Routers and Routing Intro


Some of the ideas that are covered in this section are how Cisco routers are put together, their different types of memory, their various interfaces both LAN and WAN, and their start-up processes? Allso in this section there is a review on how to configure a router, how to bring up its interfaces, and how to issue show commands to read its status. This first part of CCNA 2 also covers in an introductory way the router’s routing table, and static versus dynamic routing.

Router Memory

Similar yet different from a regular computer, the router has different kinds of memory ROM, Flash, NVRAM, and SDRAM which all have different functions:

  • ROM – POST, Bootstrap, and ROMMON
  • Flash – IOS
  • NVRAM – Configuration File
  • SDRAM – Running-Config, Routing Table, IOS (everything is loaded and executed from RAM)

Notice: The router is a computer but it does not have a traditional hard drive to store files and the operating system, this is accomplished in Flash memory and NVRAM memory.

Bootup Process

  1. POSTROM memory,
  2. BootstrapROM memory,
  3. Load the IOS – the router has an ordered routine for loading the IOS

    1. Flash Memory – the IOS is typically loaded from Flash memory
    2. TFTP – if there is no IOS in Flash, the router will search for a network TFTP server,
    3. ROM – if there is no IOS found, the router defaults to a recovery IOS called Rommon,
  4. Load the Startup-Configthe router has an ordered routine for loading the startup-config file

    1. NVRAM memory – the startup-config file is typically loaded from NVRAM memory
    2. TFTP – if there is no config file in NVRAM, the router will search for a network TFTP server,
    3. Setup-Mode – if there is no configuration file found, the router defaults to setup-mode

The Function of the Router

The router’s purpose or function is to find the best path (route) and switch out of the correct interface. The router will make the decision of the “best path” by first determining the destination network, and second by consulting its routing table.

Static Routing and Dynamic Routing

Routers can be configured to route traffic based on static routes that have to be manually entered by an administrator or by dynamic routes that are created dynamically by a routing protocol. Static routing is a good choice for networks that: never change, are small in size or have only one router, or have only one way out of the network. Dynamic routing is a good choice if a network has multiple routers, is part of a larger network, or if the network changes frequently. For instance, in a situation where the network changes, with a dynamic routing protocol if a network goes down, the routers will inform each other automatically through the routing protocol, and the route will be removed from the routing table; with static routing, if a network goes down, an administrator will have to go in and remove the the static route manually.

There is a difference between routed or routable protocols and routing protocols. A routed protocol is a protocol that is routable over multiple networks like the internet. Today the de facto routed protocol is TCP/IP. A routing protocol is a protocol used by routers to share information with each other, specifically information about available routes. Examples of routing protocols would be RIP, EIGRP, OSPF, and ISIS.

For the Cisco CCNA certification exam you will need to know how to configure an interior gateway routing protocol in a multiple router network. You will be required to know the following interior gateway routing protocols: RIPv1, RIPv2, EIGRP, and OSPF.

Routed Protocols
IPX/SPX (Novell – no longer in use)
Apple Talk (Apple – no longer in use)

 Routing Protocols

RIP v1 – interior gateway protocol, IETF – RFC1058, open standard
RIP v2 – interior gateway protocol, IETF, open standard
EIGRP – interior gateway protocol, Cisco proprietary
OSPF – interior gateway protocol, IETF, open standard
ISIS – interior gateway protocol, covered in CCNP
BGP – exterior gateway protocol, covered in CCNP

Interior Gateway Routing Protocol Types
Distance Vector     Link State  

The Routing Table and RIP

A demonstration of reading routing tables and activating the RIP routing protocol