Cisco Networking Academy Routing Concepts PDF

Chapter 6: Routing Concepts Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 1  Chapter 6 6.0 Routing Concepts 6.1 Initial Configuration of a Router 6.2 Routing Decisions 6.3 Router Operation...

Chapter 6: Routing Concepts Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 1  Chapter 6 6.0 Routing Concepts 6.1 Initial Configuration of a Router 6.2 Routing Decisions 6.3 Router Operation 6.4 Summary Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 2  Chapter 6: Objectives  Configure a router to route between multiple directly connected networks  Describe the primary functions and features of a router.  Explain how routers use information in data packets to make forwarding decisions in a small- to medium-sized business network.  Explain the encapsulation and de-encapsulation process used by routers when switching packets between interfaces.  Compare ways in which a router builds a routing table when operating in a small- to medium-sized business network.  Explain routing table entries for directly connected networks.  Explain how a router builds a routing table of directly connected networks. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 3  Chapter 6: Objectives (cont.)  Explain how a router builds a routing table using static routes.  Explain how a router builds a routing table using a dynamic routing protocol. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 4  Functions of a Router Why Routing? The router is responsible for the routing of traffic between networks. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 5  Functions of a Router Routers are Computers Routers are specialized computers containing the following required components to operate: Central processing unit (CPU) Operating system (OS) - Routers use Cisco IOS Memory and storage (RAM, ROM, NVRAM, Flash, hard drive) Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 6  Functions of a Router Routers Interconnect Networks  Routers can connect multiple networks.  Routers have multiple interfaces, each on a different IP network. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 7  Functions of a Router Routers Choose Best Paths  Routers use static routes and dynamic routing protocols to learn about remote networks and build their routing tables.  Routers use routing tables to determine the best path to send packets.  Routers encapsulate the packet and forward it to the interface indicated in routing table. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 8  Functions of a Router Routers Choose Best Paths Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 9  Connect Devices Default Gateways To enable network access devices must be configured with the following IP address information  IP address - Identifies a unique host on a local network.  Subnet mask - Identifies the host’s network subnet.  Default gateway - Identifies the router a packet is sent to to when the destination is not on the same local network subnet. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 10  Connect Devices Document Network Addressing Network Documentation should include at least the following in a topology diagram and addressing table:  Device names  Interfaces  IP addresses and subnet mask  Default gateways Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 11  Connect Devices Enable IP on a Switch  Network infrastructure devices require IP addresses to enable remote management.  On a switch, the management IP address is assigned on a virtual interface. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 12  Basic Settings on a Router Configure Basic Router Settings Basics tasks that should be first configured on a Cisco Router and Cisco Switch:  Name the device – Distinguishes it from other routers  Secure management access – Secures privileged EXEC, user EXEC, and Telnet access, and encrypts passwords to their highest level  Configure a banner – Provides legal notification of unauthorized access.  Save the Configuration Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 13  Basic Settings on a Router Configure an IPv4 Router Interface To be available, a router interface must be:  Configured with an address and subnet mask.  Must be activated using no shutdown command. By default LAN and WAN interfaces are not activated.  Optional description can be included. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 14  Verify Connectivity of Directly Connected Networks Verify Interface Settings Show commands are used to verify operation and configuration of interface:  show ip interfaces brief  show ip route  show running-config Show commands are used to gather more detailed interface information:  show interfaces  show ip interfaces Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 15  Switching Packets between Networks Router Switching Functions Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 16  Switching Packets between Networks Send a Packet Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 17  Switching Packets between Networks Forward to the Next Hop Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 18  Switching Packets between Networks Packet Routing Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 19  Switching Packets between Networks Reach the Destination Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 20  Path Determination Routing Decisions Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 21  Path Determination Best Path Best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network:  A metric is the value used to measure the distance to a given network.  Best path to a network is the path with the lowest metric. Dynamic routing protocols use their own rules and metrics to build and update routing tables:  Routing Information Protocol (RIP) - Hop count  Open Shortest Path First (OSPF) - Cost based on cumulative bandwidth from source to destination  Enhanced Interior Gateway Routing Protocol (EIGRP) - Bandwidth, delay, load, reliability Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 22  Path Determination Load Balancing When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally: Equal cost load balancing can improve network performance. Equal cost load balancing can be configured to use both dynamic routing protocols and static routes. RIP, OSPF and EIGRP support equal cost load balancing. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 23  Path Determination of the route Administrative Distance If multiple paths to a destination are configured on a router, the path installed in the routing table is the one with the lowest Administrative Distance (AD): A static route with an AD of 1 is more reliable than an EIGRP- discovered route with an AD of 90. A directly connected route with an AD of 0 is more reliable than a static route with an AD of 1. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 24  The Routing Table The Routing Table A routing table is a file stored in RAM that contains information about:  Directly connected routes  Remote routes  Network or next hop associations Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 25  The Routing Table Routing Table Sources The show ip route command is used to display the contents of the routing table:  Local route interfaces - Added to the routing table when an interface is configured. (displayed in IOS 15 or newer)  Directly connected interfaces - Added to the routing table when an interface is configured and active.  Static routes - Added when a route is manually configured and the exit interface is active.  Dynamic routing protocol - Added when EIGRP or OSPF are implemented and networks are identified. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 26  The Routing Table Routing Table Sources Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 27  The Routing Table Remote Network Routing Entries Interpreting the entries in the routing table. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 28  Directly Connected Routes Directly Connected Interfaces A newly deployed router, without any configured interfaces, has an empty routing table. An active, configured, directly connected interface creates two routing table entries:  Link Local (L)  Directly Connected (C) Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 29  Directly Connected Routes Directly Connected Example A routing table with the directly connected interfaces of R1 configured and activated. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 30  Statically Learned Routes Static Routes Static routes and default static routes can be implemented after directly connected interfaces are added to the routing table:  Static routes are manually configured  They define an explicit path between two networking devices.  Static routes must be manually updated if the topology changes.  Their benefits include improved security and control of resources.  Configure a static route to a specific network using the ip route network mask {next-hop-ip | exit-intf} command.  A default static route is used when the routing table does not contain a path for a destination network.  Configure a default static route using the ip route 0.0.0.0 0.0.0.0 {exit-intf | next-hop-ip} command. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 31  Statically Learned Routes Default Static Routes Example Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 32  Statically Learned Routes Static Routes Example Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 33  Dynamic Routing Protocols Dynamic Routing Dynamic routing is used by routers to share information about the reachability and status of remote networks. It performs network discovery and maintains routing tables. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 34  Dynamic Routing Protocols IPv4 Routing Protocols Cisco ISR routers can support a variety of dynamic IPv4 routing protocols including:  EIGRP – Enhanced Interior Gateway Routing Protocol  OSPF – Open Shortest Path First  IS-IS – Intermediate System-to-Intermediate System  RIP – Routing Information Protocol Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 35  Dynamic Routing Protocols IPv4 Routing Protocols Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 36  Chapter 6: Summary  There are many key structures and performance-related characteristics referred to when discussing networks: topology, speed, cost, security, availability, scalability, and reliability.  Cisco routers and Cisco switches have many similarities. They support a similar modal operating system, similar command structures, and many of the same commands.  One distinguishing feature between switches and routers is the type of interfaces supported by each.  The main purpose of a router is to connect multiple networks and forward packets from one network to the next. This means that a router typically has multiple interfaces. Each interface is a member or host on a different IP network. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 37  Chapter 6: Summary (cont.)  The routing table is a list of networks known by the router.  A remote network is a network that can only be reached by forwarding the packet to another router.  Remote networks are added to the routing table in two ways: either by the network administrator manually configuring static routes or by implementing a dynamic routing protocol.  Static routes do not have as much overhead as dynamic routing protocols; however, static routes can require more maintenance if the topology is constantly changing or is unstable.  Dynamic routing protocols automatically adjust to changes without any intervention from the network administrator. Dynamic routing protocols require more CPU processing and also use a certain amount of link capacity for routing updates and messages. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 38  Chapter 6: Summary (cont.)  Routers make their primary forwarding decision at Layer 3, the Network layer. However, router interfaces participate in Layers 1, 2, and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link frame and encoded into bits at Layer 1.  Router interfaces participate in Layer 2 processes associated with their encapsulation. For example, an Ethernet interface on a router participates in the ARP process like other hosts on that LAN. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 39 Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 40