On Sun, 7 Oct 2007 [EMAIL PROTECTED] wrote:
>
> The diagram looks like:
>
> Ax Bx
> | |
> Xa---Xb
> | |
> LBa--LBb
> \ /
> B{1..n} (backend) servers 1 through N
>
> On Xa, the preferred path for S is -> LBa.
> On Xb, the preferred path for S is -> LBb.
> The load balancers do not have unique IP addresses. They have the same
> IP address, call it S1. No other IP addresses from S are in use.
The load balancers in this picture are not Anycast. I think you
misunderstand both Anycast and HSRP/VRRP. The second one takes over IP
address S1 when the primary fails.
> All packets from A sent to the single IP address S1, get sent to LBa.
>
> All packets from B sent to the single IP address S1, get sent to LBb.
Sorry, this isn't what happens. LBa (the primary) acts and exchanges
HSRP or VRRP with LBb (the failover). If LBa fails, LBb takes over.
There is no Anycast. There is no 'effective' Anycast. You just
misunderstand the concepts.
> > 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.
>
> Nope. This is an instance where it is not anycast (as you seem to define
> it at least). The IP address identifies a service.
I define it as RFC 1546 defines it. Indeed, your description above is
not Anycast. But it remains true that:
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.
> There is no requirement that all packets take the same specific hop-by-hop
> path.
>
> However, if all packets from a tcp stream do not take the same *as*-path,
> then all bets are off.
There is no requirement that they take the same AS path, either. BGP is
just a protocol to exchange routes. BGP does not do the routing. So
where the routing is done, there is no knowledge of AS paths. You can
figure this out from RFC1812.
So, all 'bets' are still on. And they remain on for _Stateless_ Anycast.
They are only broken by stateFUL anycast, as RFC1546 actually notes
explicitly.
--Dean
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