问题：

[单选] Which unique characteristics of the Data Center Aggregation layer must be considered by an Enterprise Campus designer？（）

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.