Slashdot: Providers Ignoring DNS TTL?

Sat Apr 30 09:39:06 UTC 2005

> > First of all, let's ditch the term "PPLB."  The usual alternative to per 
> > packet load balancing (what's been being talked about here) is per prefix 
> > load balancing, which would also be "PPLB."  The abbreviation is therefore 
> > more confusing than anything else.
> 
> 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", they use a fully
populated forwarding table. And load balancing is automatic if you have
several equal cost routes.

> The route
> cache then causes only one of these routes to be used.  On cisco, to
> enable PPLB, you turn off the route cache.

Many modern Cisco routers can perform per-packet load balancing without
doing process switching (but this needs to be explicitly configured).

> On Juniper, you configure it
> to put multiple routes in the route table.  Its actaully more likely to
> happen on Junipers, because unless you configure additonal policies, you
> get load balancing on divergent links as well as non-divergent links.  On

Modern Juniper routers cannot do per-packet load balancing *at all*. It
is correct that the configuration statement says "per-packet", however
it is really per-flow (and this is well documented). See for instance
the description of Internet Processor II ASIC load balancing at

http://www.juniper.net/techpubs/software/junos/junos70/swconfig70-policy/html/policy-actions-config11.html#1020787

I'm afraid your statements show a certain lack of knowledge about modern
router architectures.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no