The BGP Network Command
In most IGPs, the network command starts the routing process on an interface.
In BGP, the command tells the router to originate an advertisement for
that network. The network does not have to be connected to the router; it
just has to be in the routing table. In theory, it could even be a network in a
different autonomous system (not usually recommended).
When advertising a network, BGP assumes you are using the default classful
subnet mask. If you want to advertise a subnet, you must use the optional
keyword mask and specify the subnet mask to use. Note that this is a subnet
mask, not the inverse mask used by OSPF and EIGRP network statements.
The routing table must contain an exact match (prefix and subnet mask) to
the network listed in the network statement before BGP will advertise the route.