CCNA EXPLORATION V4.0 ROUTING PROTOCOLS AND CONCEPTS ACCESSIBLE INSTRUCTOR MATERIALS POWERPOINT OBJECTIVES

Transcription

1 CCNA EXPLORATION V4.0 ROUTING PROTOCOLS AND CONCEPTS ACCESSIBLE INSTRUCTOR MATERIALS Prepared by Cisco Learning Institute June 23, 2008

2 Chapter 1 Introduction to Routing and Packet Forwarding Objectives Identify a router as a computer with an OS and hardware designed for the routing process. Demonstrate the ability to configure devices and apply addresses. Describe the structure of a routing table. Describe how a router determines a path and switches packets. Router as a Computer Describe the basic purpose of a router o Computers that specialize in sending packets over the data network. They are responsible for interconnecting networks by selecting the best path for a packet to travel and forwarding packets to their destination Routers are the network center o Routers generally have 2 connections: WAN connection (Connection to ISP) LAN connection Data is sent in form of packets between 2 end devices Routers are used to direct packet to its destination Routers examine a packet s destination IP address and determine the best path by enlisting the aid of a routing table Router components and their functions o CPU - Executes operating system instructions o Random access memory (RAM) - Contains the running copy of configuration file. Stores routing table. RAM contents lost when power is off o Read-only memory (ROM) - Holds diagnostic software used when router is powered up. Stores the router s bootstrap program. o Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP addresses (Routing protocol, Hostname of router) o Flash memory - Contains the operating system (Cisco IOS) o Interfaces - There exist multiple physical interfaces that are used to connect network. o Examples of interface types: Ethernet / fast Ethernet interfaces Serial interfaces Management interfaces Router components Major phases to the router boot-up process o Test router hardware Power-On Self Test (POST) Execute bootstrap loader June 23, 2008 Page 1

3 o Locate & load Cisco IOS software Locate IOS Load IOS o Locate & load startup configuration file or enter setup mode Bootstrap program looks for configuration file Verify the router boot-up process: o The show version command is used to view information about the router during the bootup process. Information includes: Platform model number Image name & IOS version Bootstrap version stored in ROM Image file name & where it was loaded from Number & type of interfaces Amount of NVRAM Amount of flash Configuration register Router Interface is a physical connector that enables a router to send or receive packets Each interface connects to a separate network Consist of socket or jack found on the outside of a router Types of router interfaces: o Ethernet o Fastethernet o Serial o DSL o ISDN o Cable Two major groups of Router Interfaces o LAN Interfaces: Are used to connect router to LAN network Has a layer 2 MAC address Can be assigned a Layer 3 IP address Usually consist of an RJ-45 jack o WAN Interfaces Are used to connect routers to external networks that interconnect LANs. Depending on the WAN technology, a layer 2 address may be used. Uses a layer 3 IP address Routers and the Network Layer o Routers use destination IP address to forward packets The path a packet takes is determined after a router consults information in the routing table. After router determines the best path Packet is encapsulated into a frame Frame is then placed on network medium in form of Bits June 23, 2008 Page 2

4 o Routers Operate at Layers 1, 2 & 3 o Router receives a stream of encoded bits o Bits are decoded and passed to layer 2 o Router de-encapsulates the frame o Remaining packet passed up to layer 3 Routing decision made at this layer by examining destination IP address o Packet is then re-encapsulated & sent out outbound interface Configure Devices and Apply Addresses Implementing Basic Addressing Schemes When designing a new network or mapping an existing network you must provide the following information in the form of a document: o Topology drawing that Illustrates physical connectivity o Address table that provides the following information: Device name Interfaces used IP addresses Default gateway Basic Router Configuration A basic router configuration should contain the following: o Router name - Host name should be unique o Banner - At a minimum, banner should warn against unauthorized use o Passwords - Use strong passwords o Interface configurations - Specify interface type, IP address and subnet mask. Describe purpose of interface. Issue no shutdown command. If DCE serial interface issue clock rate command After entering in the basic configuration the following tasks should be completed o Verify basic configuration and router operations. o Save the changes on a router Verify Basic Router Configuration o Issue the show running-config command o Save the basic router configuration by Issuing the copy running-config startupconfig command o 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 Routing Table Structure Routing Table is stored in ram and contains information about: June 23, 2008 Page 3

5 o Directly connected networks - this occurs when a device is connected to another router interface o Remotely connected networks - this is a network that is not directly connected to a particular router o 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 Adding a connected network to the routing table o 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 Static routes in the routing table o Includes: network address and subnet mask and IP address of next hop router or exit interface o Denoted with the code S in the routing table o 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 o When network only consists of a few routers o Network is connected to internet only through one ISP o Hub & spoke topology is used on a large network Connected and Static routes Dynamic routing protocols o Used to add remote networks to a routing table o Are used to discover networks o Are used to update and maintain routing tables Automatic network discovery o Routers are able discover new networks by sharing routing table information Maintaining routing tables o 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: o RIP o IGRP o EIGRP o OSPF Routing Table Principles o 3 principles regarding routing tables: Every router makes its decisions alone, based on the information it has in its routing table. Different routing table may contain different information A routing table can tell how to get to a destination but not how to get back June 23, 2008 Page 4

6 Effects of the 3 Routing Table Principles o Packets are forwarded through the network from one router to another, on a hop by hop basis o Packets can take path X to a destination but return via path Y (Asymmetric routing) Router Paths and Packet Switching Internet Protocol (IP) packet format contains fields that provide information about the packet and the sending and receiving hosts Fields that are importance for CCNA students: o Destination IP address o Source IP address o Version & TTL o IP header length o Precedence & type of service o Packet length MAC Layer Frame Format MAC Frames are also divided into fields. They include: o Preamble o Start of frame delimiter o Destination MAC address o Source MAC address o Type/length o Data and pad o Frame check sequence A Metric is a numerical value used by routing protocols help determine the best path to a destination o The smaller the metric value the better the path 2 types of metrics used by routing protocols are: o Hop count - this is the number of routers a packet must travel through to get to its destination o Bandwidth - this is the speed of a link also known as the data capacity of a link Equal cost metric is a condition where a router has multiple paths to the same destination that all have the same metric To solve this dilemma, a router will use Equal Cost Load Balancing. This means the router sends packets over the multiple exit interfaces listed in the routing table Path determination is a process used by a router to pick the best path to a destination One of 3 path determinations results from searching for the best path o Directly connected network o Remote network o No route determined Switching Function of Router is the process used by a router to switch a packet from an incoming interface to an outgoing interface on the same router June 23, 2008 Page 5

7 o A packet received by a router will do the following: Strips off layer 2 headers. Examines destination IP address located in Layer 3 header to find best route to destination Re-encapsulates layer 3 packet into layer 2 frame Forwards frame out exit interface As a packet travels from one networking device to another o The Source and Destination IP addresses NEVER change o The Source & Destination MAC addresses CHANGE as packet is forwarded from one router to the next o TTL field decrement by one until a value of zero is reached at which point router discards packet (prevents packets from endlessly traversing the network) Path determination and switching function details. PC1 Wants to send something to PC 2 here is part of what happens o Step 1 - PC1 encapsulates packet into a frame. Frame contains R1 s destination MAC address o Step 2 - R1 receives Ethernet frame R1 sees that destination MAC address matches its own MAC R1 then strips off Ethernet frame R1 Examines destination IP R1 consults routing table looking for destination IP After finding destination IP in routing table, R1 now looks up next hop IP address R1 re-encapsulates IP packet with a new Ethernet frame R1 forwards Ethernet packet out Fa0/1 interface Path determination and switching function details. PC1 Wants to send something to PC 2 here is part of what happens o Step 3 - Packet arrives at R2 R2 receives Ethernet frame R2 sees that destination MAC address matches its own MAC R2 then strips off Ethernet frame R2 Examines destination IP R2 consults routing table looking for destination IP After finding destination IP in routing table, R2 now looks up next hop IP address R2 re-encapsulates IP packet with a new data link frame R2 forwards Ethernet packet out S0/0 interface Path determination and switching function details. PC1 Wants to send something to PC 2 here is part of what happens o Step 4 - Packet arrives at R3 R3 receives PPP frame R3 then strips off PPP frame R3 Examines destination IP R3 consults routing table looking for destination IP June 23, 2008 Page 6

8 After finding destination IP in routing table, R3 is directly connected to destination via its fast Ethernet interface R3 re-encapsulates IP packet with a new Ethernet frame R3 forwards Ethernet packet out Fa0/0 interface o Step 5 - IP packet arrives at PC2. Frame is decapsulated & processed by upper layer protocols Summary Routers are computers that specialize in sending data over a network. Routers are composed of: o Hardware i.e. CPU, Memory, System bus, Interfaces o Software used to direct the routing process IOS Configuration file Routers need to be configured. Basic configuration consists of: o Router name o Router banner o Password(s) o Interface configurations i.e. IP address and subnet mask Routing tables contain the following information o Directly connected networks o Remotely connected networks o Network addresses and subnet masks o IP address of next hop address Routers determine a packets path to its destination by doing the following o Receiving an encapsulated frame & examining destination MAC address. o If the MAC address matches then Frame is de-encapsulated so that router can examine the destination IP address. o If destination IP address is in routing table or there is a static route then Router determines next hop IP address. Router will re-encapsulate packet with appropriate layer 2 frame and send it out to next destination. o Process continues until packet reaches destination. o Note - only the MAC addresses will change the source and destination IP addresses do not change. Chapter 2 Static Routing Objectives Define the general role a router plays in networks. Describe the directly connected networks, different router interfaces. Examine directly connected networks in the routing table and use the CDP protocol. Describe static routes with exit interfaces. June 23, 2008 Page 7

9 Describe summary and default route. Examine how packets get forwarded when using static routes. Identify how to manage and troubleshoot static routes. CISCO, INC. General Role of the Router Functions of a Router o Best Path Selections o Forwarding packets to destination Introducing the Topology o series routers connected via WAN links o Each router connected to a LAN represented by a switch and a PC Connections of a Router for WAN o A router has a DB-60 port that can support 5 different cabling standards Connections of a Router for Ethernet o 2 types of connectors can be used: Straight through and Cross-over Straight through used to connect: Switch-to-Router, Switch-to-PC, Router-to-Server, Hub-to-PC, Hub-to-Server Cross-over used to connect: Switch-to-Switch, PC-to-PC, Switch-to-Hub, Hub-to-Hub, Router-to-Router Interfaces Examining Router Interfaces o Show IP router command used to view routing table o Show Interfaces command used to show status of an interface o Show IP Interface brief command used to show a portion of the interface information o Show running-config command used to show configuration file in RAM Configuring an Ethernet interface o By default all serial and Ethernet interfaces are down o To enable an interface use the No Shutdown command Verifying Ethernet interface o Show interfaces for fastethernet 0/0 command used to show status of fast Ethernet port o Show ip interface brief o Show running-config Ethernet interfaces participate in ARP Configuring a Serial interface o Enter interface configuration mode o Enter in the ip address and subnet mask o Enter in the no shutdown command Example: o R1(config)#interface serial 0/0 o R1(config-if)#ip address o R1(config-if)#no shutdown June 23, 2008 Page 8

10 Examining Router Interfaces o Physically connecting a WAN Interface. o A WAN Physical Layer connection has sides: Data Circuit-terminating Equipment (DCE) This is the service provider CSU/DSU is a DCE device. Data Terminal Equipment (DTE) Typically the router is the DTE device Configuring serial links in a lab environment o One side of a serial connection must be considered a DCE o This requires placing a clocking signal use the clock rate command. o Example: R1(config)#interface serial 0/0 R1(config-if)#clockrate o Serial Interfaces require a clock signal to control the timing of the communications Routing Table and CDP Protocol Purpose of the debug ip routing command o Allows you to view changes that the router performs when adding or removing routes o Example: R2#debug ip routing IP routing debugging is on To configure an Ethernet interface o Example: R2(config)#interface fastethernet 0/0 R2(config-if)#ip address R2(config-if)#no shutdown When a router only has its interfaces configured & no other routing protocols are configured then: o The routing table contains only the directly connected networks o Only devices on the directly connected networks are reachable Checking each route in turn o The ping command is used to check end to end connectivity Purpose of CDP o A layer 2 cisco proprietary tool used to gather information about other directly connected Cisco devices Concept of neighbors o 2 types of neighbors Layer 3 neighbors Layer 2 neighbors CDP show commands o Show cdp neighbors command Displays the following information: - Neighbor device ID June 23, 2008 Page 9

11 - Local interface - Holdtime value, in seconds - Neighbor device capability code - Neighbor hardware platform - Neighbor remote port ID o Show cdp neighbors detail command Useful in determining if an IP address configuration error Disabling CDP o To disable CDP globally use the following command Router(config)#no cdp run CISCO, INC. Static Routes with Exit Interfaces Purpose of a static route o A manually configured route used when routing from a network to a stub network IP route command o To configure a static route use the following command: ip route o Example: Router(config)# ip route network-address subnet-mask {ip-address exitinterface Dissecting static route syntax o ip route - Static route command o Destination network address o Subnet mask of destination network o Serial 0/0/0 interface IP address on R2, which is the "next-hop" to this network Configuring routes to 2 or more remote networks o Use the following commands for R1 R1(config)#ip route R1(config)#ip route Zinin s 3 routing principles o Principle 1: "Every router makes its decision alone, based on the information it has in its own routing table. o Principle 2: "The fact that one router has certain information in its routing table does not mean that other routers have the same information. o Principle 3: "Routing information about a path from one network to another does not provide routing information about the reverse, or return path." Using Zinin s 3 routing principles, how would you answer the following? o Would packets from PC1 reach their destination? Yes, packets destined for /24 and /24 networks would reach their destination. o Does this mean that any packets from these networks destined for /24 network will reach their destination? No, because neither R2 nor R3 router has a route to the /24 network. June 23, 2008 Page 10

12 Resolving to an Exit Interface o Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet. A static route that forwards all packets to the next-hop IP address goes through the following process (reclusive route lookup) The router first must match static route s destination IP address with the Next hop address The next hop address is then matched to an exit interface Configuring a Static route with an Exit Interface o Static routes configured with an exit interface are more efficient because the routing o The routing table can resolve the exit interface in a single search instead of 2 searches o Example of syntax require to configure a static route with an exit interface Modifying Static routes o Existing static routes cannot be modified. The old static route must be deleted by placing no in front of the ip route o Example: no ip route o A new static route must be rewritten in the configuration Verifying the Static Route Configuration o Use the following commands Step 1 show running-config Step 2 verify static route has been entered correctly Step 3 show ip route Step 4 verify route was configured in routing table Step 5 issue ping command to verify packets can reach destination and that Return path is working Ethernet interfaces and ARP o If a static route is configured on an Ethernet link o If the packet is sent to the next-hop router then The destination MAC address will be the address of the next hop s Ethernet interface - This is found by the router consulting the ARP table. - If an entry isn t found then an ARP request will be sent out Summary and Default Route Summarizing routes reduces the size of the routing table. Route summarization is the process of combining a number of static routes into a single static route. Configuring a summary route o Step 1: Delete the current static route o Step 2: Configure the summary static route o Step 3: Verify the new static route June 23, 2008 Page 11

13 Default Static Route o This is a route that will match all packets. Stub routers that have a number of static routes all exiting the same interface are good candidates for a default route. Like route summarization this will help reduce the size of the routing table Configuring a default static route o Similar to configuring a static route. Except that destination IP address and subnet mask are all zeros o Example: Router(config)#ip route [exit-interface ip-address ] Static routes and subnet masks o The routing table lookup process will use the most specific match when comparing destination IP address and subnet mask Default static routes and subnet masks o Since the subnet mask used on a default static route is all packets will match Packet forwarding with static routes. (recall Zinin s 3 routing principles) Router 1 o Packet arrives on R1 s Fastethernet 0/0 interface R1 does not have a route to the destination network, /24 R1 uses the default static route Packet forwarding with static routes. (recall Zinin s 3 routing principles) Router 2 o The packet arrives on the Serial 0/0/0 interface on R2. o R2 has a static route to /24 out Serial0/0/1 Packet forwarding with static routes. (recall Zinin s 3 routing principles) Router 3 o The packet arrives on the Serial0/0/1 interface on R3. o R3 has a connected route to /24 out Fastethernet 0/1 Troubleshooting a Missing Route Tools that can be used to isolate routing problems include: o Ping tests end to end connectivity o Traceroute used to discover all of the hops (routers) along the path between 2 points o Show IP route used to display routing table & ascertain forwarding process o Show ip interface brief- used to show status of router interfaces o Show cdp neighbors detail used to gather configuration information about directly connected neighbors Solving a Missing Route Finding a missing or mis-configured route requires methodically using the correct tools o Start with PING. If ping fails then use traceroute to determine where packets are failing to arrive Issue: show ip route to examine routing table June 23, 2008 Page 12

14 o If there is a problem with a mis-configured static route remove the static route then reconfigure the new static route Solving a Missing Route Summary Routers o Operate at layer 3 o Functions include best path selection & forwarding packets Connecting Networks o WANs Serial cables are connected to router serial ports. In the lab environment clock rates must be configured for DCE. o LANs Straight through cables or cross over cables are used to connect to fastethernet port. (The type of cable used depends on what devices are being connected). Cisco Discovery Protocol o A layer 2 proprietary protocol o Used to discover information about directly connected Cisco devices Static Routes o This is a manually configured path that specifies how the router will get to a certain point using a certain path. Summary static routes o This is several static routes that have been condensed into a single static route. Default route o It is the route packets use if there is no other possible match for their destination in the routing table. Forwarding of packets when static route is used o Zinin s 3 routing principles describe how packets are forwarded. Troubleshooting static routes may require some of the following commands: o Ping o Traceroute o Show IP route o Show ip interface brief o Show cdp neighbors detail Chapter 3 Introduction to Dynamic Routing Protocol Objectives Describe the role of dynamic routing protocols and place these protocols in the context of modern network design. Identify several ways to classify routing protocols. June 23, 2008 Page 13

15 Describe how metrics are used by routing protocols and identify the metric types used by dynamic routing protocols. Determine the administrative distance of a route and describe its importance in the routing process. Identify the different elements of the routing table. Dynamic Routing Protocols Function(s) of Dynamic Routing Protocols: o Dynamically share information between routers. o Automatically update routing table when topology changes. o Determine best path to a destination The purpose of a dynamic routing protocol is to: o Discover remote networks o Maintaining up-to-date routing information o Choosing the best path to destination networks o Ability to find a new best path if the current path is no longer available Components of a routing protocol o Algorithm In the case of a routing protocol algorithms are used for facilitating routing information and best path determination o Routing protocol messages These are messages for discovering neighbors and exchange of routing information Advantages of static routing o It can backup multiple interfaces/networks on a router o Easy to configure o No extra resources are needed o More secure Disadvantages of static routing o Network changes require manual reconfiguration o Does not scale well in large topologies Classifying Routing Protocols Dynamic routing protocols are grouped according to characteristics. Examples include: o RIP o IGRP o EIGRP o OSPF o IS-IS o BGP Autonomous System is a group of routers under the control of a single authority Types of routing protocols: o Interior Gateway Protocols (IGP) June 23, 2008 Page 14

16 o Exterior Gateway Protocols (EGP) Interior Gateway Routing Protocols (IGP) o Used for routing inside an autonomous system & used to route within the individual networks themselves. o Examples: RIP, EIGRP, OSPF Exterior Routing Protocols (EGP) o Used for routing between autonomous systems o Example: BGPv4 IGP: Comparison of Distance Vector & Link State Routing Protocols o Distance vector routes are advertised as vectors of distance & direction incomplete view of network topology Generally, periodic updates o Link state complete view of network topology is created updates are not periodic Classful routing protocols o Do NOT send subnet mask in routing updates Classless routing protocols o Do send subnet mask in routing updates Convergence is defined as when all routers routing tables are at a state of consistency Routing Protocols Metrics Metric o A value used by a routing protocol to determine which routes are better than others Metrics used in IP routing protocols o Bandwidth o Cost o Delay o Hop count o Load o Reliability The Metric Field in the Routing Table Metric used for each routing protocol o RIP - hop count o IGRP & EIGRP - Bandwidth (used by default), Delay (used by default), Load, Reliability o IS-IS & OSPF Cost, Bandwidth (Cisco s implementation) Load balancing o This is the ability of a router to distribute packets among multiple same cost paths June 23, 2008 Page 15

17 Administrative Distance of a Route Purpose of a metric o It s a calculated value used to determine the best path to a destination Purpose of Administrative Distance o It s a numeric value that specifies the preference of a particular route Identifying the Administrative Distance (AD) in a routing table o It is the first number in the brackets in the routing table Dynamic Routing Protocols Directly connected routes o Have a default AD of 0 Static Routes o Administrative distance of a static route has a default value of 1 Directly connected routes o Immediately appear in the routing table as soon as the interface is configured Summary Dynamic routing protocols fulfill the following functions o Dynamically share information between routers o Automatically update routing table when topology changes o Determine best path to a destination Routing protocols are grouped as either o Interior gateway protocols (IGP) Or o Exterior gateway protocols(egp) Types of IGPs include o Classless routing protocols - these protocols include subnet mask in routing updates o Classful routing protocols - these protocols do not include subnet mask in routing update Metrics are used by dynamic routing protocols to calculate the best path to a destination. Administrative distance is an integer value that is used to indicate a router s trustworthiness Components of a routing table include: o Route source o Administrative distance o Metric Chapter 4 Distance Vector Routing Protocols Objectives Identify the characteristics of distance vector routing protocols. June 23, 2008 Page 16

18 Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP). Describe the processes to maintain accurate routing tables used by distance vector routing protocols. Identify the conditions leading to a routing loop and explain the implications for router performance. Recognize that distance vector routing protocols are in use today. Distance Vector Routing Protocols Examples of Distance Vector routing protocols: o Routing Information Protocol (RIP) o Interior Gateway Routing Protocol (IGRP) o Enhanced Interior Gateway Routing Protocol (EIGRP) Distance Vector Technology o The Meaning of Distance Vector: A router using distance vector routing protocols knows 2 things: - Distance to final destination - Vector, or direction, traffic should be directed Characteristics of Distance Vector routing protocols: o Periodic updates o Neighbors o Broadcast updates o Entire routing table is included with routing update Routing Protocol Algorithm: o Defined as a procedure for accomplishing a certain task Routing Protocol Characteristics o Criteria used to compare routing protocols includes Time to convergence Scalability Resource usage Implementation & maintenance Network Discovery Router initial start up (Cold Starts) o Initial network discovery Directly connected networks are initially placed in routing table Initial Exchange of Routing Information o If a routing protocol is configured then Routers will exchange routing information Routing updates received from other routers o Router checks update for new information If there is new information: - Metric is updated - New information is stored in routing table June 23, 2008 Page 17

19 Exchange of Routing Information o Router convergence is reached when All routing tables in the network contain the same network information o Routers continue to exchange routing information If no new information is found then Convergence is reached Convergence must be reached before a network is considered completely operable Speed of achieving convergence consists of 2 interdependent categories o Speed of broadcasting routing information o Speed of calculating routes Routing Table Maintenance Periodic Updates: RIPv1 & RIPv2 o These are time intervals in which a router sends out its entire routing table RIP uses 4 timers o Update timer o Invalid timer o Holddown timer o Flush timer Bounded Updates: EIGRP EIRPG routing updates are o Partial updates o Triggered by topology changes o Bounded o Non periodic Triggered Updates o Conditions in which triggered updates are sent Interface changes state Route becomes unreachable Route is placed in routing table Random Jitter o Synchronized updates A condition where multiple routers on multi access LAN segments transmit routing updates at the same time Problems with synchronized updates - Bandwidth consumption - Packet collisions Solution to problems with synchronized updates - Used of random variable called RIP_JITTER Routing Loops Routing loops are o A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination. Routing loops may be caused by: June 23, 2008 Page 18

20 o Incorrectly configured static routes o Incorrectly configured route redistribution o Slow convergence o Incorrectly configured discard routes Routing loops can create the following issues o Excess use of bandwidth o CPU resources may be strained o Network convergence is degraded o Routing updates may be lost or not processed in a timely manner. Count to Infinity o This is a routing loop whereby packets bounce infinitely around a network. Setting a maximum Distance Vector routing protocols set a specified metric value to indicate infinity. o Once a router counts to infinity it marks the route as unreachable. Preventing loops with holddown timers o Holddown timers allow a router to not accept any changes to a route for a specified period of time. o Point of using holddown timers Allows routing updates to propagate through network with the most current information. The Split Horizon Rule is used to prevent routing loops Split Horizon rule: o A router should not advertise a network through the interface from which the update came. Split horizon with poison reverse o The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface. IP & TTL o Purpose of the TTL field The TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network. o How the TTL field works TTL field contains a numeric value - The numeric value is decreased by one by every router on the route to the destination. If numeric value reaches 0 then Packet is discarded. Routing Protocols Today Factors used to determine whether to use RIP or EIGRP include o Network size o Compatibility between models of routers o Administrative knowledge RIP o Features of RIP: June 23, 2008 Page 19

21 Supports split horizon & split horizon with poison reverse Capable of load balancing Easy to configure Works in a multi vendor router environment EIGRP o Features of EIGRP: Triggered updates EIGRP hello protocol used to establish neighbor adjacencies - Supports VLSM & route summarization - Use of topology table to maintain all routes - Classless distance vector routing protocol - Cisco proprietary protocol Summary Characteristics of Distance Vector routing protocols o Periodic updates o RIP routing updates include the entire routing table o Neighbors are defined as routers that share a link and are configured to use the same protocol The network discovery process for D.V. routing protocol o Directly connected routes are placed in routing table 1st o If a routing protocol is configured then Routers will exchange routing information o Convergence is reached when all network routers have the same network information D.V. routing protocols maintains routing tables by o RIP sending out periodic updates o RIP using 4 different timers to ensure information is accurate and convergence is achieved in a timely manner o EIGRP sending out triggered updates D.V. routing protocols may be prone to routing loops o Routing loops are a condition in which packets continuously traverse a network o Mechanisms used to minimize routing loops include defining maximum hop count, holddown timers, split horizon, route poisoning and triggered updates Conditions that can lead to routing loops include o Incorrectly configured static routes o Incorrectly configured route redistribution o Slow convergence o Incorrectly configured discard routes How routing loops can impact network performance includes: o Excess use of bandwidth o CPU resources may be strained o Network convergence is degraded o Routing updates may be lost or not processed June 23, 2008 Page 20

22 Routing Information Protocol (RIP) o A distance vector protocol that has 2 versions RIPv1 a classful routing protocol RIPv2 - a classless routing protocol o Enhanced Interior Gateway Routing Protocol (EIGRP) A distance vector routing protocols that has some features of link state routing protocols A Cisco proprietary routing protocol Chapter 5 RIP version 1 Objectives Describe the functions, characteristics, and operation of the RIPv1 protocol. Configure a device for using RIPv1. Verify proper RIPv1 operation. Describe how RIPv1 performs automatic summarization. Configure, verify, and troubleshoot default routes propagated in a routed network implementing RIPv1. Use recommended techniques to solve problems related to RIPv1. RIPv1 RIP Characteristics o A classful, Distance Vector (DV) routing protocol o Metric = hop count o Routes with a hop count > 15 are unreachable o Updates are broadcast every 30 seconds RIP Message Format RIP header - divided into 3 fields o Command field o Version field o Must be zero Route Entry - composed of 3 fields o Address family identifier o IP address o Metric RIP Operation RIP uses 2 message types: o Request message This is sent out on startup by each RIP enabled interface Requests all RIP enabled neighbors to send routing table o Response message Message sent to requesting router containing routing table June 23, 2008 Page 21

23 IP addresses initially divided into classes o Class A o Class B o Class C RIP is a classful routing protocol o Does not send subnet masks in routing updates Administrative Distance o RIP s default administrative distance is 120 CISCO, INC. Basic RIPv1 Configuration A typical topology suitable for use by RIPv1 includes: o Three router set up o No PCs attached to LANs o Use of 5 different IP subnets Router RIP Command o To enable RIP enter: Router rip at the global configuration prompt Prompt will look like R1(config-router)# Specifying Networks o Use the network command to: Enable RIP on all interfaces that belong to this network Advertise this network in RIP updates sent to other routers every 30 seconds Verification and Troubleshooting Show ip Route To verify and troubleshoot routing o Use the following commands: show ip route show ip protocols debug ip rip show ip protocols command o Displays routing protocol configured on router Debug ip rip command o Used to display RIP routing updates as they are happening Passive interface command o Used to prevent a router from sending updates through an interface o Example: Router(config-router)#passive-interface interface-type interface-number Passive interfaces Automatic Summarization Modified Topology o The original scenario has been modified such that: Three classful networks are used: June 23, 2008 Page 22

24 / / /24 The /16 network is subnetted into three subnets: / / /24 The following devices are part of the /16 classful network address: - All interfaces on R1 - S0/0/0 and Fa0/0 on R2 Configuration Details o To remove the RIP routing process use the following command No router rip o To check the configuration use the following command Show run Boundary Routers o RIP automatically summarizes classful networks o Boundary routers summarize RIP subnets from one major network to another Processing RIP Updates o 2 rules govern RIPv1 updates: If a routing update and the interface it s received on belong to the same network then - The subnet mask of the interface is applied to the network in the routing update If a routing update and the interface it s received on belong to a different network then - The classful subnet mask of the network is applied to the network in the routing update. Sending RIP Updates o RIP uses automatic summarization to reduce the size of a routing table. Advantages of automatic summarization: o The size of routing updates is reduced o Single routes are used to represent multiple routes which results in faster lookup in the routing table. Disadvantage of Automatic Summarization: o Does not support discontiguous networks Discontiguous Topologies do not converge with RIPv1 A router will only advertise major network addresses out interfaces that do not belong to the advertised route. Default Route and RIPv1 Modified Topology: Scenario C Default routes June 23, 2008 Page 23

25 o Packets that are not defined specifically in a routing table will go to the specified interface for the default route o 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 Propagating the Default Route in RIPv1 Default-information originate command o This command is used to specify that the router is to originate default information, by propagating the static default route in RIP update. Summary RIP characteristics include: o Classful, distance vector routing protocol o Metric is Hop Count o Does not support VLSM or discontiguous subnets o Updates every 30 seconds Rip messages are encapsulated in a UDP segment with source and destination ports of 520 Commands Used by RIP o Rtr(config)#router rip Enables RIP routing process o Rtr(config-router)#network Associates a network with a RIP routing process o Rtr#debug ip rip used to view real time RIP routing updates o Rtr(config-router)#passive-interface fa0/0 Prevent RIP updates from going out an interface o Rtr(config-router)#default-information originate Used by RIP to propagate default routes o Rtr#show ip protocols Used to display timers used by RIP Chapter 6 VLSM and CIDR Objectives Compare and contrast classful and classless IP addressing. Review VLSM and explain the benefits of classless IP addressing. Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses. Introduction Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address June 23, 2008 Page 24

26 In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes IP address space was depleting rapidly The Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR) o CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space. VLSM is simply subnetting a subnet Classful and Classless IP Addressing Classful IP addressing As of January 2007, there are over 433 million hosts on internet Initiatives to conserve IPv4 address space include: o VLSM & CIDR notation (1993, RFC 1519) o Network Address Translation (1994, RFC 1631) o Private Addressing (1996, RFC 1918) The High Order Bits o These are the leftmost bits in a 32 bit address Classes of IP addresses are identified by the decimal number of the 1st octet o Class A address begin with a 0 bit Range of class A addresses = to o Class B address begin with a 1 bit and a 0 bit Range of class B addresses = to o Class C addresses begin with two 1 bits & a 0 bit Range of class C addresses = to The IPv4 Classful Addressing Structure (RFC 790) o An IP address has 2 parts: The network portion - Found on the left side of an IP address The host portion - Found on the right side of an IP address Purpose of a subnet mask o It is used to determine the network portion of an IP address Classful Routing Updates o Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates. The reason is that the Subnet mask is directly related to the network address Classless Inter-domain Routing (CIDR RFC 1517) o Advantage of CIDR: More efficient use of IPv4 address space Route summarization o Requires subnet mask to be included in routing update because address class is meaningless Recall purpose of a subnet mask: June 23, 2008 Page 25

27 - To determine the network and host portion of an IP address Classless IP Addressing CIDR & Route Summarization o Variable Length Subnet Masking (VLSM) o Allows a subnet to be further sub-netted according to individual needs o Prefix Aggregation a.k.a. Route Summarization o CIDR allows for routes to be summarized as a single route Classless Routing Protocol Characteristics of classless routing protocols: o Routing updates include the subnet mask o Supports VLSM o Supports Route Summarization Routing Protocol Routing Updates Include Subnet Mask Supports VLSM Ability to Send Supernet Routes Classful No No No Classless Yes Yes Yes VLSM Classful routing o Only allows for one subnet mask for all networks VLSM & classless routing o This is the process of subnetting a subnet o More than one subnet mask can be used o More efficient use of IP addresses as compared to classful IP addressing VLSM the process of sub-netting a subnet to fit your needs o Example: Subnet /16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask. - Mask allows for 254 host addresses per subnet - Subnets range from: / 24 to / 24 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 o Example: / 13 is the summarized route for the / 16 to / 16 classful networks Steps to calculate a route summary o List networks in binary format o Count number of left most matching bits to determine summary route s mask o Copy the matching bits and add zero bits to determine the summarized network address June 23, 2008 Page 26

28 Summary Classful IP addressing o IPv4 addresses have 2 parts: Network portion found on left side of an IP address Host portion found on right side of an IP address o Class A, B, & C addresses were designed to provide IP addresses for different sized organizations o The class of an IP address is determined by the decimal value found in the 1st octet o IP addresses are running out so the use of Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are used to try and conserve address space o Classful Routing Updates Subnet masks are not sent in routing updates o Classless IP addressing Benefit of classless IP addressing - Can create additional network addresses using a subnet mask that fits your needs Uses Classless Interdomain Routing (CIDR) CIDR o Uses IP addresses more efficiently through use of VLSM VLSM is the process of subnetting a subnet o Allows for route summarization Route summarization is representing multiple contiguous routes with a single route Classless Routing Updates o Subnet masks are included in updates Chapter 7 RIPv2 Objectives Encounter and describe the limitations of RIPv1 s limitations. Apply the basic Routing Information Protocol Version 2 (RIPv2) configuration commands and evaluate RIPv2 classless routing updates. Analyze router output to see RIPv2 support for VLSM and CIDR. Identify RIPv2 verification commands and common RIPv2 issues. Configure, verify, and troubleshoot RIPv2 in hands-on labs. Introduction Chapter focus o Difference between RIPv1 & RIPv2 RIPv1 June 23, 2008 Page 27

29 - A classful distance vector routing protocol - Does not support discontiguous subnets - Does not support VLSM - Does not send subnet mask in routing update - Routing updates are broadcast RIPv2 - A classless distance vector routing protocol that is an enhancement of RIPv1 s features. - Next hop address is included in updates - Routing updates are multicast - The use of authentication is an option o Similarities between RIPv1 & RIPv2 Use of timers to prevent routing loops Use of split horizon or split horizon with poison reverse Use of triggered updates Maximum hop count of 15 RIPv1 Limitations Lab Topology o Scenario: 3 router set up Topology is discontiguous There exists a static summary route Static route information can be injected into routing table updates using redistribution. Routers 1 & 3 contain VLSM networks o Scenario Continued VLSM - Recall this is sub netting the subnet Private IP addresses are on LAN links Public IP addresses are used on WAN links Loopback interfaces - These are virtual interfaces that can be pinged and added to routing table Null Interfaces o This is a virtual interface that does not need to be created or configured Traffic sent to a null interface is discarded Null interfaces do not send or receive traffic Static routes and null interfaces o null interfaces will serve as the exit interface for static route Example of configuring a static supernet route with a null interface R2(config)#ip route Null0 Route redistribution o Redistribution command is way to disseminate a static route from one router to another via a routing protocol June 23, 2008 Page 28

30 o Example R2(config-router)#redistribute static Verifying and Testing Connectivity o Use the following commands: show ip interfaces brief ping traceroute RIPv1 a classful routing protocol o Subnet mask are not sent in updates o Summarizes networks at major network boundaries o if network is discontiguous and RIPv1 configured convergence will not be reached Examining the routing tables o To examine the contents of routing updates use the debug ip rip command o If RIPv1 is configured then Subnet masks will not be included with the network address RIPv1 does not support VLSM o Reason: RIPv1 does not send subnet mask in routing updates RIPv1 does summarize routes to the Classful boundary o Or uses the Subnet mask of the outgoing interface to determine which subnets to advertise No CIDR Support In the diagram R2 will not include the static route in its update o Reason: Classful routing protocols do not support CIDR routes that are summarized with a smaller mask than the classful subnet mask Configuring RIPv2 Comparing RIPv1 & RIPv2 Message Formats o RIPv2 Message format is similar to RIPv1 but has 2 extensions 1st extension is the subnet mask field 2nd extension is the addition of next hop address Enabling and Verifying RIPv2 Configuring RIP on a Cisco router o By default it is running RIPv1 Configuring RIPv2 on a Cisco router - Requires using the version 2 command - RIPv2 ignores RIPv1 updates To verify RIPv2 is configured use the show ip protocols command Auto-Summary & RIPv2 RIPv2 will automatically summarize routes at major network boundaries and can also summarize routes with a subnet mask that is smaller than the classful subnet mask Disabling Auto-Summary in RIPv2 To disable automatic summarization issue the no auto-summary command June 23, 2008 Page 29