North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

RE: Static routes in an AS vs BGP advertised routes

  • From: Greg Pendergrass
  • Date: Wed Aug 08 11:45:08 2001

If you are advertising a /24 through both ISPs in each city then you won't
need them to do anything special as routers always put the most specific
route in their routing table. If you are advertising specific /24s to the
ISPs and one of the peering links drop then that /24 will be withdrawn from
that ISP's table and henceforth to the internet making the only available
route your other ISP.

The problem is if they aggregate and advertise only a /19 instead of
individual /24s In this case you could:

- Ask the pulverized ISP to withdraw the aggregate in favor of the specific
- Ask your second ISP to advertise your specific /24 in that city

I'd choose the second option because if your first ISP was truly hit by an
asteroid they'll be spending too much time
a. Attempting to make people actually believe they were hit by a piece of
ancient space debris
b. Trying to recover their network
c. Working on an appropriate press release showing lack of fault and sorrow
for the loss of several competent engineers and  d. All the surviving
employees will be trying to sign a bunch of book and movie deals about their
experience :>
All in all your second ISP will be better equipped to get your traffic up
and running.

Greg Pendergrass       Manager of Operations
BAND-X, Inc.        

-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of
Murphy, Brennan
Sent: Wednesday, August 08, 2001 10:16 AM
To: '[email protected]'
Subject: Static routes in an AS vs BGP advertised routes

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