Hello Ryel,

in BGP multiprotocol there are several address families that accomplish different tasks.

address-family ipv4 unicast:

here the network command is used to advertise a unicast prefix that can become the destination of unicast IPv4 packets.

address-family ipv4 multicast:

this address family is used to advertise possible sources of multicast traffic in an inter-AS multicast scenario. That is the nework command here is not meant to advertise possible destination of unicast traffic but to allow the device that receives the advertisement to elect the link as the link where multicast traffic with a source in the advertised prefix can be accepted (RPF check).

the address-family ipv4 multicast in BGP can be the companion of MSDP in an inter-AS multicast scenario and provides the capability to pass the RPF check.

One could ask why a separate command, the reason is to provide the capability to differentiate border links: the classic example is when two ASes are connected by two links and one link is dedicated to unicast traffic and one link has to be used for multicast traffic only.

To achieve this two eBGP sessions are formed with one activated and configured for AF ipv4 unicast on link1 and the other one activated and configured in AF ipv4 multi for multicast on link2

see for MSDP

http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_msdp_im_pim_sm_ps6350_TSD_Products_Configuration_Guide_Chapter.html

for BGP for multicast

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfmbgp.html

Hope to help

Giuseppe