Netflow differences between IOS 12.2 and 15.1

I would use ESW card in router if I had small office that needs to have 5 users connected. Then I would have router and switch in same device instead of buying a separate 2960 or such.

The only benefit I see with using EHWIC card would be if you need to use LAN features like port security, private VLAN, DHCP snooping etc. However in the first case you mentioned you would use the router as pure router and leave switching features for a real switch.

So it boils down to if you want to have routing/switching in one device or multiple. The only real use case I see for it is small office that doesn't need 24-p switch. You would only need to manage one device as well.

Like Mike said (in the comments) the network command will search for interfaces within that range and start advertising through it. It will also take the subnet of that interface and advertise it into OSPF. A cleaner way of looking at it that now a days instead of giving the network command you can go into an interface and add it to the OSPF proccess. This in turn does the same as the network command (find interface - advertise it)

If you say /24 or /21 doesn´t matter it's just a wider scope for OSPF to search for the interface.

Now back to your issue, the /21 won´t be advertised unless you have an interface as /21 and are advertising it in OSPF. What you could do is a static route (pointing to null0) on the ASA and redistribute it into OSPF (redistribute static subnets). Other methods like summary address are only available in ABR and ASBR (OSPF Border routers). Or you could, if applicable inject a default route in OSPF at the ASA.

The area part totally depends on your setup but if we are only talking OSPF within routers and ASA we can safely assume that one Area 0 will suffice. Try not to get mixed up with areas and networks here. Without going into to much detail and explinations of area and LSA just remember the golden rule, every area has to connect to the center (area 0), and area 0 can not be discontiguous.

Also remember if you want to redistribute iBGP into OSPF you need to allow it within BGP with "bgp redistribute-internal".

Based on your information i hope this is what you where looking for.

EDIT: OSPF area