Andrea Venturoli wrote:
> Jordan Gordeev wrote:
>> The only load balancing that CARP supports, to my
knowledge, is ARP
>> level load balancing. From carp(4):
>> The ARP load balancing has some limitations.
First, ARP balancing only
>> works on the local network segment. It cannot
balance traffic that
>> crosses a router, because the router itself
will always be
>> balanced to
>> the same virtual host.
>
> Forgive me for stepping in, but I had read the above
statement over
> and over trying to figure what it meant; perhaps it's
not so clear...
>
> If I understood it correctly it's not saying you should
not use CARP
> on routers. Instead it's meaning that load-balancing
won't cross a
> third router which is on cascade of the two CARP
routers.
...
Andrea, you are correct. Jordan is pointing out the main
limitation of
CARP, which is that it operates only within a broadcast
domain. I
should point out such a feature is out of scope for VRRP,
CARP, IPMP or
other Layer 2 IP sharing protocol. However this behaviour is
just fine
for load balancing a router, in which case one relies on
next-hop
reachability anyway.
The thing to remember with CARP is that it relies on the
ability of the
interface to go into promiscuous mode to pick up traffic for
its virtual
MAC addresses. More modern cards may support more than one
station
address in hardware, which avoids the need for promiscuous
mode
processing, however we don't currently support this hardware
feature.
If one wishes to load balance across Layer 3 hops (rather
than within
the same broadcast domain), what one is asking for is a
feature like
BGP4 Anycast, IPv6 Anycast, or OSPF-based Anycast which
relies on
cooperating routers to inject a route into the Layer 3
routing domain
for a given 'virtual' IP address.
There is a daemon out there which uses the OSPF API in
Quagga to flood
OSPF domains with virtual host routes for anycasting
services using
Opaque LSAs but I forget its name. XORP has the potential to
do the same
but requires some development effort to do so.
If one wishes to load balance specific requests for an
application layer
service, one enters the wonderful world of 'middleware' and
competing
commercial solutions to the problem.
And this is where money comes into play...
Regards,
BMS
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