The Routing Table: A Closer Look

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Describe the various route types found in the routing esc be e a ous ou e ypes ou d e ou g table structure Describe the routing table lookup process. Describe routing behavior in routed networks.

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The Routing Table: A Closer Look Routing Protocols and Concepts – Chapter 8 ITE PC v4.0 1 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Objectives Describe the various route types found in the routing esc table structure Describe the routing table lookup process. Describe the routing table lookup process. Describe routing behavior in routed networks. ITE PC v4.0 2 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Introduction Chapter Focus – Structure of the routing table 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). IP Routing by Alex Zinin (ISBN ITE PC v4.0 3 Chapter 1 © 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 172 172.16.2.0/24 subnets. 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 also has 172 subnet which is disconnected or discontiguous, from the 172.16.0.0 network that R1 and R2 share. In a later section, we will configure the interfaces for R2 ITE PC v4.0 4 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure The figure shows routing table entries come from the figure shows routing table entries come from the following sources -Directly connected networks connected networks -Static routes -Dynamic routing protocols routing protocols ITE PC v4.0 5 Chapter 1 © 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 routing table debug ip routing ip routing ITE PC v4.0 6 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure Ci Cisco IP routing table IP is a hierarchical structure structure –The reason for this is to speed up lookup to speed up lookup process –The hierarchy hierarchy includes several levels. •level 1 •level 2 ITE PC v4.0 7 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure Routing Table Structure Level 1 Routes –Have a subnet mask equal to or less than the classful 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 for class C networks, such as the 192.168.1.0 network. th 192 Level 1 route can function as –Default route •A default route is a static route with the address 0.0.0.0/0. –Supernet route •A supernet route is a network address with a mask less than the classful mask. –Network route 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 source of the level route can be directly connected network, static route, or a dynamic routing protocol. ITE PC v4.0 8 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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 -An exit interface The directly connected network 192.168.1.0/24 –It is a level 1 network route because it has a subnet mask that is the same as It it th th 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 9 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Parent and Child Routes Parent and Child Routes A parent route is a level 1 route –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. –A parent route is actually a heading that indicates the presence of level 2 routes, also known as child routes. ITE PC v4.0 10 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure Routing Table Structure A level 1 parent route is automatically created any time subnet is added to created any time a subnet is added to the routing table. –In other words, a parent route is created whenever route with mask created whenever a route with a mask greater than the classful mask is entered into the routing table. –172.16.0.0/24 is subnetted, 1 subnets A level 2 route is a route that is a subnet of a classful network address. – Child routes are level 2 routes – Child routes are a subnet of a classful network address –C 172.16.3.0 is directly connected, FastEthernet0/0 ITE PC v4.0 11 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure Routing Table Structure The parent route contains the 172.16.0.0 - The classful network address for our subnet. Level 2 child routes contain 172.16.3.0, route source & the network address of the route –Notice that the subnet mask is not included with the subnet, the level that the subnet mask is not included with the subnet the level 2 child route. The subnet mask for this child route (subnet) is the /24 mask included in its parent route, 172.16.0.0 Level 2 child routes are also considered ultimate routes child routes considered ultimate routes –Reason: they contain the next hop address &/or exit interface ITE PC v4.0 12 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure Routing Table Structure The figure shows the configuration of the Serial configuration of the Serial 0/0/0 interface on R2. –The routing table shows routing table shows two child routes for the same 172.16.0.0/24 parent route route. •Both 172.16.2.0 and 172.16.3.0 are members of the same parent route, •because they are both members of the th 172.16.0.0/16 classful network ITE PC v4.0 13 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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 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 child route. Thev4.0role of the parent route will be examined when we discuss the route lookup process. ITE PC 14 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure In classless networks, child routes do not have to share the same subnet mask –Whenever there are two or more child routes with different subnet masks belonging to the same classful network the subnet masks belonging to the same classful network, the routing table presents a slightly different view, which states that this parent network is variably subnetted. ITE PC v4.0 15 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure St Parent & Child Routes: Classless Networks Child Routes: Classless Networks ITE PC v4.0 16 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Structure Parent Child Routes: classful and classless Networks Parent & Child Routes: classful and classless Networks classful Network Parent Term Includes Subnet Type route’s variably the # of mask Classful subnetted different included mask is is seen in masks of with each Displayed parent child child route route in routes entry routing routing table Class- No No No No classless ful Class- Yes Yes Yes Yes less ITE PC v4.0 17 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Lookup Process Routing Table Lookup Process The Route Lookup Process 1. Examine level 1 routes • If best match a level 1 ultimate route and is not a parent route this route is used to forward packet • If the best match is a level 1 parent the best match is level parent route, proceed to Step 2 2. Router examines level 2 (child) routes • If there is a match with level 2 child route then that subnet is used to forward packet • If no match then proceed to Step 3 3. Router determines classful or classless routing behavior • If classful then packet is dropped • If classless then router searches level classless then router searches level one supernet and default routes 4. If there exists a level 1 supernet or default route match then Packet is forwarded 5. If not packet is dropped ITE PC v4.0 18 Chapter 1 © 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 match is 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. For example, in the figure we have a packet destined for 172 172.16.0.10. Many possible routes could match this packet. Three Many possible routes could match this packet Three possible routes are shown that do match this packet: 172.16.0.0/12, 172.16.0.0/18, and 172.16.0.0/26. Of the three routes, 172 172.16.0.0/26 has the longest match. has the longest match ITE PC v4.0 19 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Routing Table Lookup Process Routing Table Lookup Process Finding the subnet mask used to determine the used to determine the longest match Scenario: –PC1 pings 192.168.1.2 –Router examines level 1 route for best match –There exist a match between192.168.1.2 & 192.168.1.0 / 24 –Router forwards packets out s0/0/0 ITE PC v4.0 20 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public