Slashdot: Providers Ignoring DNS TTL?
Lincoln Dale
ltd at interlink.com.au
Sun May 1 08:36:51 UTC 2005
> > > Err. No, that would be worse. "Per prefix" load balancing is an
> > > artifact of the Cisco route cache. The route engine (ie the route
> > > table) isn't queried for every packet. Instead the route
> in the route cache is used.
> > > One doesn't configure "per prefix" load balancing. One configures
> > > load balancing, which adds multiple routes into the route table.
> >
> > Modern Cisco routers do not use a "route cache",
>
> You'll need to define what you mean by "modern" with respect
> to cisco.
> This statement seems to be incorrect.
the statement is largely correct -- at least from an operational standpoint.
it is true that IOS still has 'route-cache'-based forwarding and
'flow'-based forwarding schemes (ip route-cache, ip-route-cache flow), BUT
given we're talking about internet routing here, you would defintely want to
be using CEF which isn't a cache demand-populated method.
the distinction between demand-populated forwarding (FIB) versus
prepopulated forwarding tables is relatively straight-forward, as are the
reasons why it is a "good thing"<tm>. of course, hindsight is a wonderful
thing.
> > they use a fully populated forwarding table. And load balancing is
> > automatic if you have several equal cost routes.
>
> This sounds very much like the Juniper description for the
> Internet Processor ASIC behavior. I'd say that's worse.
umm, no, i'd say it "isn't worse".
i can't speak for how J does it (or what methods they may use for
loadbalancing across distributed forwarding hardware and/or multiple
switch-fabric(s)), but in the case of C, the default (per-prefix)
loadbalancing provides deterministic loadbalancing which won't reorder
packets within the same src/dst tuple (tuple could be L3 or L3+L4-based).
> > Many modern Cisco routers can perform per-packet load balancing
> > without doing process switching (but this needs to be
> explicitly configured).
>
> Well, 7500 and 7200 have interface processors that can route
> packets using the route cache without interrupting the main
> processor. So, if you don't consider 7500's and 7200s to be
> "modern", this feature above doesn't seem like a big deal:
> They could do that before. It was called CEF and DCEF.
umm, what you're saying is largely orthogonal to what Steinar is saying.
distributed versus centralized forwarding is a different topic of
discussion.
you seem familiar with the methods commonly used to gain per-packet
loadbalancing from about 6 years ago. CEF can provide the same
functionality but without 'process-switching'.
> I'm afraid your statements show a certain lack of knowledge
> about whats being used in datacenters to route packets. And
> perhaps some arrogance about whats "modern". I'd still call
> cisco 7500 and 7200 series routers "modern", and they have
> route caches.
"best practice" would be to use CEF for pre-populated Forwarding Tables
rather than 'fast-switching' methods which use demand-based population
methods.
cheers,
lincoln.
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