question about BGP default routing

Tue Jan 6 19:34:36 UTC 2009

cK> Date: Tue, 6 Jan 2009 07:40:16 -0500 (EST)
cK> From: chloe K

cK> Why it needs default routes when running BGP?

If you have a full table, you do not need default.  It's even desirable
to drop road-to-nowhere packets inside your network, before they clog up
your connections.

However, consider that you may encounter some problems -- such as
insufficient RAM to deal with growing table size -- that leave you
forced to take a partial table.  Then what?

If you're running BGP, you probably have more than one upstream, so you
don't want static defaults (unless the next hop is a serial interface).
To deal with this, you can have your providers originate default _and_
send a full table.

Under normal circumstances, use a route map that nukes 0/0.  If you find
yourself in a jam, replace the route map with one that allows 0/0 and
discard long paths, AS_PATHs that you consider troublesome, et cetera.
You still have the benefit of directing certain routes to a specific
provider, but with a smaller (partial) table.

Finally, note that not every router needs full tables.  Consider a
peering router that exchanges traffic between a network's peers and
customers.

Eddy
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