A. to provide service internetworking
B. to support transparency for Layer 2 frames
C. as a connection identifier to indicate destination
D. as a mapping to the DLCI in service internetworking
E. to provide a trunk by which all VLANs can navigate from one site to one or multiple sites

A. Layer 3 routing between the Access and Aggregation layers facilities the ability to span VLANsacross multiple access switches， which is a requirement for many server virtualization andclustering technologies.
B. “East-west” server-to-server traffic can travel between aggregation modules by way of the core，but backup and replication traffic typically remains within an aggregation module.
C. Load balancing， firewall service， and other network service are commonly integrated by the useof service modules that are inserted in the aggregation switched.
D. Virtualization tools allow a cost effective approach for redundancy in the network design byusing two or four VDCs from the same physical switch.