BGP vs. static routing (Re: Why Vadim likes statics)

Robert E. Seastrom rs at
Tue Apr 25 21:37:06 UTC 1995

> So it does not make sense for IBM or Sony to run dynamic routing in 
> their internal networks?!? 

Well.... it probably doesn't make sense for IBM or Sony to assign a different 
AS number to each router in their network and speak BGP between them.  It's a 
matter of degree we're talking about here; of course it doesn't make sense to 
run full routing everywhere, just as it doesn't make sense to manually set up 
static routes everywhere.  The point we're debating is what point along the 
line it makes most sense to set the slider.

Most corporate networks are dendritic; as Vix noted, you run dynamic routing 
protocols in the center where there are multiple paths, and on hosts on the 
leaf nets you point default at the first router in the direction of the core 
and then forget about it (simple arrangements like this tend not to break and 
mess up your day).  Maybe if you're feeling generous you rip a default route 
into the leaf nets so that if someone gets a new machine that's running evil 
routed and doesn't know about default routes and such they won't lose.  On the 
other hand, maybe you would consider such a move to be anti-Darwinian and 
encouraging sloped-forehead, knuckle-dragging behavior.

> > The border router does aggregation outbound and points the aggregates 
> > at Null 0 with a high metric. 
> True. 
> > This is for cases in which there is no other router participating 
> > within the customer iBGP mesh, and where there are N (N>=1) 
> > upstream providers, and where dynamic routing must take place within 
> > the ISP's routing domain for various reasons (portable dialup 
> > links, links that are time-sensitive, etc.) 
> The assumption in this case is a common egress point. 

What percentage of the Internet's end-user customers have a single egress 
point for their networks?  At a guess, I'd say 95%.  If taking the preemptive 
step of installing pull-ups for those networks could reduce route flap by 75%, 
I submit that doing so expeditiously would get out of the woods at least for 
the time being.  This is one of those cases in which the 90% solution is 
indeed the Right Thing.  No, it won't scale if every end-user decides to 
become multihomed, but I don't see any great rush in that direction and 
compared to getting custom hacks put into the router code, it is very cost-

If space to hold the statics in the configuration memory of the routers is an 
issue, you don't even have to do that -- a PC running BSDI with a trivially-
hacked gated and connected to the fddi ring or "utility ethernet" in your POP 
can dynamically broadcast the pull-ups into your routers via rip or ospf, and 
would probably be easier to maintain and blow automatically-generated configs 
into than loading up the statics directly on the routers (at least you'd only 
have to do it once per POP instead of once per router).


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Robert E. Seastrom -- rs at
Network Engineer, Digex International
My posting, my opinions, not speaking for the company, etc. etc.

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