Hi,
I have seen a few operators adding static routes like:
0.0.0.0/1 some next-hop and
128.0.0.0/1 some next-hop.
Why would anyone want to add such static routes? What does 0.0.0.0/1
mean. Note that the netmask is 1 and not 0.
Thanks,
Glen
Protection against learning a bad default route through whatever routing protocol they are learning, since these two routes would be more specific than any typical default route. They probably got burned learning a default route.
Wouldn't it make more sense to filter in bound default? or use a single
static default if you where worried about that?
-jim
It means half the IPv4 internet goes one way. Half goes the other way.
Well considering that native multicast isn't enabled end to end Internet wide, and class E address space isn't used, it's more like half your IPv4 Internet goes one way, and ~38% goes the other way... :-b
Stefan Fouant
JNCIE-M, JNCIE-ER, JNCIE-SEC, JNCI
Technical Trainer, Juniper Networks
Follow us on Twitter @JuniperEducate
there's lots of smarter things you COULD do 
this, it seems to me,
is a great thing for the operations bcp folks to work out though
Wouldn't it make more sense to filter in bound default? or use a single
static default if you where worried about that?
Yes, the aesthetics of using a "/1 route" for that purpose are very poor.
Don't implement design objectives using subtle side-effects, when a
proper tool
is available -- human errors later are likely.
Using a /1 static to achieve a "longer prefix" to override a
default falls in that
category, when routers have a filtering mechanism capable of
explicitly expressing
the desired policy
I found I had to do this many years ago on some Cisco routers to get them to
load balance (per packet) across two links. Adding 0.0.0.0/0 routes across
both links just resulted in traffic routing across one link. Broke it into
two /1's per link and it worked perfectly.
Two other reasons for this too:
1) Something won't redistribute 0.0.0.0/0 on the network. Either because the person doesn't know the command to tell the router to do it, or because the router simply won't redistribute a default route.
2) Could also be failover. One router might be advertising 0.0.0.0/0 on one end of the network. A different router on a different part of the network might be advertising the two /1's. The /1's would be used unless they became unreachable.
Joel, Glen,
Le 24/09/2011 03:18, Joel Maslak a �crit :
Protection against learning a bad default route through whatever
routing protocol they are learning, since these two routes would
be more specific than any typical default route. They probably
got burned learning a default route.
Having a default route, or rather having a route to every possible
adresses, is required when you expunge your routing tables of some
prefixes yet you still wish to contact them relying on the next-hop's table.
Simple application is to filter incoming routes longer than /20 or /21
to free up some memory on your routers (reducing the global table from
377k to less than 100k routes is a nice perspective 
)
But a default route is an obvious move and could easily be leeked by an
upstream, yet replacing yours if not properly filtered. So, using more
precise routes (/1s to /8s) helps avoiding these risks and yet lets you
roughly balance load to several gateways.