Sem 1v2 Chapter 11 Routing Protocols

1. Sem 1v2 Chapter 11 Routing Protocols

2. A simple definition of router functionality -- a device which makes best path routing decisions based on layer 3 addressing. The students have just finished studying the dominant Layer 3 Addressing scheme -- IP addressing. The purpose of this chapter is to illuminate how those best path decisions are made.

3. It should be noted that while routers make their decisions based on Layer 3 addresses, Layer 2 addresses remain important. For example, the router will strip off layer 2 source addresses and replace them with its OWN layer 2 source address when forwarding a packet. Bridges and switches use physical addresses.

4. This seemingly simple example -- one router connecting two simple LANs -- must be fully understood if the student is to progress very far in their understanding of more complex and realistic networks. The router switches packets to the appropriate interface based on the destination IP address. It should also be noted that the router interfaces themselves must have addresses.

5. This target indicator stresses the point that routers connect separate networks and that each of the connections to those network -- called interfaces or ports -- must have its own IP address. If this seems odd, make the point that just as hosts need NICs to connect to the network, the router has NIC-like modules in it, called interfaces, to put signals onto the media.

6. The students have probably been convinced of the importance of IP addresses. But an important question about them has been left unaddressed -- how does a host obtain its IP address? Four different methods for obtaining an IP addressed are described.

7. There are essentially two methods for assigning IP addresses - statically and dynamically. Static AddressingIf you assign IP addresses statically, you must go to each individual device and configure it with an IP address. This method requires you to keep very meticulous records. Dynamic AddressingThere are a few different methods that you can use to assign IP addresses dynamically. Examples of these are: BOOTP, RARP, and DHCP.

8. There are a few different methods that you can use to assign IP addresses dynamically Reverse Address Resolution Protocol (RARP)Reverse address resolution protocol (RARP) binds MAC addresses to IP addresses. A network device might know its MAC address but not its IP address in diskless workstations or dumb terminals. Devices using RARP require that a RARP server A RARP request consists of a MAC header, an IP header, and an ARP request message. Workstations running RARP have codes in ROM that direct them to start the RARP process, and locate the RARP server.

9. There are a few different methods that you can use to assign IP addresses dynamically BOOTstrap Protocol (BOOTP) A device uses BOOTstrap protocol(BOOTP) when it starts up, to obtain an IP address. BOOTP uses UDP to carry messages; the UDP message is encapsulated in an IP datagram. A computers uses BOOTP to send a broadcast IP datagram (using a destination IP address of all 1s - 255.255.255.255), a BOOTP server receives the broadcast and then sends a broadcast. The client receives a datagram and checks the MAC address, if it finds its own MAC address in the destination address field, it then takes the IP address in that datagram.

10. There are a few different methods that you can use to assign IP addresses dynamically Dynamic Host Configuration Protocol (DHCP) Dynamic host configuration protocol (DHCP) has been proposed as a successor to BOOTP. Unlike BOOTP, DHCP allows a host to obtain an IP address quickly and dynamically. All that is required using DHCP is a defined range of IP addresses on a DHCP server (commonly referred to as a scope). As hosts come online they contact the DHCP server and request an address. The DHCP server chooses an address and allocates it to that host

11. When DHCP clients boot, they enter an initialize state. They send a DHCPDISCOVER broadcast messages, which are UDP packets with the port number set to the BOOTP port. After sending the DHCPDISCOVER DHCPOFFER responses from DHCP server. DHCP server by sending DHCPREQUEST packets The DHCP servers acknowledge client requests with DHCPACK packets.

12. This target indicator provides an overview of the key components of the Internet Protocol -- the IP datagram, the Address Resolution Protocol, and the Internet Control Message Protocol. IP, ARP, and ICMP are all related Layer 3 protocols which a fundamental to understanding how the entire Internet works.

13. The TCP/IP suite has a protocol, called ARP, that can automatically detect the MAC address. ARP enables a computer to find the MAC address of the computer that is associated with an IP address. The basic unit of data transfer in IP is the IP datagram. Datagram processing occurs in software, which means that content and format are not hardware dependent. Another major component of IP is Internet Control Message Protocol (ICMP). This protocol is used by a device to report a problem to the sender of a message. One of their many features of ICMP is echo-request/echo-reply, which is a component that tests whether a packet can reach a destination by pinging the destination.

14. A data packet must contain both, a destination MAC address and a destination IP address. If it lacks one or the other, the data will not pass from Layer 3 to the upper layers. In this way, MAC addresses and IP addresses act as checks and balances for each other

15. They are called Address Resolution Protocol (ARP)tables, and they map IP addresses to the corresponding MAC addresses. ARP tables are sections of RAM memory, in which the cached memory is maintained automatically on each of the devices.

16. 11.2.5.1. Describe ARP operation within a subnet. If a source device wants to send data to another device, it must know the destination IP address. If it is unable to locate a MAC address for the destination, in its own ARP table, the device initiates a process called an ARP request, that enables it to discover the destination MAC address.

17. To ensure that all devices see the ARP request, the source uses a broadcast MAC address. [in hexadecimal] would have the form FF-FF-FF-FF-FF-FF.) The frame header can be further subdivided into a MAC header and an IP header. Once the originating device receives the ARP reply, it extracts the MAC address from the MAC header, and updates its ARP table.

18. 11.3.1.1. Describe a default gateway In order for a device to communicate with another device on another network, you must supply it with a default gateway. A default gatewayis the IP address of the interface, on the router, that connects to the network segment that it needs. The default gateway’s IP address must be in the same logical network as its own. If no default gateway is defined, communication is possible only on the device’s own logical network segment

19. Two general problems of internetworking. For both delivery and handling after delivery, hosts on different subnetworks must have protocols that have features beyond the LAN protocols discussed earlier

20. There is the notion that ARP uses broadcast packets in order to find a destination MAC address. But routers do not forward broadcast packets and thus a destination host on another subnetwork will not receive the ARP broadcast. This is actually a desirable property of routers; they create separate, smaller broadcast domains. Instead the host relies on the router interface which is the default gateway for the host. The default gateway router will reply to the host's ARP.

21. The process of mapping two IP addresses to one physical address is also called spoofing, and is sometimes done on networks to intercept packets. This is an important troubleshooting technique. Proxy ARP, another form of ARP, maps a single IP address to multiple MAC addresses. Routers running proxy ARP capture ARP packets and respond with the appropriate MAC addresses. Proxy ARP is generally used on subnetted networks, not on complex networks.

22. The concept of routed protocols is introduced. Without routable protocols, internetworking is impossible. Addressing and protocols are layer 3 issues. Protocols that provide support for the network layer are called routed or routable protocols.

23. Three important routable protocols, IP, IPX, and AppleTalk, are introduced. By far the most important, since it is part of the TCP/IP protocol stack and the informal "official" protocol of the Internet is IP.

24. 11.4.3.1. Name some routable and non-routable protocols. Protocols such as IP, IPX/SPX and AppleTalk provide Layer 3 support and are, therefore, routable. However, there are protocols that do not support Layer 3; these are classed as non-routable protocols. The most common of these non-routable protocols is NetBEUI.

25. In order for a protocol to be routable, it must provide the ability to assign a network number, as well as a host number. IPX, only require that you assign a network number; they use a MAC address for the physical number. IP, require that you provide a complete address, as well as a subnet mask. The network address is obtained by ANDing the address with the subnet mask.

26. Routing protocols determine the paths that routed protocols follow to their destinations. Examples of routing protocols include the Routing Information Protocol(RIP), the Interior Gateway Routing Protocol (IGRP), the Enhanced Interior Gateway Routing Protocol (IGRP), and Open Shortest Path First(OSPF) .

27. 11.5.2.1. Define routing protocolRIP is the most common protocol used to transfer information between routers on the same network Emphasize to students that without routing protocols to update each other about the state of the network topology, disruptions in that topology (which grow more likely as the internetwork grows larger) become fatal to packets (they become undeliverable for lack of a path) trying to traverse the network. Routing protocols, when properly running, assure the routers have a consistent and up-to-date way to decide how to choose the best path.

28. 11.5.3.1. Describe the sequence of encapsulation during routing A router receives the frame, strips off the frame header, then checks the destination IP address in the IP header. The router then looks for that destination IP address in its routing table, encapsulates the data in a data link layer frame, and sends it out to the appropriate interface. If it does not find the destination IP address, it drops the packet.

29. The students are introduced to the definition of multi-protocol routing. This flexible feature of routers allows them to inter-connect a diverse array of networks. The reality of the computing world is diversity -- many different vendors and protocols -- so it is important for routers to be able to process. The analogy is that the router is "multilingual."

30. 11.6.1.1. Describe connectionless network services. The concept of connectionless network services is introduced. This is a fundamental property of the Internet -- packets can take various paths to get to their destination and the destination is not contacted before a packet is sent. This helps ensure delivery if one path becomes unavailable for some reason. The students should be reminded of the postal system analogy, where zip codes are like IP addresses and where the post office performs the routing functions.

31. 11.6.2.1. Describe connection-oriented network services. The concept of connection-oriented network services is introduced there must be a connection made between the sender and the receiver. The telephone system, which relies on connection -- real physical circuits between source and destination -- is given as an example. Point out to students a potential flaw in connection-oriented systems -- if at any point the circuit is disrupted, the communication stops. The connection-oriented network is often referred as circuit switched.

32. This target indicator identifies another contrast between connectionless and connection-oriented network processes -- information can arrive out of order in a connectionless system, whereas information arrives sequentially in a connection-oriented system. Thus connectionless systems must have some provision for correctly ordering data as it arrives at the destination host. IP uses the Transport layer to determine whether packets need retransmission.

33. IP is identified as a connectionless network service. This has as its historical roots the fact the Department of Defense wanted a network that could survive a war which destroyed parts of the network. For such a network to ensure that messages could still be delivered as parts of the network were being destroyed, the concept of packet-switching and the specific implementation of IP were developed.

34. You have learned that the port where a router connects to a network, is considered part of that network, therefore, the router interface connected to the network has an IP address for that network. Routers and hosts on the network maintain APR tables. Routers send and receive data on the network, and build ARP tables that map IP addresses to MAC addresses.

35. 11.7.2.1. Explain how router ARP tables differ from ARP tables kept by other networking devices.

36. There are two differences to how router ARP tables differ from other ARP tables. First, router ARP tables contain MAC Address -- IP Address pairs from multiple networks (whereas a given host will keep ARP tables of the hosts on its network only). Secondly, the router ARP table keeps track of which interface is the path to a given MAC Address -- IP Address pair. This is of course necessary for the router to perform its jobs of best path selection and switching of packets.

37. There are MAC Address -- IP Address entries in the routers ARP table other than those of hosts. There are also entries for other ROUTERS. This is a crucial aspect of internetworking. Even if a given router does not know the exact location of the destination, it can forward a packet to other routers likely to have that information. The Internet is built on an complex hierarchy of routers which pass packets along until a router is found who can help deliver the packet.

38. 11.7.4.1. Describe what happens when a device does not know the MAC address of the router it wants to use to perform indirect routing services. ARP is used only on a local network. What would happen if a network device wanted to ask a non-local router to provide indirect routing services, but did not know the MAC address of the non-local router? When a source does not know the MAC address of a non-local router, the source issues an ARP request. A router that is connected to the same network as the source picks up the ARP request. This router issues an ARP reply to the device that originated the ARP request. The reply contains the MAC address of the non-local router. Without the ARP request ever going beyond the local network, the source is able to obtain the addressing information it needs in order to send data to devices located on distant networks.

39. 11.7.5.1. Explain what occurs when a subnetwork device does not know the destination MAC address of a device on another subnetwork. A host on one network cannot send ARP request to devices on other networks because ARP requests are broadcasts and hence are not forwarded by routers. Recall that the connection of separate networks must be achieved by a router. Working through a third party is called proxy ARP, and it allows the router to act as a default gateway.

40. When a source resides on a network that has a different network number than that of the desired destination, and when it does not know the MAC address of the destination, it must use the services of a router, in order for its data to reach the destination. A router that is used for this purpose is called a default gateway.

41. 11.8.1.1. Describe the difference between routed protocols and routing protocols.

42. Routing protocols enable routers to draw a map, internally, of the entire Internet for the purposes of routing. Such maps become part of each router's routing table

43. 11.8.2.1. Describe the difference between IGPs and EGPs.Two types of routing protocols are the Interior Gateway Protocols (IGPs) and the Exterior Gateway Protocols (EGPs). • Interior gateway protocols route data into an autonomous system. Examples of IGPs are: • RIP • IGRP • EIGRP • OSPF Exterior Gateway Protocols (EGP, BGP) are used to route packets between autonomous systems. Since the Internet is a complex combination of autonomous systems, EGPs are used by routers which form the Internet backbone.

44. The most common method, within a network, to transfer routing information between routers that are located on the same network, is RIP. RIP update their routing tables at programmable intervals every thirty seconds. Because it is constantly connecting neighboring routers, this can cause network traffic to build. RIP allows routers to determine which path it will use, to send data, based on a concept known as vector- distance. Because hop count is the only routing metric used by RIP in determining best paths, it is not necessarily the fastest path If the destination network is more than fifteen routers away, it is considered unreachable

45. IGRP and EIGRP are routing protocols that were developed by Cisco Systems, therefore, they are considered proprietary routing protocols. Like RIP, IGRP is a distance vector protocol, however, when determining the best path, it also takes into consideration such things as bandwidth, load, delay, and reliability EIGRP is an advanced version of IGRP. EIGRP provides superior operating efficiency and combines the advantages of link state protocols with those of distance vector protocols.

46. OSPF means "open shortest path first". A better description, however, might be determination of optimum path, because this interior gateway protocol actually uses several criteria to determine the best route to a destination. These include cost metrics, which factor in such things as route speed, traffic, reliability, and security

47. The two basic ways routers recognize networks -- by static and dynamic routing Static--Manual routing-table entries can be useful whenever a network administrator wants to control which path a router will select. For example, routing tables that are based on static information could be used to test a particular link in the network, or to conserve wide area bandwidth. stub networks

48. 11.8.8.1. Describe an example of dynamic routing. Adaptive, or dynamic, routing occurs when routers send periodic routing update messages to each other. Each time a router receives a message containing new information, it recalculates the new best route, and sends the new update information to other routers. By using dynamic routing, routers can adjust to changing network conditions. RIP, IGRP, EIGRP, and OSPF are all examples of dynamic routing protocols because they allow this process to occur. The END of Chapter 11

49. 11.9.1.1. Learn to use protocol analyzer software. The END of Chapter 11