On Sat, 6 Oct 2007, Brian Dickson wrote:
> Consider the following set-up:
>
> A single prefix is announced by a single ASN, for each of which there
> is only one instance. (I.e. non-anycast.)
>
> The prefix is used solely for offering services that are front-ended
> by a stateful load-balancer pair. There are two LB's for redundancy
> reasons. The LB's participate in the IGP of the ASN, also for
> redundancy.
>
> The ASN (call it X) has two upstream ASNs, call them A and B. Each ISP
> is connected to a separate router, for redundancy.
A B
\ /
X
Ok.
> Inbound traffic from upstream ASN A hits router X-A, and inbound
> traffic from upstream B hits router X-B.
Ax Bx
\ /
Xa--Xb
Still with you.
> Router X-A prefers LB-A, and router X-B prefers LB-B.
Not sure what you mean by 'LB' Assuming you mean prefer 'Link B to
destination in A', I'm with you.
That just means that you prefer the link to B. That's pretty common,
too. No problem with that.
> LB-A and LB-B are state-independent. The LB support for stateful
> traffic (e.g. TCP) works only if all incoming packets for a particular
> TCP session hit the same LB.
>
> This is *not* anycast to the world. Some *may* consider it to be
> IGP-anycast.
Since you haven't reused the same IP addresses anywhere in this example,
you don't have bgp anycast. IGP anycast isn't known. There may or may
not be anycast within an AS. You example doesn't show that.
> It is an extremely common set-up, perhaps *the* most common
> configuration for load balancers.
I'll grant that one ISP with 2 upstreams is very common.
However, each AS has its own load balancer with a unique IP address for
each load balancer. When loadbalancers like the LTM redirect to other
sites, (as the LTM example shows) it uses http redirection. It does not
use Anycast.
> Anyone using PPLB between the two AS paths, the one containing A
> andthe one contain B, *will* absolutely have problems using TCP, as
> in, it won't work except in unusual circumstances (one of the two
> paths is withdrawn, for instance.)
Absolutely wrong. In _every_ non-anycast case, _all_ packets are
delivered to _unique_ IP addresses, which identify individual, _unique_
hosts. There is not requirement that all packets take the same path.
The _only_ way that subsequent packets can arrive at the wrong host is
to use Anycast. If you haven't used Anycast, there is no problem no
matter how many different paths are used.
PPLB won't harmed anything in the non-Anycast case. RFC 1812 indicates
that one can load balance packets across multiple paths. PPLB and the
LTM will work just fine.
RFC 1546 notes that Anycast can only work for stateless protocols.
But it is apparent that you still don't understand the difference
between Anycast and the methods of load balancing implemented by load
balancers such as the LTM.
--Dean
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