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 router is 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 using 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 to determine the best path to a given destination The term metric refers to the method or measurement used by the routing protocol logic to determine the best path to a given network

20. Routing Protocols A distance vector routing protocol usually uses hop count as its metric 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 connected peer routers

21. Routing Protocols Convergence is the time it takes for a given set of routers to learn routes to all networks Convergence describes the time it takes a set of routers to learn of a change in the network Distance vector routing protocols generally take longer to converge than link state protocols because they use a periodic route advertisement schedule

22. Routing Protocols A routing loop occurs when routers get confused during update operations, causing frames to bounce back 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 fastest path 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 and multicast 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