There are a number of ways to have RIP advertise a default route.
see
http://www.cisco.com/en/US/tech/tk36...80094374.shtml
RIP advertises a route to 0.0.0.0 if a gateway of last resort is
selected using the ip default-network command. This network specified
in the ip default-network command need not be explicitly advertised
under RIP
In earlier versions of RIP, the default route created using the ip
route 0.0.0.0 0.0.0.0 was automatically advertised by RIP routers. In
Cisco IOS Software Release 12.0T and later, RIP does not advertise the
default route if the route is not learned via RIP. It may be necessary
to redistribute the route into RIP.
================================================== ===========================================
router rip
redistribute static
network 192.168.1.0
ip route 0.0.0.0 0.0.0.0 Null0
*Mar 1 00:06:31.681: RIP: build update entries
*Mar 1 00:06:31.681: subnet 0.0.0.0 metric 1
*Mar 1 00:06:59.547: RIP: sending v1 update to 255.255.255.255 via
FastEthernet1/0 (192.168.1.1)
================================================== ===============================================
router rip
network 192.168.1.0
ip default-network 192.168.2.0
MERV1#debug ip rip
RIP protocol debugging is on
MERV1#
*Mar 1 00:15:45.717: RIP: sending v1 update to 255.255.255.255 via
FastEthernet
1/0 (192.168.1.1)
*Mar 1 00:15:45.717: RIP: build update entries
*Mar 1 00:15:45.717: subnet 0.0.0.0 metric 1
================================================== ==============================================
router rip
network 192.168.1.0
default-information originate
*Mar 1 00:18:06.220: RIP: sending v1 update to 255.255.255.255 via
FastEthernet
1/0 (192.168.1.1)
*Mar 1 00:18:06.220: RIP: build update entries
*Mar 1 00:18:06.220: subnet 0.0.0.0 metric 1