from wendell, r&s
The BGP network router subcommand differs significantly from the network command used by
IGPs. The BGP network command instructs that router’s BGP process to do the following:
Look for a route in the router’s current IP routing table that exactly matches the
parameters of the network command; if the IP route exists, put the equivalent NLRI into
the local BGP table.
With this logic, connected routes, static routes, or IGP routes could be taken from the IP routing
table and placed into the BGP table for later advertisement. When the router removes that route
from its IP routing table, BGP then removes the NLRI from the BGP table, and notifies neighbors
that the route has been withdrawn.
Note that the IP route must be matched exactly when the no auto-summary command is
configured or used by default.
the withdrawn route is part of an update… see bgp update below…