cisco learning

noreply@blogger.com (andreas) — Fri, 22 May 2009 03:04:00 +0000

in the first cisco learning we will learn about what is a router, and the components and their functions.

Define Router

A router is a computer, just like any other computer including a PC. The very first router, used for the Advanced Research Projects Agency Network (ARPANET), was the Interface Message Processor (IMP). The IMP was a Honeywell 316 minicomputer; this computer brought the ARPANET to life on August 30, 1969.

Note: The ARPANET was developed by Advanced Research Projects Agency (ARPA) of the United States Department of Defense. The ARPANET was the world's first operational packet switching network and the predecessor of today's Internet.

Routers have many of the same hardware and software components that are found in other computers including:
- CPU
- RAM
- ROM
- Operating System

Routers determine the best path
The primary responsibility of a router is to direct packets destined for local and remote networks by:
- Determining the best path to send packets
- Forwarding packets toward their destination

The router uses its routing table to determine the best path to forward the packet. When the router receives a packet, it examines its destination IP address and searches for the best match with a network address in the router's routing table. The routing table also includes the interface to be used to forward the packet. Once a match is found, the router encapsulates the IP packet into the data link frame of the outgoing or exit interface, and the packet is then forwarded toward its destination.

It is very likely that a router will receive a packet that is encapsulated in one type of data link frame, such as an Ethernet frame and when forwarding the packet, the router will encapsulate it in a different type of data link frame, such as Point-to-Point Protocol (PPP). The data link encapsulation depends on the type of interface on the router and the type of medium it connects to. The different data link technologies that a router connects to can include LAN technologies, such as Ethernet, and WAN serial connections, such as T1 connection using PPP, Frame Relay, and Asynchronous Transfer Mode (ATM).

Static routes and dynamic routing protocols are used by routers to learn about remote networks and build their routing tables. These routes and protocols are the primary focus of the course and will be discussed in detail in later chapters along with the process that routers use in searching their routing tables and forwarding the packets.

Router Components and their Functions
Like a PC, a router also includes:
- Central Processing Unit (CPU)
- Random-Access Memory (RAM)
- Read-Only Memory (ROM)

Roll over components in the figure to see a brief description of each.

CPU
The CPU executes operating system instructions, such as system initialization, routing functions, and switching functions.

RAM
RAM stores the instructions and data needed to be executed by the CPU. RAM is used to store these components:
Operating System: The Cisco IOS (Internetwork Operating System) is copied into RAM during bootup.

Running Configuration File: This is the configuration file that stores the configuration commands that the router IOS is currently using. With few exceptions, all commands configured on the router are stored in the running configuration file, known as running-config.
IP Routing Table: This file stores information about directly connected and remote networks. It is used to determine the best path to forward the packet.
ARP Cache: This cache contains the IPv4 address to MAC address mappings, similar to the ARP cache on a PC. The ARP cache is used on routers that have LAN interfaces such as Ethernet interfaces.

Packet Buffer: Packets are temporarily stored in a buffer when received on an interface or before they exit an interface.

ROM
ROM is a form of permanent storage. Cisco devices use ROM to store:
- The bootstrap instructions
- Basic diagnostic software
- Scaled-down version of IOS.

Flash Memory
Flash memory is nonvolatile computer memory that can be electrically stored and erased. Flash is used as permanent storage for the operating system, Cisco IOS. In most models of Cisco routers, the IOS is permanently stored in flash memory and copied into RAM during the bootup process, where it is then executed by the CPU. Some older models of Cisco routers run the IOS directly from flash. Flash consists of SIMMs or PCMCIA cards, which can be upgraded to increase the amount of flash memory.

NVRAM
NVRAM (Nonvolatile RAM) does not lose its information when power is turned off. This is in contrast to the most common forms of RAM, such as DRAM, that requires continual power to maintain its information. NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file (startup-config). All configuration changes are stored in the running-config file in RAM, and with few exceptions, are implemented immediately by the IOS. To save those changes in case the router is restarted or loses power, the running-config must be copied to NVRAM, where it is stored as the startup-config file. NVRAM retains its contents even when the router reloads or is powered off.

Bootup Process
There are four major phases to the bootup process:
1. Performing the POST
The Power-On Self Test (POST) is a common process that occurs on almost every computer during bootup. The POST process is used to test the router hardware. When the router is powered on, software on the ROM chip conducts the POST. During this self-test, the router executes diagnostics from ROM on several hardware components including the CPU, RAM, and NVRAM. After the POST has been completed, the router executes the bootstrap program.
Loading the Bootstrap Program

2. After the POST, the bootstrap program is copied from ROM into RAM. Once in RAM, the CPU executes the instructions in the bootstrap program. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM.

3. Locating and Loading Cisco IOS
Locating the Cisco IOS software. The IOS is typically stored in flash memory, but can also be stored in other places such as a TFTP (Trivial File Transfer Protocol) server.
If a full IOS image can not be located, a scaled-down version of the IOS is copied from ROM into RAM. This version of IOS is used to help diagnose any problems and can be used to load a complete version of the IOS into RAM.

4. Locating and Loading the Configuration File
Locating the Startup Configuration File. After the IOS is loaded, the bootstrap program searches for the startup configuration file, known as startup-config, in NVRAM. This file has the previously saved configuration commands and parameters including:
- interface addresses
- routing information
- passwords
- any other configurations saved by the network administrator
If the startup configuration file, startup-config, is located in NVRAM, it is copied into RAM as the running configuration file, running-config.

noreply@blogger.com (andreas) — Thu, 21 May 2009 11:36:00 +0000

Network discovery with CDP

Cisco Discovery Protocol (CDP) is a powerful network monitoring and troubleshooting tool. CDP is an information-gathering tool used by network administrators to get information about directly connected Cisco devices. CDP is a proprietary tool that enables you to access a summary of protocol and address information about Cisco devices that are directly connected. By default, each Cisco device sends periodic messages, which are known as CDP advertisements, to directly connected Cisco devices. These advertisements contain information such as the types of devices that are connected, the router interfaces they are connected to, the interfaces used to make the connections, and the model numbers of the devices.

Most network devices, by definition, do not work in isolation. A Cisco device frequently has other Cisco devices as neighbors on the network. Information gathered from other devices can assist you in making network design decisions, troubleshooting, and making changes to equipment. CDP can be used as a network discovery tool, helping you to build a logical topology of a network when such documentation is missing or lacking in detail.

Familiarity with the general concept of neighbors is important for understanding CDP as well as for future discussions about dynamic routing protocols.

Layer 3 Neighbors

At this point in our topology configuration, we only have directly connected neighbors. At Layer 3, routing protocols consider neighbors to be devices that share the same network address space.

Layer 2 Neighbors

CDP operates at Layer 2 only. Therefore, CDP neighbors are Cisco devices that are directly connected physically and share the same data link. In the CDP Protocol figure, the network administrator is logged in to S3. S3 will receive CDP advertisements from S1, S2, and R2 only.

noreply@blogger.com (andreas) — Mon, 11 May 2009 03:03:00 +0000

RIP Timers

In addition to the update timer, the IOS implements three additional timers for RIP:
- Invalid
- Flush
- Holddown

Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing table until the flush timer expires.

Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.

Holddown Timer. This timer stabilizes routing information and helps prevent routing loops during periods when the topology is converging on new information. Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology to learn about the unreachable network. By default, the holddown timer is set for 180 seconds. The holddown timer is discussed in more detail later in this chapter.

noreply@blogger.com (andreas) — Mon, 11 May 2009 02:49:00 +0000

Static Routing

A static route includes the network address and subnet mask of the remote network, along with the IP address of the next-hop router or exit interface. Static routes are denoted with the code S in the routing table as shown in the figure. Static routes are examined in detail in the next chapter.

When to Use Static Routes :
Static routes should be used in the following cases:
- A network consists of only a few routers. Using a dynamic routing protocol in such a case does not present any substantial benefit. On the contrary, dynamic routing may add more administrative overhead.
- A network is connected to the Internet only through a single ISP. There is no need to use a dynamic routing protocol across this link because the ISP represents the only exit point to the Internet.
- A large network is configured in a hub-and-spoke topology. A hub-and-spoke topology consists of a central location (the hub) and multiple branch locations (spokes), with each spoke having only one connection to the hub. Using dynamic routing would be unnecessary because each branch has only one path to a given destination-through the central location.

Set Static Routing Protokols
On the routers, enter global configuration mode and configure the basic global configuration commands including:
-hostname
- enable secret
Example :
-R1(config)#interface fastethernet 0/0
-R1(config-if)#ip address 172.16.3.1 255.255.255.0
-R1(config-if)#no shutdown
Configure the IP address as specified in the Topology Diagram.
- R1(config-if)#ip address 172.16.3.1 255.255.255.0

To configure static routes with a next-hop specified, use the following syntax:
- Router(config)# ip route network-address subnet-mask ip-address
- network-address:—Destination network address of the remote network to be added to the
routing table.
- subnet-mask—Subnet mask of the remote network to be added to the routing table. The subnet mask can be modified to summarize a group of networks.
- ip-address—Commonly referred to as the next-hop router’s IP address.
- R3(config)#ip route 172.16.1.0 255.255.255.0 192.168.1.2

noreply@blogger.com (andreas) — Mon, 11 May 2009 02:08:00 +0000

Internetwork Operating System

The operating system software used in Cisco routers is known as Cisco Internetwork Operating System (IOS). Like any operating system on any computer, Cisco IOS manages the hardware and software resources of the router, including memory allocation, processes, security, and file systems. Cisco IOS is a multitasking operating system that is integrated with routing, switching, internetworking, and telecommunications functions.

Although the Cisco IOS may appear to be the same on many routers, there are many different IOS images. An IOS image is a file that contains the entire IOS for that router. Cisco creates many different types of IOS images, depending upon the model of the router and the features within the IOS. Typically the more features in the IOS, the larger the IOS image, and therefore, the more flash and RAM that is required to store and load the IOS. For example, some features include the ability to run IPv6 or the ability for the router to perform NAT (Network Address Translation).

As with other operating systems Cisco IOS has its own user interface. Although some routers provide a graphical user interface (GUI), the command line interface (CLI) is a much more common method of configuring Cisco routers. The CLI is used throughout this curriculum.

Upon bootup, the startup-config file in NVRAM is copied into RAM and stored as the running-config file. IOS executes the configuration commands in the running-config. Any changes entered by the network administrator are stored in the running-config and are immediately implemented by the IOS. In this chapter, we will review some of the basic IOS commands used to configure a Cisco router. In later chapters, we will learn the commands used to configure, verify, and troubleshoot static routing and various routing protocols such as RIP, EIGRP, and OSPF.

noreply@blogger.com (andreas) — Mon, 11 May 2009 01:02:00 +0000

Link-state

Link-state routing protocols are also known as shortest path first protocols and built around Edsger Dijkstra's shortest path first (SPF) algorithm. The SPF algorithm will be discussed in more detail in a later section.

The IP link-state routing protocols are shown in the figure:
- Open Shortest Path First (OSPF)
- Intermediate System-to-Intermediate System (IS-IS)

Link-state routing protocols have the reputation of being much more complex than their distance vector counterparts. However, the basic functionality and configuration of link-state routing protocols is not complex at all. Even the algorithm itself can be easily understood, as you will see in the next topic. Basic OSPF operations can be configured with a router ospfprocess-id command and a network statement, similar to other routing protocols like RIP and EIGRP.

There are two link-state routing protocols used for routing IP today:
- Open Shortest Path First (OSPF)
OSPF was designed by the IETF (Internet Engineering Task Force) OSPF Working Group, which still exists today. Most of the work on OSPF was done by John Moy, author of most of the RFCs regarding OSPF. His book, OSPF, Anatomy of an Internet Routing Protocol, provides interesting insight to the development of OSPF.

- Intermediate System-to-Intermediate System (IS-IS)
IS-IS was designed by ISO (International Organization for Standardization) and is described in ISO 10589.IS-IS was originally designed for the OSI protocol suite and not the TCP/IP protocol suite. Later, Integrated IS-IS, or Dual IS-IS, included support for IP networks. Although IS-IS has been known as the routing protocol used mainly by ISPs and carriers, more enterprise networks are beginning to use IS-IS.

OSPF and IS-IS share many similarities and also have many differences. There are many pro-OSPF and pro-IS-IS factions who discuss and debate the advantages of one routing protocol over the other. Both routing protocols provide the necessary routing functionality needed. You can learn more about IS-IS and OSPF in CCNP and begin to make your own determination if one protocol is more advantageous than the other.

noreply@blogger.com (andreas) — Mon, 11 May 2009 01:01:00 +0000

Basic Router Configuration

When configuring a router, certain basic tasks are performed including:
- Naming the router
- Setting passwords
- Configuring interfaces
- Configuring a banner
- Saving changes on a router
- Verifying basic configuration and router operations

The first prompt appears at user mode. User mode allows you to view the state of the router, but does not allow you to modify its configuration. Do not confuse the term "user" as used in user mode with users of the network. User mode is intended for the network technicians, operators, and engineers who have the responsibility to configure network devices.
The enable command is used to enter the privileged EXEC mode. This mode allows the user to make configuration changes on the router. The router prompt will change from a ">" to a "#" in this mode.

Hostnames and Passwords
- Router#config t
- Router(config)#enable secret class
Next, configure the console and Telnet lines with the password cisco.
- R1(config)#line console 0
- R1(config-line)#password cisco
- R1(config-line)#login
- R1(config)#line vty 0 4
- R1(config-line)#password cisco
- R1(config-line)#login

Configuring a Banner
From the global configuration mode, configure the message-of-the-day (motd) banner. A delimiting character, such as a "#" is used at the beginning and at the end of the message.
R1(config)#banner motd #
Enter TEXT message. End with the character '#'.

Verifying Basic Router Configuration
Currently in the example, all of the previous basic router configuration commands have been entered and were immediately stored in the running configuration file of R1. The running-config file is stored in RAM and is the configuration file used by IOS. The next step is to verify the commands entered by displaying the running configuration with the following command:

R1#show running-config

Now that the basic configuration commands have been entered, it is important to save the running-config to the nonvolatile memory, the NVRAM of the router. That way, in case of a power outage or an accidental reload, the router will be able to boot with the current configuration. After the router's configuration has been completed and tested, it is important to save the running-config to the startup-config as the permanent configuration file.

- Router1#copy running-config startup-config

After applying and saving the basic configuration, you can use several commands to verify that you have correctly configured the router. Click the appropriate button in the figure to see a listing of each command's output. All of these commands are discussed in detail in later chapters. For now, begin to become familiar with the output.

- Router1#show startup-config
This command displays the startup configuration file stored in NVRAM.

- Router1#show ip route
This command displays the routing table that the IOS is currently using to choose the best path to its destination networks.

- Router1#show interfaces
This command displays all of the interface configuration parameters and statistics. Some of this information is discussed later in the curriculum and in CCNP

- Router1#show ip interface brief
This command displays abbreviated interface configuration information, including IP address and interface status

noreply@blogger.com (andreas) — Mon, 11 May 2009 01:00:00 +0000

Distance vector

As the name implies, distance vector means that routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface. A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network.
Distance vector routing protocols include RIP, IGRP, and EIGRP.

RIP
Over the years, RIP has evolved from a classful routing protocol (RIPv1) to a classless routing protocol (RIPv2). RIPv2 is a standardized routing protocol that works in a mixed vendor router environment. Routers made by different companies can communicate using RIP.
Features of RIP:
- Supports split horizon and split horizon with poison reverse to prevents loops.
- Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.
RIPv2 introduced the following improvements to RIPv1:
- Includes the subnet mask in the routing updates, making it a classless routing protocol.
- Has authentication mechanism to secure routing table updates.
- Supports variable length subnet mask (VLSM).
- Uses multicast addresses instead of broadcast.
- Supports manual route summarization.

IGRP
Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco. IGRP has the following key design characteristics:
Bandwidth, delay, load and reliability are used to create a composite metric.
Routing updates are broadcast every 90 seconds, by default.
IGRP is the predecessor of EIGRP and is now obsolete.

EIGRP
Enhanced IGRP (EIGRP) was developed from IGRP, another distance vector protocol. EIGRP is a classless, distance vector routing protocol with features found in link-state routing protocols.
EIGRP features include:
- Triggered updates (EIGRP has no periodic updates).
- Use of a topology table to maintain all the routes received from neighbors (not only the best paths).
- Establishment of adjacencies with neighboring routers using the EIGRP hello protocol.
- Support for VLSM and manual route summarization. These allow EIGRP to create hierarchically structured large networks.
Advantages of EIGRP:
- Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path rather than hop count.
- Fast convergence due to Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers.
- Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes.

noreply@blogger.com (andreas) — Mon, 11 May 2009 00:58:00 +0000

Autonomous System

An autonomous system (AS) is a collection of networks under the administrative control of a single entity that presents a common routing policy to the Internet. In the figure, companies A, B, C, and D are all under the administrative control of ISP1. ISP1 "presents a common routing policy" for all of these companies when advertising routes to ISP2.

The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930. AS numbers are assigned by the Internet Assigned Numbers Authority (IANA), the same authority that assigns IP address space. You learned about IANA and its Regional Internet Registries (RIRs) in a previous course. The local RIR is responsible for assigning an AS number to an entity from its block of assigned AS numbers. Prior to 2007, AS numbers were 16-bit numbers, ranging from 0 to 65535. Now 32-bit AS numbers are assigned, increasing the number of available AS numbers to over 4 billion.

Who needs an autonomous system number? Usually ISPs (Internet Service Providers), Internet backbone providers, and large institutions connecting to other entities that also have an AS number. These ISPs and large institutions use the exterior gateway routing protocol Border Gateway Protocol, or BGP, to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.

The vast majority of companies and institutions with IP networks do not need an AS number because they come under the control of a larger entity such as an ISP. These companies use interior gateway protocols such as RIP, EIGRP, OSPF, and IS-IS to route packets within their own networks. They are one of many independent and separate networks within the autonomous system of the ISP. The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems

noreply@blogger.com (andreas) — Mon, 11 May 2009 00:45:00 +0000

RIP

RIP is the oldest of the distance vector routing protocols. Although RIP lacks the sophistication of more advanced routing protocols, its simplicity and continued widespread use is a testament to its longevity. RIP is not a protocol "on the way out." In fact, an IPv6 form of RIP called RIPng (next generation) is now available.

RIP Characteristics :
- RIP is a distance vector routing protocol.
- RIP uses hop count as its only metric for path selection.
- Advertised routes with hop counts greater than 15 are unreachable.
- Messages are broadcast every 30 seconds.

To enter the router configuration mode for RIP, enter router rip at the global configuration prompt. Notice that the prompt changes from a global configuration prompt to the following:

- R1(config-router)#

This command does not directly start the RIP process. Instead, it provides access to configure routing protocol settings. No routing updates are sent.
To enable RIP routing for a network, use the network command in the router configuration mode and enter the classful network address for each directly connected network.

- Router(config-router)#network directly-connected-classful-network-address

The network command:
- Enables RIP on all interfaces that belong to a specific network. Associated interfaces will now both send and receive RIP updates.
- Advertises the specified network in RIP routing updates sent to other routers every 30 seconds.
To enable RIP routing for a network, use the network command in the router configuration mode and enter the classful network address for each directly connected network.

- Router(config-router)#network directly-connected-classful-network-address

Command for RIP configurations
Example :
- Router(config)#router rip
- Router(config-router)#network 192.168.17.0
- Router(config-router)#network 192.168.18.0

Use the version 2 command to enable RIP version 2
- Router(config)#router rip
- Router(config-router)#version 2

noreply@blogger.com (andreas) — Mon, 11 May 2009 00:34:00 +0000

Classful Routing Protocols and Classless Routing Protocols

Classful Routing Protocols

Classful routing protocols do not send subnet mask information in routing updates. The first routing protocols such as RIP, were classful. This was at a time when network addresses were allocated based on classes, class A, B, or C. A routing protocol did not need to include the subnet mask in the routing update because the network mask could be determined based on the first octet of the network address.

Classful routing protocols can still be used in some of today's networks, but because they do not include the subnet mask they cannot be used in all situations. Classful routing protocols cannot be used when a network is subnetted using more than one subnet mask, in other words classful routing protocols do not support variable length subnet masks (VLSM). There are other limitations to classful routing protocols including their inability to support discontiguous networks.
Classful routing protocols include RIPv1 and IGRP.

Classless Routing Protocols

Classless routing protocols include the subnet mask with the network address in routing updates. Today's networks are no longer allocated based on classes and the subnet mask cannot be determined by the value of the first octet. Classless routing protocols are required in most networks today because of their support for VLSM, discontiguous networks and other features which will be discussed in later chapters.
In the figure, notice that the classless version of the network is using both /30 and /27 subnet masks in the same topology. Also notice that this topology is using a discontiguous design..
Classless routing protocols are RIPv2, EIGRP, OSPF, IS-IS, BGP.

noreply@blogger.com (andreas) — Mon, 11 May 2009 00:03:00 +0000

Switching Function

After the router has determined the exit interface using the path determination function, the router needs to encapsulate the packet into the data link frame of the outgoing interface.

The switching function is the process used by a router to accept a packet on one interface and forward it out another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.

What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps:

1. Decapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer.

2. Examines the destination IP address of the IP packet to find the best path in the routing table.

3. Encapsulates Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.

noreply@blogger.com (andreas) — Sun, 10 May 2009 23:59:00 +0000

Routing Table Principles

At times in this course we will refer to three principles regarding routing tables that will help you understand, configure, and troubleshoot routing issues. These principles are from Alex Zinin's book, Cisco IP Routing.

1. Every router makes its decision alone, based on the information it has in its own routing table.

2. The fact that one router has certain information in its routing table does not mean that other routers have the same information.

3. Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.

Asymmetric Routing

Because routers do not necessarily have the same information in their routing tables, packets can traverse the network in one direction, using one path, and return via another path. This is called asymmetric routing. Asymmetric routing is more common in the Internet, which uses the BGP routing protocol than it is in most internal networks.

This example implies that when designing and troubleshooting a network, the network administrator should check the following routing information:
Is there a path from source to destination available in both directions?
Is the path taken in both directions the same path? (Asymmetrical routing is not uncommon, but sometimes can pose additional issues)

noreply@blogger.com (andreas) — Sun, 10 May 2009 23:56:00 +0000

Routers Operate at Layers 1, 2, and 3

A router makes its primary forwarding decision at Layer 3, but as we saw earlier, it participates in Layer 1 and Layer 2 processes as well. After a router has examined the destination IP address of a packet and consulted its routing table to make its forwarding decision, it can forward that packet out the appropriate interface toward its destination. The router encapsulates the Layer 3 IP packet into the data portion of a Layer 2 data link frame appropriate for the exit interface. The type of frame can be an Ethernet, HDLC, or some other Layer 2 encapsulation - whatever encapsulation is used on that particular interface. The Layer 2 frame is encoded into the Layer 1 physical signals that are used to represent bits over the physical link.

To understand this process better, refer to the figure. Notice that PC1 operates at all seven layers, encapsulating the data and sending the frame out as a stream of encoded bits to R1, its default gateway.

R1 receives the stream of encoded bits on its interface. The bits are decoded and passed up to Layer 2, where R1 decapsulates the frame. The router examines the destination address of the data link frame to determine if it matches the receiving interface, including a broadcast or multicast address. If there is a match with the data portion of the frame, the IP packet is passed up to Layer 3, where R1 makes its routing decision. R1 then re-encapsulates the packet into a new Layer 2 data link frame and forwards it out the outbound interface as a stream of encoded bits.

R2 receives the stream of bits, and the process repeats itself. R2 decapsulates the frame and passes the data portion of the frame, the IP packet, to Layer 3 where R2 makes its routing decision. R2 then re-encapsulates the packet into a new Layer 2 data link frame and forwards it out the outbound interface as a stream of encoded bits.

This process is repeated once again by router R3, which forwards the IP packet, encapsulated inside a data link frame and encoded as bits, to PC2.

Each router in the path from source to destination performs this same process of decapsulation, searching the routing table, and then re-encapsulation. This process is important to your understanding of how routers participate in networks. Therefore, we will revisit this discussion in more depth in a later section.

noreply@blogger.com (andreas) — Sun, 10 May 2009 11:39:00 +0000

Download Example Configuration

Download EIGRP

Download OSPF

Download Dinamic Routing

Download RIP

Download Static Routing

noreply@blogger.com (andreas) — Sun, 10 May 2009 11:25:00 +0000

Dynamic Routing

Dynamic routing protocols are used by routers to share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including:
Network discovery
- Updating and maintaining routing tables
- Automatic Network Discovery

Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of configuring static routes to remote networks on every router, a dynamic routing protocol allows the

Automatic Network Discovery
Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of configuring static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks - and the best path to each network - are added to the router's routing table and denoted as a network learned by a specific dynamic routing protocol.

Maintaining Routing Tables
After the initial network discovery, dynamic routing protocols update and maintain the networks in their routing tables. Dynamic routing protocols not only make a best path determination to various networks, they will also determine a new best path if the initial path becomes unusable (or if the topology changes). For these reasons, dynamic routing protocols have an advantage over static routes. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.

noreply@blogger.com (andreas) — Sun, 10 May 2009 11:21:00 +0000

Management Ports

Routers have physical connectors that are used to manage the router. These connectors are known as management ports. Unlike Ethernet and serial interfaces, management ports are not used for packet forwarding. The most common management port is the console port. The console port is used to connect a terminal, or most often a PC running terminal emulator software, to configure the router without the need for network access to that router. The console port must be used during initial configuration of the router.

Another management port is the auxiliary port. Not all routers have auxiliary ports. At times the auxiliary port can be used in ways similar to a console port. It can also be used to attach a modem. Auxiliary ports will not be used in this curriculum.

The figure shows the console and AUX ports on the router.

Router Interfaces

The term interface on Cisco routers refers to a physical connector on the router whose main purpose is to receive and forward packets. Routers have multiple interfaces that are used to connect to multiple networks. Typically, the interfaces connect to various types of networks, which means that different types of media and connectors are required. Often a router will need to have different types of interfaces. For example, a router usually has FastEthernet interfaces for connections to different LANs and various types of WAN interfaces to connect a variety of serial links including T1, DSL and ISDN. The figure shows the FastEthernet and serial interfaces on the router.

Like interfaces on a PC, the ports and interfaces on a router are located on the outside of the router. Their external location allows for convenient attachment to the appropriate network cables and connectors.

Note: A single interface on a router can be used to connect to multiple networks; however, this is beyond the scope of this course and is discussed in a later course.

Like most networking devices, Cisco routers use LED indicators to provide status information. An interface LED indicates the activity of the corresponding interface. If an LED is off when the interface is active and the interface is correctly connected, this may be an indication of a problem with that interface. If an interface is extremely busy, its LED will always be on. Depending on the type of router, there may be other LEDs as well. For more information on LED displays on the 1841, see the link below.

noreply@blogger.com (andreas) — Sun, 10 May 2009 06:52:00 +0000

Configuring Interface Router

Configuring an Ethernet Interface

As shown, R1 does not yet have any routes. Let's add a route by configuring an interface and explore exactly what happens when that interface is activated. By default, all router interfaces are shutdown, or turned off. To enable this interface, use the no shutdown command, which changes the interface from administratively down to up.

- R1(config)#interface fastethernet 0/0
- R1(config-if)#ip address 192.169.17.1 255.255.255.0
- R1(config-if)#no shutdown
Note: Although enabled with no shutdown, an Ethernet interface will not be active, or up, unless it is receiving a carrier signal from another device (switch, hub, PC, or another router).

Configuring Serial Interface

- R1(config)#interface serial 0/0/0
- R1(config-if)#ip address 172.168.18.1 255.255.255.0
- R1(config-if)#no shutdown

If we now issue the show interfaces serial 0/0/0 command on either router, we still see that the link is up/down.

The physical link between R1 and R2 is up because both ends of the serial link have been configured correctly with an IP address/mask and enabled with the no shutdown command. However, the line protocol is still down. This is because the interface is not receiving a clock signal. There is still one more command that we need to enter, the clock rate command, on the router with the DCE cable. The clock rate command will set the clock signal for the link. Configuring the clock signal will be discussed in the next section.

Commands to Verify Interface Configuration:

1. Click show ip interface brief in the figure.
The show ip interface brief command also shows verifies this same information. Under the status and protocol, you should see "up".

2. The show running-config command shows the current configuration of this interface. When the interface is disabled, the running-config command displays shutdown; however, when the interface is enabled, no shutdown is not displayed.

noreply@blogger.com (andreas) — Sun, 10 May 2009 06:17:00 +0000

Download Lab Activity

Basic RouterConfiguration

Basic Configuration OSPF

Subnetting 1

Basic RIP configuration

VLSM

Basic Static Route Configuration

Subnetting 3

Basic RIP ver 2 Configuration

noreply@blogger.com (andreas) — Sun, 10 May 2009 01:35:00 +0000

Multiple Routing Sources

We know that routers learn about adjacent networks that are directly connected and about remote networks by using static routes and dynamic routing protocols. In fact, a router might learn of a route to the same network from more than one source. For example, a static route might have been configured for the same network/subnet mask that was learned dynamically by a dynamic routing protocol, such as RIP. The router must choose which route to install.
Although less common, more than one dynamic routing protocol can be deployed in the same network. In some situations it may be necessary to route the same network address using multiple routing protocols such as RIP and OSPF. Because different routing protocols use different metrics, RIP uses hop count and OSPF uses bandwidth, it is not possible to compare metrics to determine the best path.

So, how does a router determine which route to install in the routing table when it has learned about the same network from more than one routing source?

The Purpose of Administrative Distance

Administrative distance (AD) defines the preference of a routing source. Each routing source - including specific routing protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when it learns about the same destination network from two or more different routing sources.

Administrative distance is an integer value from 0 to 255. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed.

It is possible to modify the administrative distance for static routes and dynamic routing protocols. This is discussed in CCNP.

An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table

noreply@blogger.com (andreas) — Sun, 10 May 2009 01:05:00 +0000

IGP and EGP

Interior Gateway Protocols (IGP) are used for intra-autonomous system routing - routing inside an autonomous system. Exterior Gateway Protocols (EGP) are used for inter-autonomous system routing - routing between autonomous systems

Characteristics of IGP and EGP Routing Protocols
IGPs are used for routing within a routing domain, those networks within the control of a single organization. An autonomous system is commonly comprised of many individual networks belonging to companies, schools, and other institutions. An IGP is used to route within the autonomous system, and also used to route within the individual networks themselves. For example, CENIC operates an autonomous system comprised of California schools, colleges and universities. CENIC uses an IGP to route within its autonomous system in order to interconnect all of these institutions. Each of the educational institutions also uses an IGP of their own choosing to route within its own individual network. The IGP used by each entity provides best path determination within its own routing domains, just as the IGP used by CENIC provides best path routes within the autonomous system itself. IGPs for IP include RIP, IGRP, EIGRP, OSPF, and IS-IS.

Routing protocols, and more specifically the algorithm used by that routing protocol, use a metric to determine the best path to a network. The metric used by the routing protocol RIP is hop count, which is the number of routers that a packet must traverse in reaching another network. OSPF uses bandwidth to determine the shortest path.

EGPs on the other hand, are designed for use between different autonomous systems that are under the control of different administrations. BGP is the only currently-viable EGP and is the routing protocol used by the Internet. BGP is a path vector protocol that can use many different attributes to measure routes. At the ISP level, there are often more important issues than just choosing the fastest path. BGP is typically used between ISPs and sometimes between a company and an ISP. BGP is not part of this course or CCNA; it is covered in CCNP.

noreply@blogger.com (andreas) — Sun, 10 May 2009 00:41:00 +0000

The Metric Parameters

Different routing protocols use different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol. Two different routing protocols might choose different paths to the same destination due to using different metrics.

Metrics used in IP routing protocols include:
- Hop count - A simple metric that counts the number of routers a packet must traverse
Bandwidth - Influences path selection by preferring the path with the highest bandwidth
- Load - Considers the traffic utilization of a certain link
- Delay - Considers the time a packet takes to traverse a path
- Reliability - Assesses the probability of a link failure, calculated from the interface error count or previous link failures
- Cost - A value determined either by the IOS or by the network administrator to indicate preference for a route. Cost can represent a metric, a combination of metrics or a policy.

The Metric Field in the Routing Table

The metric for each routing protocol is:
- RIP: Hop count - Best path is chosen by the route with the lowest hop count.
- IGRP and EIGRP: Bandwidth, Delay, Reliability, and Load - Best path is chosen by the route with the smallest composite metric value calculated from these multiple parameters. By default, only bandwidth and delay are used.
- IS-IS and OSPF: Cost - Best path is chosen by the route with the lowest cost. . Cisco's implementation of OSPF uses bandwidth. IS-IS is discussed in CCNP.

Routing protocols determine best path based on the route with the lowest metric.

Refer to the example in the figure The routers are using the RIP routing protocol. The metric associated with a certain route can be best viewed using the show ip route command. The metric value is the second value in the brackets for a routing table entry. In the figure, R2 has a route to the 192.168.8.0/24 network that is 2 hops away.