link avoidance

William Herrin bill at
Wed May 6 23:36:50 UTC 2015

On Wed, May 6, 2015 at 6:56 PM, Randy Bush <randy at> wrote:
> a fellow researcher wants
>     > to make the case that in some scenarios it is very important for a
>     > network operator to be able to specify that traffic should *not*
>     > traverse a certain switch/link/group of switches/group of links
>     > (that's true right?). Could you give some examples? Perhaps point
>     > me to relevant references?
> if so, why? security?  congestion?  other?  but is it common?  and, if
> so, how do you do it?

Hi Randy,

Depends on the context of the question. There's a simple concept a
surprising number of routing researchers don't fully grasp: we like to
be paid.

Scenario: a free peer and a paying customer can swap packets via my
links but two free peers may not. A free peer definitely should not
have access to the upstream transit links I have to buy. If nobody is
paying me for that packet, I'd like it to take the long way around.
Any way but through my network.

And yes, as you know it is very common for ISPs to strenuously
disapprove of unpaid transit. And we mainly do it by limiting the
propagation of free peer routes we received via BGP.

Seems like this should be so obvious as to need no mention. It's not.

Bill Herrin

William Herrin ................ herrin at  bill at
Owner, Dirtside Systems ......... Web: <>

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