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Re: CARP and switches
On Sep 29, 2005, at 4:26 PM, Charles Sprickman wrote:
This is somewhat off-topic, but the question has really been
nagging me ever since someone brought it up at NYCBSDCon (http://
www.nycbsdcon.org/index.php?NAV=Speakers) after Jason Dixon's CARP
demo. The demo was really cool, BTW - failover with IPSEC.
Between Jason's demo and my being laid off and thus having more time
on my hands, I built a lab to test/build a nice HA firewall. I'm
happy to report that (a) it really does work, (b) it is easy to do,
(c) best of all I didn't have to waste a real public IP on either of
the external facing NICs. Instead they each have private
(172.16.x.x) addresses while the IP of the CARP interface associated
to those NICs is public and routable. I have a pretty diagram if
anyone wants to see it. Email me.
The question that was posed was along the lines of "how does a
standard ethernet switch handle carp?". The questioner wasn't too
clear and I'm not sure Jason really knew exactly what the guy was
asking. So I'll ask it here in the hopes of understanding how this
You have two OpenBSD boxes plugged into a switch, and the OBSD
boxes are running PF/CARP. Each one has a "real" IP and MAC
address, and there is a "virtual" IP and MAC that your hosts
plugged into the same switch use as their gateway. Basic failover
Now during normal operation with both boxes up, how does the switch
deal with seeing the same "virtual" MAC address on two ports? My
simple understanding of a dumb switch is that it builds a list of
what MAC addresses are on what ports and uses that list to
determine which ports to forward traffic to. The design seems to
assume that one MAC address can only exist on one port at a time,
correct? How does this jibe with CARP's "virtual" IP and MAC?
Same question for HSRP or VRRP really.
Am I missing something? Does only one box use the "virtual" MAC
address until failover?
Assuming you're not using net.inet.carp.arpbalance (i.e. Jason's demo
was not) then only the master carp interface will respond to ARP
requests. When the other carp interface becomes master then it will
respond to arp requests. Thus no confusion for the switch.
I can't comment on the arp balance and having multiple MAC addresses
on a switch port. Probably more a question for the switch vendor(s).
Sorry for posting something so basic, I'm just now getting my feet
wet in the more interesting pf features. I generally have been
using ipf on FBSD as a simple host firewall, so I'm not up to speed
on the neat stuff.
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