lack of sending the arp is a result of
delays in negotiating on the wire.  Does this problem turn up only the
first time you raise the interface, or every time you change the IP
address on the interface? 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at      Senior Research Scientist, McAfee Research

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