North American Network Operators Group

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Static routes in an AS vs BGP advertised routes

  • From: Murphy, Brennan
  • Date: Wed Aug 08 10:20:38 2001

Suppose you had a scenario where your AS is multi-homed
to two ISPs in, for example, 8 cities around North America.
Every site is connected to both ISPs and all of your address
space occupies a /19. To your ISPs, you advertise specific
/24s in each city. In turn, each ISP advertises your /19 to
the rest of the Internet.  

Everything is going along smoothly until an asteroid crashes
into the data center of one of your ISPs in one of the cities.
It causes extensive damage and a certain amount of
hysteria but to you, it means that only one of your ISPs
can truly reach your full /19.  That is, the /24 you were
advertising in that city is now only reachable via one
of the ISPs. But the ISP taken out by the asteroid is
still advertising the full /19.  

Under the circumstances (loss of life, hysteria, sub-optimal 
routing), would it be appropriate to ask the unfortunate
ISP to create a static route on their network to push
traffic destined for that particular /24 over to the other
ISP's network?  This way, the /19 advertisements can
be maintained and when traffic destined for that
one /24 reaches the asteroid ISP, it can get passed
over to the non-asteroid ISP.   The route wouldnt be
advertised to other carriers.....just used to make sure
traffic reached the correct final destination. 

Will ISPs make these types of accommodations? Suppose
the reason was less unexpected than an asteroid.  For example,
suppose you open a site in a 9th city, but only one carrier
is available there. In other words, are ISPs reluctant to slap in static
for every customer who comes along with some sob story
about poor planning and/or unexpected growth?

Your comments on this theoretical scenario are much