The Cisco Catalyst 3550-24, 3550-48, and 3550-24 PWR switches can be ordered with the SMI or the EMI. The Catalyst 3550-24-DC and 3550-24-FX can only be ordered with the SMI pre-installed, but as with all switches running the SMI, they can be upgraded to the EMI version using an EMI upgrade kit. The Cisco Catalyst 3550-12T and 3550-12G switches come installed with EMI. The EMI enables a richer set of enterprise-class features including, advanced hardware-based IP unicast and multicast routing, and the Web Cache Communication Protocol (WCCP).
　　
　　You can also configure the SMI image to perform Layer 3 Routing. There is support for basic IP unicast routing via Static and RIPv1/v2 using the SMI image. The EMI image provides advanced IP unicast and multicast routing. These advanced routing protocols are Open Shortest Path First (OSPF), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), Border Gateway Protocol version 4 (BGPv4), and Protocol Independent Multicast (PIM).
　　
　　Both the SMI and the EMI allow for Layer 3 and Layer 4 lookups for QoS and Security.
　　
　　The following features and functionality are only supported with the EMI:
　　
　　- Advanced IP routing protocols for load balancing and constrUCting scalable LANs (OSPF, IGRP, EIGRP, BGPv4)
　　
　　- WCCP to allow switch interaction with a cache for the purpose of redirecting content requests to a cache and performing basic load balancing across multiple caches
　　
　　- PIM for IP multicast routing within a network that enables the network to receive the multicast feed requested and for switches not participating in the multicast to be pruned梥upport for PIM sparse mode (PIM-SM), PIM dense mode (PIM-DM), and PIM sparse-dense mode
　　
　　- Distance Vector Multicast Routing Protocol (DVMRP) tunneling for interconnecting two multicast-enabled networks across non multicast networks
　　
　　这几句说的最明白
　　The following features and functionality are only supported with the EMI:
　　
　　- Advanced IP routing protocols for load balancing and constructing scalable LANs (OSPF, IGRP, EIGRP, BGPv4)
　　
　　- WCCP to allow switch interaction with a cache for the purpose of redirecting content requests to a cache and performing basic load balancing across multiple caches
　　
　　- PIM for IP multicast routing within a network that enables the network to receive the multicast feed requested and for switches not participating in the multicast to be pruned梥upport for PIM sparse mode (PIM-SM), PIM dense mode (PIM-DM), and PIM sparse-dense mode
　　
　　- Distance Vector Multicast Routing Protocol (DVMRP) tunneling for interconnecting two multicast-enabled networks across non multicast networks
　　
　　这里提到的WCCP，已经作为CCIE的考点了