ISP Filter Policies--Effect is what?

Adrian Chadd adrian at
Tue May 8 17:18:06 UTC 2001

On Tue, May 08, 2001, Stephen J. Wilcox wrote:

> if you have a /16 why would it be broken down to /24? i would assume the
> only reason you advertise /24 is because that is the size of your
> assignment from the NIC, in which case you cannot advertise the /16.
> if you do own the /16 then yes of course you can advertise it.

Not necessarily. I've been in a position where I've had $LARGEBLOCK
and I've had to break it into smaller chunks for per-POP customers.
The trick there is trying to handle your network being split down
the middle and still have traffic flowing to each POP right.
The only real way to do that is to split the /16 up into smaller
per-POP chunks and announce them seperately.

Yes, then you run into provider filtering blackholing you.

So, the comprimise is to announce the /16 and the /24s that make
it up, and bite the bullet.

You're also (possibly) helped by default routes in providers,
since the default route catches any routes being ignored due to
some form of routing policy. You then just hope that the packet
can get to you somehow.

(Default routes are even more fun - have any of you wondered how
different the net would be if noone who spoke full BGP had a default
route in their network? :-)


Adrian Chadd			"How could we possibly use sex to get
<adrian at>	  what we want?
			 	   Sex _IS_ what we want!" -- Fraser

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