Hello Ho,
the most efficient and current way to perform inter vlan routing is the use of SVI interfaces on multilayer switches.
SVI interfaces are L3 interfaces associated to the corresponding L2 broadcast domain.
So interface Vlan1 is associated to L2 broadcast domain Vlan1, interface Vlan2 to L2 broadcast domain vlan2 and so on.
An SVI is a logical interface that is considered up/up if at least one L2 port is up/up and in spanning-tree in forwarding state in the associated L2 Vlan (broadcast domain). The L2 port can be an access port with a PC connected to it or a trunk port that is member of multiple vlans at the same time,
The SVI provides L3 services to hosts in the Vlan and typically acts as the default gateway for end user devices connected to the Vlan.
The use of an external router with vlan based subinterfaces connected to a switch trunk is limited to those cases where a L2 only switch has been deployed like C2950 or C2960 (last IOS images of C2960 allows for some inter-vlan routing with limited features).
There are some special cases that still require the use of subinterface on a router in the field of VOIP to connect IP phones of a remote office to a router that can provide limited signalling features in a remote branch office when the office is isolated from the HQ callmanager ( VOIP signalling server) this is called Survivable remote site telephony (SRST).
A router may be needed for other special cases like network address translation between different parts of the network. But this aspect goes beyond inter vlan routing.
The use of external router without trunk link/subinterfaces with per Vlan dedicated interface is really rare and limited to cases where a L2 only switch is connected to an old router that is not able to support subinterfaces, as it is clearly very poor in scalability as it requires a router interface for each served Vlan and routers haven't many ports.
The use of EIGRP or other interior gateway protocol like OSPF provides the capability to advertise IP subnets to multiple devices and has the benefits of dynamic routing
You take advantage in using a routing protocol when multiple router and multilayer switches devices are present in your network and it is a good move to have them exchange routing information that includes the IP subnets associated to client Vlans.
The high performance in inter vlan routing is achieved by using multilayer switches as stated above.
The cost of multilayer switch is now really low specially considering cost per port and they work in hardware this explains their high performance.
Note:
we should speak of fastest way to perform forwarding of packets in a inter vlan context this is the meaning I have given to your question.
Hope to help
Giuseppe
