Routers use RAM to store the current configuration file and other important data collected by the router (such as Cisco Express Forwarding [CEF] tables and Address Resolution Protocol [ARP] entries, to name a few). This data includes the IP routing table and buffer information. Buffers temporarily store packets before they are processed. All Cisco IOS processes, such as routing algorithms (Open Shortest Path First [OSPF] and Border Gateway Protocol [BGP], for example), also run in RAM.
RAM information is lost if the router power cycles (when a router loses and regains power) or is restarted by an administrator. To view a router's current configuration, use the show running-config IOS command. Before Cisco IOS version 10.3, administrators used the write terminal command to show a router's configuration. The write terminal command is still valid in today's Cisco IOS releases, although Cisco IOS releases 12.2T and above now provide a warning to use the new commands only.
Cisco IOS software is hardware-specific, and the image loaded on various router platforms varies from platform to platform. For example, the image on a Cisco 4500 (end of sale in 2004) will not run on a Cisco 3600, nor will an image designed for an 1800 run on the 3800. Also, IOS images contain certain features, such as Internetwork Packet Exchange (IPX) or Data Encryption Standard (DES) encryption. For example, you can load only Cisco IOS software that supports IP or IP plus DES encryption, and so forth.
Visit the following Cisco website for more details on Cisco IOS images and platform requirements: http://www.cisco.com/public/sw-center/sw-ios.shtml.
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