CCNA Study Guide Chapter 6 Summary

Chapter 6 began with a look at the external features of a Cisco 2500 series router. This included an overview of each port, including its numbering, physical connectors, and characteristics. An overview of the router’s Ethernet port provided insight into AUI connections, transceivers, and their purpose.

A closer look a router’s serial ports included an explanation of synchronous communications, DTE and DCE devices, and the physical layer communications standards that are used to connect between these devices. A basic introduction to CSU/DSUs provided insight into its role in the WAN communication process.

The purpose of both the asynchronous console and auxiliary ports was looked at next, including the ways in which these ports are connected to devices using rollover cables and adapters. The pinouts for a rollover cable were also looked at, in order to allow you to create your own as necessary. A review of the router’s LED lights provided insight into the types of information that these provide.

A look at a router’s internal elements focused on the four main memory storage areas used on Cisco routers, including Flash, RAM, ROM, and NVRAM. The information contained within each was examined, as were their associated characteristics.

An overview of the Cisco IOS provided insight into features sets and the different types of IOS releases provided by Cisco. The ability to distinguish a major release from an early deployment release was discussed, as were the purposes of maintenance and general deployment releases.

Cisco IOS Versions

Before we get into the configuration of a Cisco router in Chapter 7, we should first understand a little more about the Cisco IOS. This is an area that many people find confusing, especially when just starting out. Between the various IOS versions and release codes, it’s easy to understand why.

Note: You are not explicitly required to be familiar with the specifics of IOS versions for either the CCNA or CCDA exams. This information is provided for practical purposes only, and to give you some insight into how IOS images are named.

The Cisco IOS is specialized operating system software that was originally designed for Cisco’s routers, but is now also used on much of their switching equipment. The key thing to understand is that Cisco IOS software is specific to the particular hardware platform that you are using. For example, while a release such as IOS version 12.0 will work across many platforms, the specific images provided for the Cisco 2500 series are not for use on 2600 series routers.

Cisco breaks its IOS software releases into groupings referred to as “feature sets”. Many different feature sets will be available for a given hardware platform, and allow different capabilities depending upon the requirements of a given environment. For example, the basic IP feature set image does not include IPSec encryption capabilities, but the IP Plus IPSec 56 feature set image does. In any network environment, required features will vary according to the protocols, security requirements, and functionality needed. When purchasing or upgrading the IOS for a router, it is important to be sure that you have chosen a feature set image that meets the needs of your environment. Of course, the IOS feature set can always be changed (at a cost) if your requirements change.

Cisco releases three major types of IOS versions – major releases, early deployment (ED) releases, and general deployment (GD) releases.

A major release is developed to ensure high quality and stable software for customer networks. Once issued, no new features are ever added to a major release – changes are limited to bug fixes, which are implemented in the form of maintenance updates. For example, you may come across IOS software version 12.1(3). In this case, the number 12.1 identifies the major release. The bracketed 3 identifies the fact that it is the third maintenance release. When any new bug fixes are applied and integrated into a release, a new maintenance number will be associated with it. In this case, the next would be 12.1(4). Maintenance releases are intended to add to a major release’s stability. They tend to be issued approximately every 8 weeks in the early portion of the major release’s lifecycle.

In order to satisfy customers who require access to new or emerging features and functions, Cisco also releases what are referred to as early deployment (ED) releases. These releases include all of the functionality of the associated major release, but also include additional features (and potentially bug fixes for those features). ED releases always end with a capitalized letter – in most cases, the letter “T”. 12.1(3)T represents major release 12.1, maintenance release 3, and an ED release. In any given maintenance release, new features may also be added. A variety of trailing capitalized letters can refer to an ED release. For example, releases ending in “X” represent a one-time-only release.

The final status that Cisco applies to an IOS version is general deployment. While a major release will be deployed to customers along with updated maintenance versions, a product is not considered to be certified for General Deployment (GD) until it has been qualified through extensive exposure to customer networks, analyzed by Cisco engineering groups, and customer feedback has been assessed. A cross-functional team at Cisco decides when the IOS version receives GD status. In the 12.1 IOS major release, everything from maintenance release 13 forward was granted GD status. Simply, GD status means that the stability of the release is considered proven by Cisco and validated by customer experience.

Non-Volatile RAM (NVRAM)

Non-Volatile Random Access Memory (NVRAM) is used as the storage location for the router’s startup configuration file. After the router loads its IOS image, the settings found in the startup configuration are applied. When changes are made to a router’s running configuration, they should always be saved to the startup configuration (stored in NVRAM) or they will be lost when the router shuts down. Remember that the running configuration is stored in RAM, which is erased when the router is powered down. On a Cisco 2500 series router, NVRAM is a relatively tiny 32KB in size.

Figure: Cisco router memory elements and purposes. 

Read-Only Memory (ROM)

In older Cisco router models, Read-Only Memory (ROM) chips were used to store the IOS software. In newer models, this is no longer the case. As mentioned previously, the IOS image is now stored in Flash memory (it can also be stored on a TFTP server, as I’ll discuss in the next chapter). ROM is now used as the memory area from which a Cisco router begins the boot process, and is made up of a number of elements. These elements are implemented via microcode, a set of programming instructions that are contained in ROM.

  • Power-on Self Test (POST). When the router is powered up, microcode stored in ROM performs a POST sequence. This is used to ensure that elements such as the CPU, memory, and interfaces are capable of functioning correctly.
  • Bootstrap Program. The bootstrap program is used to initialize the CPU and boot functions of the router. The bootstrap program is responsible for locating and loading the router’s IOS.
  • ROM Monitor. A special diagnostic environment used for the purpose of troubleshooting or special configuration. For example, this mode can be used to transfer an IOS image over a console connection.
  • RxBoot. When a valid IOS image cannot be found in Flash or on a TFTP server, this limited IOS version is loaded for the purpose of installing a new IOS image into Flash. It is also sometimes referred to as the boot loader, boot image, or helper image. The command set provided is only a subset of normal IOS commands.

On Cisco 2500 series routers, ROM is 2MB in size. In cases where ROM needs to be upgraded (which is rare), the actual chips needs to be replaced on the router’s motherboard. When a router is powered down, the contents of ROM are always retained.

Random Access Memory (RAM)

Random Access Memory (RAM) represents the non-permanent or volatile working area of memory on a Cisco router. When the router is powered down, the contents of RAM are lost.

By default, RAM is broken up into two main areas – Main Processor Memory, and Shared I/O Memory. Main Processor Memory is where the routing table, ARP tables, and running configuration are stored. Shared I/O Memory is used as a buffer location for temporarily storing packets prior to processing. Most Cisco 2500 routers will have 2MB of RAM soldered to the system board (this amount, however, depends on the revision number of the router), along with one SIMM slot to add additional RAM. The maximum amount of RAM that can be added to a Cisco 2500 is 16MB. If 16MB is added, that provides a maximum of 18MB of available RAM. In cases where a RAM SIMM is installed, its capacity will be used as Main Processor Memory, while the onboard RAM (2MB) will be used as Shared I/O memory. If no SIMM chip is present, that 2MB of on-board RAM will be split between both areas, providing each with 1MB of working space. This should be avoided for performance reasons.

Flash Memory

Flash memory is implemented on a Cisco 2500 using two Single Inline Memory Module (SIMM) slots that hold erasable programmable read-only memory (EPROM). Flash memory is used to store and run the Cisco IOS software. When a system is powered down, the contents of Flash memory are not lost. However, its contents can be upgraded by “flashing” the chip, a concept that we’ll look at in more detail in Chapter 12. While a router is running, the contents of Flash are set to a read-only mode.

Flash memory for a Cisco 2500 series router ranges in size from a minimum of 4MB up to a maximum of 16MB. You might consider adding additional Flash memory to meet the space requirements of the IOS version that you have chosen to run. For a Cisco 2501, the base IP version of IOS 12.0 requires a minimum of 8MB of Flash memory. If your Cisco 2501 shipped with only 4MB of Flash, you would require at least one additional 4MB SIMM. For IOS versions with more advanced feature sets, it is not uncommon to require at least 16MB of Flash.

When installing or upgrading Flash using multiple SIMMs, it is important to note that they must be the same size. For example, if you already have 4MB of Flash and wish to upgrade, you can either replace the 4MB SIMM with an 8MB SIMM, or simply add a second 4MB SIMM. You cannot mix and match SIMMs with different storage capabilities. As such, you cannot have one 4MB and one 8MB SIMM installed at the same time – their storage capabilities must equal.

Router Internals

The internal components of the router are where the real magic takes place. Think of a Cisco router are being really no more than a specialized computer running a custom operating system. In this case, it is a computer optimized to provide routing and related functions. Instead of relying on a hard disk for storage, a Cisco router relies on different types of memory, each with a different purpose. In this section we’ll take a look at each of these different storage areas, and the functions they are responsible for.

There are four main memory areas within a Cisco router that you’ll need to be familiar with – Flash, RAM, ROM, and NVRAM.

LED Lights on a Cisco Router

On any given Cisco router, the LED lights provide quick insight into whether the system or ports are functioning correctly. On a Cisco 2500 series router, each port (with the exception of the console and auxiliary ports) has an associated LED that indicates port activity, in a fashion similar to what you might expect to see on a switch or hub. Another LED, to the right of the auxiliary port, is referred to as the “System OK” LED. When this LED is lit, it indicates that the router is functioning correctly.

In a normal working state, port LEDs will flicker to indicate activity. In cases where the port LED is not active, it suggests either a connectivity issue, or potentially a problem with the port.

Obviously a router needs to be powered. In cases where the router needs to be opened in order to change or upgrade components, not only should it be powered down using the power switch, but the Alternating Current (AC) cable should also be disconnected. Cisco routers are usually also available in Direct Current (DC) versions, since many telecommunications carriers require DC-powered devices. In cases where DC devices need to be powered down, the power switch should be turned off, and then power should be disconnected at the circuit breaker.

Auxiliary Port on a Cisco Router

The purpose of the auxiliary port is for connecting to an external modem. Once configured, this modem can be used as a backup demand-dial connection to another location, or as a way to dial in to the router for troubleshooting purposes should regular connectivity fail. Unlike the console port, the auxiliary port supports hardware flow control, which ensures that the receiving device receives all data before the sending device transmits more. In cases where the receiving device’s buffers become full, it can pass a message to the sender asking it to temporarily suspend transmission. This makes the auxiliary port capable of handling the higher transmission speeds of a modem.

Much like the console port, the auxiliary port is also an asynchronous serial port with an RJ-45 interface. Similarly, a rollover cable is also used for connections, using a DB-25 adapter that connects to the modem. Typically, this adapter is labeled “MODEM”.

Rollover Cables

In order to connect to the console port, you’re going to require what Cisco calls a rollover cable. While these are usually provided with a router, they are also exceptionally simple to create. All that you require is a reasonable length of twisted pair cabling, some RJ-45 connectors, and the ability to remember what rollover means.

The pinouts of a rollover cable are simple to remember because they relate directly to the cable name – the ends of the cable are actually “rolled”, with the pins directly opposite one another in terms of their connections. In that way, pin 1 on one end connects to pin 8 on the other, while pin 2 connects to pin 7, and so forth. There is no need to pay attention to colors when creating a rollover cable. Just ensure that the cables on one end are in reverse order on the other end.

As a best practice, make a point of identifying the various cables you create to avoid confusion. Use different colored cables if possible, or at least consider adding letters such as “X” (for crossover) or “R” (for rollover) to the cable ends using a permanent marker.