Routing occurs when a layer 3 devices makes a forwarding decision base on network address information .
The role of routing in an Entreprise :
Part of the campus network that provide access to the users.Security ( authentification ) Layer 2 switching is typically user at this layer with in conjuction VLANs.
This layer is part of the campus network that aggregates building access switches . Multilayer switches or often used here.
This layer is concerned with high speed transfer of data throught the network. High end multilayer switches are often used here.
This layer is part of the campus network and serves as the ingress and egress point of all traffic into and out of the campus network.
Routers or multilayer switches are appropriate devices for this layer.
This layer connect the campus to the internet ( ISP’s )with Routers with single or multi connections .
This layer contains routers that connect the campus network out to remote offices.examples MPLS ( Multiprotocol Label Switching ).