null0 default bgp...

the following example is from r2 in the last tshoot lab 10...  the routing is static... pay particular attention to the default route, and the advertised network in bgp... r2 below in the lab represents the isp:

R2#sh run | b router
router bgp 65502
 no synchronization
 bgp log-neighbor-changes
 network 0.0.0.0
 neighbor 192.168.1.1 remote-as 65501
 neighbor 192.168.1.1 ebgp-multihop 2
 neighbor 192.168.1.1 update-source Loopback0
 no auto-summary!
ip route 0.0.0.0 0.0.0.0 Null0
ip route 10.1.0.0 255.255.0.0 209.165.200.225
ip route 192.168.1.1 255.255.255.255 209.165.200.225

on r1, the customer (enterprise), all 0's is the advertised bgp network... 10.1.0.0 is the internal enterprise network... r1's 192.168.1.1 is connected to 192.168.2.1...

R1#sh tcp brie all
TCB       Local Address           Foreign Address        (state)
84D5CF00  192.168.1.1.32348       192.168.2.1.179        ESTAB


R1#sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 0.0.0.0          192.168.2.1              0             0 65502 i

the effectiveness of the default to null0 in conjunction with the statics is clearly shown here... of course within the confines of the lab it is meaningless but we have our imagination hats on, correct? 

from doyle:

Configuring a Default Route to BGP Neighbors
router bgp 100
network 0.0.0.0
neighbor 192.168.1.210 remote-as 300
neighbor 192.168.1.222 remote-as 100
neighbor 192.168.1.225 remote-as 200
!
ip route 0.0.0.0 0.0.0.0 Null0
 
A default route to the Null0 interface is created statically, and the route is advertised with the network command...

 any destination address that cannot be matched to a more-specific route matches the static route and is dropped.

from halabi:

Less-Specific Routes of a Network’s Own Aggregate

A specific rule of routing states that, for the sake of preventing routing loops, a network must not follow a less-specific route for a destination that matches one of its own aggregated routes. A routing loop occurs when traffic circles back and forth between network elements, never reaching its final destination. Default routes 0.0.0.0/0 are a special case of this rule. A network should not follow the default route to reach destinations that are part of its aggregated advertisements. This is why routing protocols that handle aggregation of routes should always keep a bit bucket (Null0 route in Cisco parlance) to the aggregate route itself. Traffic sent to the bit bucket will be discarded, which prevents
potential looping situations.

this is wildly important...