The oldest available version of Security Device Manager (SDM) is 2.2.1.
SDM 2.2.1 supports Cisco 3620 routers running Cisco IOS Software Release 12.3(1)M or later. An upgrade to this Cisco IOS version is required in order to support SDM. This version of SDM requires 5.2M Flash at a minimum, so a Flash upgrade is also necessary to support it.
The Cisco 3620 model router can use up to 32MB Flash, with onboard Single In-line Memory Modules (SIMMs). There is an option to increase this capacity further, to 40MB, with the Personal Computer Memory Card Industry Association (PCMCIA) Flash cards.
For an online training module that describes how to secure Cisco IOS, refer to Securing Cisco Routers 1.0.Then click on the desired unit, either Unit 1: Cisco Router Security or Unit 2: Advanced Cisco Router Security, to launch the respective training course.