Subject: Re: [vserver] ping vs. IP address vs. MAC (address)
From: Herbert Poetzl <herbert@13thfloor.at>
Date: Fri, 1 May 2009 23:49:50 +0200

On Fri, May 01, 2009 at 11:02:21PM +0200, Dennis Roos wrote:
> What you could do is clear the arp entry for the address on your 
> router/switch and generate some traffic (eg. ping) on the interface.
> Mostly it takes 5 minutes for a router to learn about the new address, 
> as it caches the information for that amount of time.

a trick often used by HA folks is to send an
unsolicited or gratuitous ARP, with e.g.

 arping -U -I eth0 <guest-ip>

this will update all directly connected routers
with the new arp information

HTH,
Herbert

> Best regards,
> Dennis Roos
> 
> 
> Roderick A. Anderson wrote:
> >I'm asking for a clue stick here.  First time I've run into this issue 
> >... hard.  Be a few times where it took a bit for outbound traffic to 
> >get a route but can't remember running into this problem.
> >
> >The short question is: does the network/NIC _know_ which IP address it 
> >is responding for when a ping request comes in or does it just respond 
> >to the ping request at the MAC (hardware?) level?
> >
> >I have two hosts and each of their guests has its own IP address. I am 
> >trying to move the guest's services from one host to the other.  The 
> >new and old guests both use the same IP address.  So I shut down the 
> >old guest and then start up the new guest on the other system.  
> >Network traffic to the new guest, except pings, doesn't work.  
> >Iptables, ip addr ls, ifconfig, etc., all appear to be good.
> >
> >I'm guessing the routing is sending the ping request to the old 
> >guest's MAC.
> >
> >Thoughts, Ideas?
> >
> >\\||/
> >Rod