BGP4 on a /20

Bradley Dunn bradley at
Sat Sep 20 03:04:21 UTC 1997

On Fri, 19 Sep 1997, Phil Howard wrote:

> Statically coded route?  They have a /19 (or shorter) and it is all chopped
> up (and perhaps filled up) with customers using small networks (/20 or
> longer prefixes).  They do the announcing.  What would change if I want
> to do my own and I am a /20 in there?

They setup a BGP session with you. Then they ensure your routes are
allowed to be announced to their peers/providers. After the BGP session is
stable it is generally a good idea to have them delete the static routes
they have for your prefixes. They only announce your more specific 
prefixes when they hear about them via BGP.

> But that isn't just automatic, surely.  Don't they have to remove the
> statements from the configuration of the router that originates the
> announcements?

If you setup BGP with them YOUR router originates the announcements.

> I'm not expecting their routes to flap based on downstream.  I am trying
> to figure out if they will announce their /19 regardless.  But I have
> determined this is not an issue as long as more specific routes take
> precendence over fewer hops.  Next concern is making sure my more specific
> route really gets through the provider that also announces the aggregate.
> That could depend on provider policy.

Isn't the provider announcing the aggregate one of your upstreams?

> Someone said always.  Someone else said usually.  Does the RFC say always?

The latest BGP draft says:
   When overlapping routes are present in the same Adj-RIB-In, the more
   specific route shall take precedence, in order from more specific to
   least specific.

See section 9.1.4 of

> There is a list of providers (perhaps incomplete) that block longer
> networks.  Is there a list of providers that block from their customers
> or is it safe to assume every provider has no problem with this?

I haven't heard of providers not accepting routes from their customers,
provided the routes are aggregated as much as possible and the customer is
authorized to announce them.

> I did mention before in another posting that these are extra routes that
> would not have to be here were it not for some provider filtering out
> long networks.  Funny how trying to reduce routes on their own networks
> gives them less of the advantage expected and imposes more burden on
> everyone else.

I don't follow. You would have to announce routes covering your address
space regardless of other providers' policy, no?

> Suppose my link to them is down.  Suppose they do allow the more specific
> routes to come in via peers.  Now also suppose that some branch of their
> network has to go through the origin of the block I am in, such as others
> in the same locality whose links have not gone down (e.g. my circuit got
> backhoed or something).  Will the routes even go through the originating
> router?

If your provider is letting the routes in from peers, then the routes
should be present on all of their transit routers. If the routes are not,
then their iBGP isn't meshed properly.

"Seems she thought of me as some mystic, fatalistic, mystical guru
 Me, I haven't got a clue."
	-- Tears for Fears, "Cold"

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