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Chapter 8 Routing

Chapter 8 Routing. Introduction. Look at: Routing Basics (8.1) Address Resolution (8.2) Routing Protocols (8.3) Administrative Classification (8.4) Hierarchical Routing (8.5). Introduction. When networking was first introduced, only a small number of devices were interconnected

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Chapter 8 Routing

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  1. Chapter 8 Routing

  2. Introduction • Look at: • Routing Basics (8.1) • Address Resolution (8.2) • Routing Protocols (8.3) • Administrative Classification (8.4) • Hierarchical Routing (8.5)

  3. Introduction • When networking was first introduced, only a small number of devices were interconnected • As networks grew, broadcasts and collisions caused significant issues • In order to reduce broadcasts, Layer 3 devices, protocols, and addresses are used to segment the network

  4. Routing Basics • Reasons to segment a network at Layer 3 include: • Creation of small troubleshooting areas • Creation of small administrator management areas • Interconnection of remote offices using WAN technologies • Grouping clients together with similar network resources

  5. Routing Basics • A router is required to properly forward data between clients on different segments. • A routeris a device that forwards data based on a logical Layer 3 address • Many routers support the use of different protocols

  6. Routing Basics • The routing process usually occurs between physical network interfaces but can also be accomplished between logical network interfaces • When a single physical network interface has more than one address assigned, it is said to have a logical interface • A router can be any network device with the proper software to make routing decisions

  7. Routing Basics • Two key pieces of information are required for any device to route packets: • A route to one or more networks • A destination Layer 3 address • The routes to a given network are stored in the memory of the router and are referred to as a routing table

  8. Routing Basics • On a Router there is an entry for each network and its corresponding network interface kept in memory • The table allows the router to properly forward frames out the correct interface • This occurs once the frame is received and the destination address has been processed

  9. Routing Basics • In standard routing, the route table is consulted every time a frame is received and so it plays a fundamental role in the proper delivery of data • A routing table only maintains the best possible route to a destination, not all possible routes

  10. Routing Basics • Routing table entries have the following functions: • Network destination • Netmask • Gateway • Interface • Metric

  11. Routing Basics • The task of populating the routing table is accomplished by using either dynamic routing or static routing • Dynamic routing uses routing protocols to build route tables automatically • Static routing requires manual route table entries and updates to different networks

  12. Routing Basics • Layer 3 addressing is critical for end-to-end reachability and does not change throughout the routing process • The Layer 2  address moves the packet from one hand-off point or hop to the next • The router’s Layer 2 address is the frame’s destination • The Layer 3 address remains constant through each hop

  13. Address Resolution • Address resolution is the mapping of one address to another • It is generally a mapping between a Layer 3 network address (logical) and a Layer 2 hardware address (physical) • The reverse process is also address resolution

  14. Address Resolution • Address resolution is accomplished in one of the following ways: • Table lookup is a rarely used method of address resolution   • Closed-form computation is only used in very specific networks and is time consuming to configure • Dynamic message exchange is the most common and involves an exchange of information between two hosts

  15. Address Resolution • Address Resolution Protocol (ARP) is used when an IP host has a known destination IP address (Layer 3) and it needs to retrieve the corresponding Layer 2 MAC address from the destination host • The ARP cache is used to further reduce the need for broadcasts by storing the IP-to- MAC mapping in memory for a specified duration

  16. Address Resolution • The ARP Process: • Client A sends out an ARP broadcast • All clients receive and process the broadcast frame but only Machine B responds • Client A receives the response and places Machine B’s MAC address in its ARP cache

  17. Address Resolution • ARP locates the Layer 2 address when the Layer 3 address is known • Reverse Address Resolution Protocol (RARP) finds the Layer 3 address when the Layer 2 address is known • A good example of RARP is found in TCP/IP address reservations and the Boot Protocol (BootP) • Using BootP, IP hosts are automatically assigned their configuration information through a BootP server

  18. Routing Protocols • Dynamic routing uses routing protocols • Purpose of routing protocols is to build a routing table with the best routes • Routing protocols are categorized into two types: • Distance Vector • Link State

  19. Routing Protocols • Distance vector routing protocols are simple • Generally they are easy to configure • They use simple logic (algorithms) to determine the “best path” to a given destination • The term “metric” refers to the method or measurementused by the routing protocol logic to determine the best path to a given network – e.g., hops, bandwidth, latency, etc.

  20. Routing Protocols • A distance vector routing protocol usually uses hop count as its metric (RIP and RIPv.2). [IGRP – Cisco proprietary – on the other hand, uses 4 metrics and MTU, Maximum Transmission Unit, as a tie-breaker. The four metrics are Bandwidth, Distance, Latency and Reliability]. • A distance vector routing protocol is characterized by how it communicates with other routing devices • Distance vector routing protocols use broadcasts to advertise their entire routing table to directly connectedpeer routers. (With RIP, the broadcasts are every 30 seconds; with IGRP it’s every 90 seconds. This is very bandwidth-intensive and one reason that link-state routing protocols are preferred in large networks with many devices. The more devices there are, the more broadcasts will be clogging the network.)

  21. Routing Protocols • “Convergence” is the time it takes for a given set of routers to learnroutes to all networks in a give area known as the internetwork. • Convergence describes the time it takes a set of routers to learn of a change in the network – devices added, or being turned off, or going down by malfunction, etc. • Distance vector routing protocols generally take longer to converge than link state protocols because they use a periodic route advertisementschedule.

  22. Routing Protocols • A routing loop occurs when routers get confused during update operations, causing frames to bounceback and forth between a set of interfaces • Two easy methods to identify routing loops: • Tracert or traceroute TCP/IP utilities • View the routing table and the metric associated with the network

  23. Routing Protocols • Prevent routing loops by using the following software-based methods: • Split horizon • Hold-down timers • Triggered updates • Hop count limits • Poisoning

  24. Routing Protocols • Link state routing protocols are more intelligent than distance vector protocols • The metric used by most link state protocols is bandwidth allowing more complex routing configurations • Routing protocols capable of making complex decisions use a mathematical formula or algorithm for deriving the best path or route to a given network

  25. Routing Protocols • Some link state protocols  are capable of determining the best route to a destination network based on the following: • Delay • Load • Reliability • MTU

  26. Routing Protocols • When more than one metric is used it is referred to as a composite metric • Link state protocols only send updates when changes occur, and they only send the changes, not the entire route table • Link state protocols use multicast and unicast traffic instead of broadcast traffic • Link state routers also develop an overall picture of the networks available by establishing neighbor relationships

  27. Administrative Classification • Routing protocols are also separated by an administrative classification based on where they are used in the networking environment: • Interior routing protocols or interior gateway protocols • Exterior routing protocols or exterior gateway protocols

  28. Administrative Classification • Interior gateway protocols (IGPs) are used within a company’s network infrastructure to maintain routing tables and policies set by the network administrators • The two industry standard IGPs are: • Routing Information Protocol • Open Shortest Path First

  29. Administrative Classification • RIP is a distance vector protocol that uses hop count for its metric when determining the best route to a given network • In most implementations, RIP uses split horizon, hop count limit, and poisoning for routing loop prevention • RIP is a classful routing protocol

  30. Administrative Classification • The shortest path as measured by Open Shortest Path First (OSPF) is actually the fastestpath based on bandwidth • Shortest refers to the shortest time • OSPF is used in large networks and ones requiring more intelligence than distance vector routing protocols

  31. Administrative Classification • OSPF communicates using unicast andmulticast packets • It only transmits changes or updates to the routing table when they occur • It uses hello packets to determine the current state of a link between itself and its neighbors • It utilizes a link state database to maintain a local view of the entire routing environment

  32. Administrative Classification • The configuration possibilities using OSPF • Areas • Autonomous system (AS) • Backbone router • Area border router (ABR) • Autonomous system boundary router (ASBR)

  33. Administrative Classification • The decision making process of EGPs is far more complex than that of internal protocols • The power and routing flexibility associated with EGPs requires knowledge and understanding of the complex nature of your network and its traffic • EGPs can let you influence and manage traffic only as it enters or leaves your AS

  34. Administrative Classification • One member of EGPs is the Exterior Gateway Protocol (EGP) • EGP was the first protocol developed that allowed isolation of autonomous systems • EPG is not used today and is replaced by the Border Gateway Protocol

  35. Administrative Classification • Border Gateway Protocol (BGP) version 4 is the most widely used exterior protocol in the world • BGP is a well established standard and commonly used by ISPs and in very large companies • there are actually two different classifications of BGP • internal • external

  36. Administrative Classification • iBGP is used for internal routing • eBGP is used for external routing • iBGP functions under different rules than eBGP • If two routers running BGP are in the same AS, they are running an iBGP connection

  37. Administrative Classification • BGP communication starts by establishing peers • Once the peers have been established, BGP routing information is exchanged and updated as necessary • BGP is an advanced distance vector protocol that uses triggered updates for communicating changes in the routing environment • Routing loops in BGP are avoided by using the AS-path attribute

  38. Administrative Classification • BGP uses active TCP sessions that are setup and continuously maintained. Convergence in the routing environment is very fast • BGP has features that you can use to help speed the convergence of the network routes under your control

  39. Hierarchical Routing • Hierarchical routing depends on hierarchical addressing • It is a routing technique originally designed to help reduce the size of the routing tables on the Internet as well as speed up the overall routing process • The concept uses an address block or blocks to represent different sections of a network

  40. Hierarchical Routing • Summarizing routes is often referred to as supernetting networks • The process of summarization is built around the binary bit patterns just as in subnetting • The difference is that rather than extending the subnet mask by adding bits, we remove bits

  41. Hierarchical Routing • By using summarization, you reduce the routing tables on each router • To accomplish the summarization, you need to determine how many bits to unmask or un-subnet in order to make the networks appear as one big address block • The routing protocol must transmit the network prefix along with the network address during route advertisements

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