6.1 VLAN Issues
6.1.7 External routers
You can use existing high-end routers in conjunction with the NetFlow feature card (NFFC) or NFFC II on a CatalystŪ 5000 Family Switch to implement multilayer switching. The router must be directly attached to the switch either by multiple Ethernet connections (one per subnet) or by a Fast Ethernet connection using a trunking protocol such as ISL.

According to the main Figure, traffic from clients on Switch A to clients on Switch B has to traverse the router, unless a link is missing in the Figure from Switch A to Switch B. 

The high-end routers supporting multilayer switching include the 7500, 7200, 4500, and 4700 Series Routers. These routers must have the MultiLayer Switch Protocol (MLSP) software and Cisco IOSŪ 11.3.4 or later software installed to provide the Layer 3 services to the switch.