Chapter 1. Resume. Introduction to Routing and Packet Forwarding
Routers have many of the same
hardware and
software components that are found in other computers including:
~ CPU
~ RAM
~ ROM
~ Operating System
When a router receives an IP packet on one interface, it
determines which
interface to use to forward the packet onto its destination. T
he interface that the router uses to forward the packet may be the
network of the final destination of the packet (the network with the destination IP address of this packet), or it may be a
network connected to another router that is used to reach the destination network.
Routers determine the best path.
The primary responsibility of a router is to direct packets destined for local and remote networks by:
1. Determining the best path to send packets
2. Forwarding packets toward their destination.
Static routes and
dynamic routing protocols are used by routers to learn about
remote networks and build their routing tables.
Like a PC, a router also includes:
~ Central Processing Unit (CPU)
The CPU executes operating system instructions, such as system initialization, routing functions, and switching functions.
~ Random-Access Memory (RAM)
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.
~
Read-Only Memory (ROM)
The bootstrap instructions
Basic diagnostic software
Scaled-down version of IOS
~ 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.
Flash memory does not lose its contents when the router loses power or is restarted
~ NVRAM (Nonvolatile RAM) does not lose its information when power is turned off.
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.
Internetwork Operating System
The operating system software used in Cisco routers is known as Cisco Internetwork Operating System (IOS).(is a multitasking operating system that is integrated with routing, switching, internetworking, and telecommunications functions.)
Bootup Process
There are four major phases to the
bootup process:
1. Performing the POST
The Power-On Self Test (POST) process is used to test the router hardware(CPU, RAM, and NVRAM.)
2. Loading the bootstrap program.
After the POST, the bootstrap program is copied from ROM into RAM.
3. Locating and loading 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.
4. Locating and loading the startup configuration file or entering setup mode.
Locating the Startup Configuration File.
Startup-config in NVRAM 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.
Executing the Configuration File. If a startup configuration file is found in NVRAM, the IOS loads it into RAM as the running-config and executes the commands in the file, one line at a time. The running-config file contains interface addresses, starts routing processes, configures router passwords and defines other characteristics of the router.
Management Ports.
Routers have physical connectors that are used to manage the router -
management ports. Unlike
Ethernet and
serial interfaces, management ports are not used for packet forwarding. The most common management port is the
console port( 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.
Routers have multiple interfaces that are used to connect to multiple networks.
Router interfaces can be divided into two major groups:
~ LAN interfaces - such as Ethernet and FastEthernet
~ WAN interfaces - such as serial, ISDN, and Frame Relay
Routers and the Network Layer
The main purpose of a router is to
connect multiple networks and
forward packets destined either for its own networks or other networks.
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.
Routers Operate at Layers 1, 2, and 3
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.
Implementing basing adressing schemes.
When designing a new network or mapping an existing network, document the network.:
~ topology diagram that indicates the physical connectivity
~ an addressing table that lists all of the following information:
~ Device names
~ Interfaces used in the design
~ IP addresses and subnet masks
~ Default gateway addresses for end devices, such as PCs.
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 Routing Table
A routing table is a data file in RAM that is used to store route information about directly connected and remote networks. The routing table contains network/next hop associations.
The network/exit-interface association can also represent the destination network address of the IP packet. This association occurs on the router's directly connected networks.
A remote network is a network that is not directly connected to the router
Remote networks are added to the routing table using either a dynamic routing protocol or by configuring static routes.
When a PC is configured with a host IP address and subnet mask, the PC uses the subnet mask to determine what network it now belongs to. This is done by the operating system
ANDing the host
IP address and
subnet mask. A router uses the same logic when an interface is configured.
A PC is normally configured with a single host IP address because it only has a single network interface, usually an Ethernet NIC. Routers have multiple interfaces; therefore, each interface must be a member of a different network.
After the router's interface is configured and the interface is activated with the no shutdown command, the interface must receive a carrier signal from another device (router, switch, hub, etc.) before the interface state is considered "up." Once the interface is "up," the network of that interface is added to the routing table as a directly connected network.
Static and dynamic routes cannot exist in the routing table without a router's own directly connected networks.
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.
IP Routing Protocols
~RIP (Routing Information Protocol)
~IGRP (Interior Gateway Routing Protocol)
~EIGRP (Enhanced Interior Gateway Routing Protocol)
~OSPF (Open Shortest Path First)
~IS-IS (Intermediate System-to-Intermediate System)
~BGP (Border Gateway Protocol)
Routing Table Principles
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- Asymmetric Routing
MAC Layer Frame Format.
Preamble - Seven bytes of alternating 1s and 0s, used to synchronize signals
Start-of-frame (SOF) delimiter - 1 byte signaling the beginning of the frame
Destination address - 6 byte MAC address of the sending device on the local segment
Source address - 6 byte MAC address of the receiving device on the local segment
Type/length - 2 bytes specifying either the type of upper layer protocol (Ethernet II frame format) or the length of the data field (IEEE 802.3 frame format)
Data and pad - 46 to 1500 bytes of data; zeros used to pad any data packet less than 46 bytes
Frame check sequence (FCS) - 4 bytes used for a cyclical redundancy check to make sure the frame is not corrupted
