Why 3550 and SMI v/s EMI

Before I discuss difference between SMI and EMI ios on Cisco catalyst switch (3550), lets first talk about a big Question WHY 3350 switch ?

1. First Switch which can be a Router
2. Some other features support like Uplink Fast / Backbone Fast / Rapid STP
3. HUGE bandwidth optimization using Layer-2 and Layer-3 Etherchannel
4. Advance QoS Features

Models

1. 3550-24
2. 3550-48
3. 3550-12T
4. 3550-12G

3550-24 and 48 are either 10-Base, 100-Base or 1000-Base. While 3550-12T provides 10-ports of having speed either 10, 100 or 1000 Base while having 2-GBIC ports which is used for Fiber connectivity. On the other hand 3550-12G provides 10-GBIC ports and two Cat-5 ports of respective speed.

Now Lets talk about the ios that 3550 switch has, it will be either 3550-EMI IOS or 3550-SMI IOS so we should know the difference between these two so that we can decide which ios to go for:

The 3550 is either a Layer-2 or Layer-3 switch, which depends on the software version and feature set that you install. The naming conventions for 3550 images can be any one:

ipbase (Formerly SMI): Cisco IOS IP base image and device manager files. This image has Layer 2+ and basic Layer 3 routing (Static, RIP) features.

ipservices(Formerly EMI): Cisco IOS IP services image and device manager files. This image has Layer 2+ and full Layer 3 features.

ipbasek9: Cisco IOS IP base cryptographic image and device manager files. This image has the Kerberos, Secure Shell (SSH), Layer 2+, and basic Layer 3 routing features.

ipservicesk9: Cisco IOS IP services cryptographic image and device manager files. This image has the Kerberos, SSH, Layer 2+, and full Layer 3 features.


The differences between the two are:

SMI

The SMI image is essentially an L2-only image. However, SMI adds basic L3 functionality to the image. This L3 functionality includes static unicast routing, the Routing Information Protocol (RIP), and other features.

EMI

The EMI image is an L2 image in combination with a full L3 feature set. This feature set includes:
Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP)
Open Shortest Path First (OSPF) Protocol
Border Gateway Protocol Version 4 (BGP4)
Hot Standby Router Protocol (HSRP)
Protocol Independent Multicast (PIM)
Other advanced services

I Hope this will be informative for you !

Cheers :)