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RE: Static routes in an AS vs BGP advertised routes

  • From: Murphy, Brennan
  • Date: Thu Aug 09 16:47:58 2001

If the theoretical AS advertised its /19 itself
to the ISPs, and then one of the /24 networks became
inaccessible via the asteroid ISP, wouldn't
withdrawing the /19 take all traffic off of the
asteroid ISP? What if the outage was not an asteroid
but something more common, like uncontrolled testing of
the network while the stock market is in session?
More specifically, what if there was reason to believe the affected ISP
could still deliver service in the unaffected areas?

I have examined the responses to my query thus far and
it seems there are two options: 
 1) have both ISPs advertise both the /19 and /24s all the time
 2) change nothing til the asteroid hits. call the unaffected ISP
    and have them send out the /24 for the affected site

Option 1 creates larger route tables but automatically handles
the asteroid situation. Option 2 respects the desire for smaller
route tables but requires a manual process of phoning in a service
request.  In the meantime, the customers served by that connection
may receive sub-optimal routing, ie, if the /19 is still advertised
by the asteroid ISP, traffic reaches that AS and where does it go?

Am I understanding this correctly? If so, this seems to be a case where
being internet friendly conflicts with a theoretical entity's desire for
Maybe I am missing something fundamental here though.  

Comments?   Thanks much to those who responded! 

-----Original Message-----
From: Brandon Ross [mailto:[email protected]]
Sent: Thursday, August 09, 2001 1:25 PM
To: Greg Pendergrass
Cc: '[email protected] Edu'
Subject: RE: Static routes in an AS vs BGP advertised routes

On Wed, 8 Aug 2001, Greg Pendergrass wrote:

> 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
> /24s
> - Ask your second ISP to advertise your specific /24 in that city

Better yet, advertise your own /19 aggregate, mark your /24's with
communities that mean to your provider that they shouldn't be propagated
outside of their network and their customers (if they can't do this, fire
them) to be a good neighbor by not polluting the global routing table with
unnecessary routes.  Then when the asteroid hits, just pull down the /19
advertisement (or if you're lucky, they are so destroyed they won't get
the announcement out to the world anymore anyway).

Brandon Ross                                                 404-522-5400
EVP Engineering, NetRail                 
AIM:  BrandonNR                                             ICQ:  2269442
Read RFC 2644!