RE: Slashdot: Providers Ignoring DNS TTL?

["Lincoln Dale" <ltd () interlink com au>]

> > > 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.