© 2007 Cisco Systems, Inc. All rights reserved.Cisco Public ITE PC v4.0 Chapter 1 1 Introduction to Routing and Packet Forwarding Routing Protocols and Concepts – Chapter 1

ITE PC v4.0 Chapter 1 2 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Objectives  Functions of a router  Configuring Route  Structure of a routing table.  Dynamic Routing and Routing Protocols

ITE PC v4.0 Chapter 1 3 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Router as a Computer Routers are responsible for interconnecting networks by selecting the best path for a packet to travel and forwarding packets to their destination using IP address Routers generally have 2 connections: -WAN connection (Connection to ISP) -LAN connection

ITE PC v4.0 Chapter 1 4 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Configure Devices and Apply Addresses  Basic Router Configuration  A basic router configuration should contain the following: -Router name - Host name should be unique -Banner - At a minimum, banner should warn against unauthorized use -Passwords - Use strong passwords -Interface configurations – Specify interface type, IP address and subnet mask. Issue no shutdown command. If DCE serial interface issue clock rate command. Refer to worksheet on basic router configuration

ITE PC v4.0 Chapter 1 5 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Configure Devices and Apply Addresses  Verify Basic Router Configuration -Issue the show running-config command -Save the basic router configuration by Issuing the copy running-config startup-config command -Additional commands that will enable you to further verify router configuration are:  Show running-config - Displays configuration currently in RAM  Show startup-config - Displays configuration file NVRAM  Show IP route - Displays routing table  Show interfaces - Displays all interface configurations  Show IP int brief - Displays abbreviated interface configuration information

ITE PC v4.0 Chapter 1 6 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Routing Table is stored in ram and contains information about:  Directly connected networks - this occurs when a device is connected to another router interface  Remotely connected networks - this is a network that is not directly connected to a particular router  Detailed information about the networks include source of information, network address & subnet mask, and Ip address of next-hop router  Show ip route command is used to view a routing table

ITE PC v4.0 Chapter 1 7 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Adding a connected network to the routing table -Router interfaces  Each router interface is a member of a different network  Activated using the no shutdown command  In order for static and dynamic routes to exist in routing table you must have directly connected networks

ITE PC v4.0 Chapter 1 8 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Static routes in the routing table -Includes: network address and subnet mask and IP address of next hop router or exit interface -Denoted with the code S in the routing table -Routing tables must contain directly connected networks used to connect remote networks before static or dynamic routing can be used  When to use static routes -When network only consists of a few routers -Network is connected to internet only through one ISP -Hub & spoke topology is used on a large network

ITE PC v4.0 Chapter 1 9 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Connected and Static routes

ITE PC v4.0 Chapter 1 10 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Dynamic routing protocols -Used to add remote networks to a routing table -Are used to discover networks -Are used to update and maintain routing tables  Automatic network discovery -Routers are able discover new networks by sharing routing table information

ITE PC v4.0 Chapter 1 11 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Maintaining routing tables -Dynamic routing protocols are used to share routing information with other router & to maintain and up date their own routing table.  IP routing protocols. Example of routing protocols include: -RIP -IGRP -EIGRP -OSPF

ITE PC v4.0 Chapter 1 12 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Router Paths and Packet Switching  A Metric is a numerical value used by routing protocols help determine the best path to a destination –The smaller the metric value the better the path  2 types of metrics used by routing protocols are: -Hop count - this is the number of routers a packet must travel through to get to its destination -Bandwidth - this is the “speed” of a link also known as the data capacity of a link

ITE PC v4.0 Chapter 1 13 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Routing table entries come from the following sources -Directly connected networks -Static routes -Dynamic routing protocols

ITE PC v4.0 Chapter 1 14 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Structure  Parent and Child Routes -A parent route is a level 1 route, i.e., it does not contain any next-hop IP address or exit interface information  Level 2 child routes contain route source & the network address of the route  Diagram illustrates 2 child networks belonging to the parent route / 24

ITE PC v4.0 Chapter 1 15 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Table Lookup Process  Longest Match: Level 1 Network Routes –Best match is also known as the longest match –The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.

ITE PC v4.0 Chapter 1 16 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Classful and Classless IP Addressing  Classful Routing Updates classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates as the Subnet mask is directly related to the network address

ITE PC v4.0 Chapter 1 17 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Behavior  Classful Routing Behavior – Search Process  An example of when classful routing behavior is in effect and why the router drops the Packet -The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits. This means packet is dropped

ITE PC v4.0 Chapter 1 18 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Behavior  Classless Routing Behavior: ip classless  Route lookup process when ip classless is in use -If classless routing behavior in effect then  Search level 1 routes  Supernet routes Checked first -If a match exists then forward packet  Default routes Checked second If there is no match or no default route then the  Packet is dropped

ITE PC v4.0 Chapter 1 19 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Routing Behavior  Classless Routing Behavior – Search Process  If no match is found in child routes of previous slide then Router continues to search the routing table for a match that may have fewer bits in the match

ITE PC v4.0 Chapter 1 20 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Classless Inter-Domain Routing (CIDR)  Route summarization done by CIDR Routes are summarized with masks that are less than that of the default classful mask Example: / 13 is the summarized route for the / 16 to / 16 classful networks

ITE PC v4.0 Chapter 1 21 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Classless Inter-Domain Routing (CIDR)  Steps to calculate a route summary -List networks in binary format -Count number of left most matching bits to determine summary route’s mask -Copy the matching bits and add zero bits to determine the summarized network address

ITE PC v4.0 Chapter 1 22 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Default Route  Default routes Packets that are not defined specifically in a routing table will go to the specified interface for the default route Example: Customer routers use default routes to connect to an ISP router. Command used to configure a default route is #ip route s0/0/1

ITE PC v4.0 Chapter 1 23 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Default Route and Static route