The Routing Table: A Closer Look Routing Protocols and Concepts – Chapter 8 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public ITE PC v4.0 Chapter 1 1 Objectives  D esc ri be th e v a ri ous r ou t e t ypes f ou n d in th e r ou tin g esc be e a ous ou e ypes ou d e ou g table structure  Describe the routing table lookup process. Describe the routing table lookup process.  Describe routing behavior in routed networks. ITE PC v4.0 Chapter 1 2 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Introduction  Chapter Focus – Structure of the routing table Structure of the routing table •Will examine the format of the routing table and learn about level 1 and level 2 routes. – Lookup process of the routing table – Classless and classful routing behaviors Cisco IP Routing by Alex Zinin (ISBN 0 - 201 - 60473 - 6) Cisco IP Routing , by Alex Zinin (ISBN 0 201 60473 6) . ITE PC v4.0 Chapter 1 3 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Routing Table Structure Routing Table Structure  Lab Topology  3 router setup -R1 and R2 share a common 172.16.0.0/16 network with 172 16 2 0/24 subnets 172 . 16 . 2 . 0/24 subnets . -R2 and R3 are connected by the 192.168.1.0/24 network. - R3 also has a 172 16 4 0/24 subnet which is disconnected or R3 also has a 172 . 16 . 4 . 0/24 subnet , which is disconnected , or discontiguous, from the 172.16.0.0 network that R1 and R2 share. ITE PC v4.0 Chapter 1 4 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public In a later section, we will configure the interfaces for R2 Routing Table Structure  The figure shows routing table entries come from the  The figure shows routing table entries come from the following sources - Directly connected networks - Directly connected networks -Static routes Dynamic routing protocols - Dynamic routing protocols ITE PC v4.0 Chapter 1 5 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Routing Table Structure Routing Table Structure  The figure shows what happens as the Serial 0/0/1 interface for R2 is configured with the 192.168.1.1/24 address. – R1 and R3 already have their interfaces configured with the appropriate IP addresses and subnet masks. –We will now configure the interfaces for R2 and use debug ip routing to view the routing table process that is used to add these entries.  As soon as the “no shutdown” command is issued the route is added to routin g table g debug ip routing debug ip routing ITE PC v4.0 Chapter 1 6 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Routing Table Structure Ci IP ti t bl  Ci sco IP rou ti ng t a bl e is a hierarchical structure structure –The reason for this is to speed up lookup to speed up lookup process The hierarchy – The hierarchy includes several levels. •level 1 • level 2 ITE PC v4.0 Chapter 1 7 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public level 2 Routing Table Structure Routing Table Structure  Level 1 Routes – Have a subnet mask e q ual to or less than the classful q mask of the network address. –192.168.1.0/24 is a level 1 network route, because the subnet mask is equal to the network's classful mask. /24 f l C t k h th 192 168 1 0 t k f or c l ass C ne t wor k s, suc h as th e 192 . 168 . 1 . 0 ne t wor k .  Level 1 route can function as – Default route •A default route is a static route with the address 0.0.0.0/0. – Supe rn e t r ou t e Supeeoue •A supernet route is a network address with a mask less than the classful mask. – Network route Network route •A network route is a route that has a subnet mask equal to that of the classful mask.  The source of the level 1 route can be a directly ITE PC v4.0 Chapter 1 8 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public  The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol. Routing Table Structure Routing Table Structure  The level 1 route 192.168.1.0/24 can be further defined as an ultimate route . ultimate route . ultimate route includes either: -A next-hop ip address (another path) OR OR -An exit interface  The directly connected network 192.168.1.0/24 It i l l 1 t k t b it h b t k th t i th – It i s a l eve l 1 ne t wor k rou t e b ecause it h as a su b ne t mas k th a t i s th e same as its classful mask. –This same route is also an ultimate route because it contains the exit interface Serial 0/0/1 Serial 0/0/1 . ITE PC v4.0 Chapter 1 9 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Parent and Child Routes  A parent route is a level 1 route Parent and Child Routes – A parent route does not contain any next-hop IP address or exit interface information  When the 172.16.3.0 subnet was added to the routing table, another route , 172.16.0.0 , was also added. ,, –The first entry, 172.16.0.0/24, does not contain any next-hop IP address or exit interface information. or exit interface information. –This route is known as a level 1 parent route. Attitllhdi – A paren t rou t e i s ac t ua ll y a h ea di ng that indicates the presence of level 2 routes, also known as child routes. ITE PC v4.0 Chapter 1 10 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public [...]... match match ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 19 Routing Table Lookup Process Finding the subnet mask used to determine the longest match Scenario: –PC1 pings 192.1 68. 1.2 –Router examines level 1 route for best match t f b t t h –There exist a match between192.1 68. 1.2 & 192.1 68. 1.0 / 24 –Router forwards packets out s0/0/0 ITE PC v4.0 Chapter 1 © 2007 Cisco... PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 21 Routing Table Lookup Process Finding Fi di a match between packet’s destination IP address t hb t k t’ d ti ti dd and the next route in the routing table –The figure shows a match between the destination IP of 192 1 68 1 0 The 192.1 68. 1.0 and the level one IP of 192.1 68. 1.0 / 24 then packet forwarded out s0/0/0 –Not only... default –The command “no ip classless” means that the route lookup process uses classful routing table lookups by default default Classless routing behavior works for -Discontiguous networks Di ti t k And -CIDR supernets CIDR t ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 30 Routing Behavior “no ip classless” no classless Classful Routing Behavior – Search Process –when... Systems, Inc All rights reserved Cisco Public 26 Routing Behavior Classful & classless routing protocols Influence how routing table is populated Classful & classless routing behaviors Determines how routing table is searched after it is filled ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 27 Routing Behavior Classful Routing Behavior: no ip classless pp What happens... administrator does not understand th diff d t d the difference b t between classful and classless routing behavior ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 33 Routing Beha ior Ro ting Behavior “ip classless” Classless Routing Behaviorg ip lassless Step 3: If classless routing behavior in effect then, continue searching level 1 supernet routes in the routing table for a... child routes of the parent? -Router must determine if the routing behavior is g classless or classful -If router is utilizing classful routing behavior then -Lookup process is terminated and packet is dropped ITE PC v4.0 Chapter 1 ip classless and no ip classless © 2007 Cisco Systems, Inc All rights reserved Cisco Public 28 Using the ip classless command (cont.) g ( ) http://www.networkking.net/out/IPClassless.htm.. .Routing Table Structure A level 1 parent route is automatically created any time a subnet is added to the routing table –In other words, a parent route is created whenever a route with a mask greater than the classful mask is entered into the routing table –1 72.16.0.0/24 i subnetted, 1 subnets 172 16 0 0/24 is b tt d b t A level 2 route is a route that is a subnet of a classful network address –. .. the subnetting Therefore, if the subnet was not in the routing table, the subnet did not exist and packet was dropped The routing table process will not use the default route, 0.0.0.0/0, or any other route route ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 32 Routing Behavior R ti B h i “no ip classless” The routing table p g process will not use the default route,... classless" command prevents the existence of a single "subnet" route from blocking access via the p p g g default route to other subnets of the same old-style network Default only works with single-homed ISPs RFC 187 9 IP classless command is not easy to understand, we know that But I bet, after you read the following lines, you will understand what it is all about First, you must understand a very simple... first –If a match exists then forward packet Default routes Checked second Step 5: If there is no match or no default route then the Packet is dropped t th th ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 34 Routing Behavior R ti B h i “ip classless” Classless Routing Behavior – Search Process g Router begins search process by finding a match between destination IP and . The Routing Table: A Closer Look Routing Protocols and Concepts – Chapter 8 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public ITE PC v4.0 Chapter 1 1 Objectives . Public Introduction  Chapter Focus – Structure of the routing table Structure of the routing table •Will examine the format of the routing table and learn about level 1 and level 2 routes. – Lookup. networks -Static routes Dynamic routing protocols - Dynamic routing protocols ITE PC v4.0 Chapter 1 5 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public Routing Table Structure Routing Table