Subject: Re: CARP Committed (correctly presented)
To: None <>
From: John R. Shannon <>
List: current-users
Date: 05/19/2006 12:57:02
David Young wrote:
> On Thu, May 18, 2006 at 10:18:45AM -0600, Herb Peyerl wrote:
>> On 18-May-06, at 10:12 AM, Jeff Rizzo wrote:
>>> I'm not familiar with keepalived, but what CARP does is to present a
>>> separate floating MAC address common to all the redundant  
>>> interfaces to
>>> the upstream, so failover occurs as soon as the carp-implementing
>>> interfaces decide amongst themselves that it should.
>> That seems like it would have the same problem then.  The upstream  
>> switch will still have associated the virtual mac address with a  
>> physical port and the only way it will know the mac address has moved  
>> to a different physical port is to wait for some sort of traffic from  
>> the new master.  Though I'm just talking out of my posterior orifice  
>> at this point... Maybe this works better in practice.
> In general, it doesn't sound to me like it should work.  I figure it
> works 9 times out of 10 by chance: some packet just happens to update the
> switch's forwarding table in enough time that you don't notice a hiccup.
> hostapd sends a "802.2 Type 1 LLC XID Update" to update a switch's
> forwarding table when a wireless client moves from one AP to another.
> See src/dist/hostapd/iapp.c.  I believe CARP should send the same type
> of update.
> Dave

If you consider that it's primarily seen by firewalls, and that 
firewalls pass outgoing packets frequently, the firewalls gateway should 
update it's arp table in short order.

John R. Shannon, CISSP