The Routing Table: A Closer Look pot

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 [...].. .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 –172.16.0.0/24 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... –Router forwards packets out s0/0/0 ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 20 Routing Table Lookup Process The process of matching –1st there must be a match made between the parent route & destination IP •If a match is made then an attempt at finding a match between the destination IP and the child route is made •Do at least 16 of the left-most bits of the parent... Routes –Best match is also known as the longest match Best 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 For example, in the figure we have a packet destined for 172.16.0.10 172 16 0 10 Many possible routes could match this packet Three packet possible routes are shown that do match this packet: 172.16.0.0/12,... Inc All rights reserved Cisco Public 31 Routing Behavior R ti B h i “no ip classless” Classful Routing Behavior – Search Process P The reason why the router will not search beyond the child routes y At the beginning of the Internet's growth, networks were all classful This meant an organization could subnet a major network address and “enlighten” all the organization’s routers about the subnetting Therefore,... Public 13 Routing Table Structure Both child routes have the same subnet mask -This means the parent route maintains the /24 mask Note: If there is only a single level 2 child route and that route is removed, the level 1 parent route will be automatically deleted A level 1 parent route exists only when there is at least one level 2 child route route Thev4.0 role of the parent route will be examined when... 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 if there is not a match with any level... routes in the routing table If a packet doesn't match a child route for the parent network route, then the router drops the packet packet R2 receives a packet destined for PC3 at 172.16.4.10 –Even with the default route configured The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits This Thi means packet i d k t is dropped d ITE PC v4.0 Chapter 1... with the longest match First, the router examines the parent route for a match The router checks the last child route for 172.16.3.0/24 and finds a match The first 24 bits do match The routing table process will use this route, 172.16.3.0/24, to forward the packet with the destination IP address of 172.16.3.10 out the exit interface of Serial 0/0/0 R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0... in the universe RIP's 10 4 0 1 10.x.x.x universe Otherwise RIP will drop the packet, even if there is a default route RIP will not care about your default route or last resort gateway; it will drop your packet How do you ask RIP not to drop your packet and send the unknown destinations to the default route, although you have been so mean to him and have broken its class? You tell him: please, please,... classless! If no ip classless, drop the packet i l l d th k t If ip classless, send the packet to the default ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc All rights reserved Cisco Public 29 Routing Behavior ip Classless Beginning with IOS 11.3, “ip classless” was configured by default The command “no ip classless” means that the route lookup process uses classful routing table lookups by default . 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 –. is added to the routing table. –In other words, a parent route is created whenever a route with a mask created whenever a route with a mask greater than the classful mask. 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