1. [Networking Fundamentals] [Y. K. Choi]
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Week Four - ARP and RARP
[Networking Fundamentals]
[Y. K. Choi]
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2. Review on Information Flow
You learnt that as information works its way down through the layers described by the OSI model, it is transformed from language people understand into language computers can understand
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5. How do devices know when to copy data so that it goes above the network layer of the OSI model?
the 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 be passed to the upper levels.
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6. Highlight those things that made progress possible
How do devices on a local area network know the MAC addresses and IP addresses of other devices?
Some devices keep tables containing all the MAC addresses and IP addresses of other devices connected to the same LAN. These tables are nothing more than a section of RAM memory on each device.
They are called ARP tables because a language known as the Address Resolution Protocol is used to map IP addresses to MAC addresses. For the most part ARP tables are cached in memory and are maintained automatically. List achievements and progress since last status update was given
address schedule implications
Highlight those things that made progress possible
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7. ARP Table
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8. Function of ARP Table
Whenever a network device wants to send data on the network it uses information provided by its ARP table to accomplish this.
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9. How do devices use ARP tables to send data?
Once the source has determined the IP address for the destination, the source's internet protocol will look into its ARP table in order to locate the MAC address for the destination
If the internet protocol locates a mapping of destination IP address to destination MAC address in its table, it will bind the IP address with the MAC address and use them to encapsulate the data.
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10. Example
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15. What happens if the device is unable to locate the destination MAC address in its ARP table?
the device initiates a process called an ARP request to discover what the destination MAC address is.
First, the device builds an ARP request packet and sends it to all devices on the network using a broadcast MAC address.
The broadcast address that is used in a MAC addressing scheme results when all places are set to F.
Thus, a MAC broadcast address would have the form FF-FF-FF-FF-FF-FF.
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16. What information is contained in an ARP request?
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17. Frame Header
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18. What happens when the source sends the ARP request out over the network?
When source is asking for the MAC address of the destination with an IP address of
destination would pick up the ARP request and respond with an ARP reply containing its MAC address.
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19. The Result in Frame Header
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20. What happens when the ARP reply is sent back to the originating device on the network?
Once the device that originated the ARP request receives the ARP reply, it extracts the MAC address from the MAC header and updates its ARP table.
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21. What do other network devices do when an ARP reply is sent?
Any device on the network that received the broadcast ARP request sees the information supplied in the ARP request. Devices use the source information to update their ARP tables.
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22. Why is it important for network devices to update their ARP tables?
If devices did not keep ARP tables, the process of issuing an ARP request and an ARP reply would have to take place each time for each device.
This would be extremely inefficient and could result in too much traffic on the network.
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23. What is RARP?
The protocol that a device uses when it does not know its own IP address is the Reverse Address Resolution Protocol, or as it is more commonly called, RARP.
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24. What kinds of devices would need to use RARP?
This occurs in diskless workstations or dummy terminals.
Each time a diskless workstation ends a session and is shut down, what memory cache it possesses disappears, because it has no hard drive
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25. What do devices using RARP require?
Devices using RARP require that a RARP server be present on the network to answer RARP requests.
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26. What is a RARP request?
Imagine that a source device wants to send data to another device. The source knows its own MAC address, but is unable to locate its own IP address in its ARP table.
If the destination is to retain the data, pass it to the higher layers of the OSI model, and respond to the device that originated the data, the source must include both its MAC address and IP address.
Therefore the source initiates a process called a RARP request that is designed to help it discover what its IP address is
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27. What information is contained in a RARP request?
RARP requests have the same structure as ARP requests.
Thus, RARP requests consist of a MAC header, an IP header, and the ARP request message.
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28. What happens when the source sends the RARP request out over the network?
Since the RARP request frame is sent out in broadcast mode, all devices on the network will see it.
However, only a designated RARP server can respond to the RARP request.
The designated RARP server replies by sending a RARP reply containing the IP address of the device that originated the RARP request.
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29. What structure do RARP replies have and what information is contained in each of the headers?
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30. What does the device that originated the RARP request do when it receives the RARP reply?
When the device that originated the RARP request receives the RARP reply
it copies its IP address into its memory cache where it will reside for as long as the session lasts.
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31. What internetworking devices have ARP tables?
The router interface connected to the network has an IP address for that network.
Routers, just like every other device on the network, send and receive data on the network, they build ARP tables that map IP addresses to MAC addresses.
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32. How are ARP tables kept by routers different from ARP tables kept by other devices on a network?
Because routers build tables that describe all networks connected to them,
ARP tables kept by routers can contain IP addresses and MAC addresses of devices located on more than one network.
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33. Example
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35. What other addresses do routers contain in their tables?
What happens if a data packet reaches a router destined for a network that the router is not connected to?
In addition to IP addresses and MAC addresses of devices located on networks it is connected to, a router also possesses IP addresses and MAC addresses of other routers.
It uses these addresses to direct data toward its final destination.
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37. What happens when a device on a subnetwork does not know the destination MAC address of a device on another subnetwork?
Generally speaking, a device on one network cannot send an ARP request to a device on another network. Can you think of a reason why this is so?
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38. When does a device seek out the services of a router?
If the source resides on a network with a different network number than that of the desired destination, and if the source does not know the MAC address of the destination,
it will have to use the services of a router in order for its data to reach the destination.
When a router is used in this manner, it is called a default gateway.
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40. What happens if the device does not know the MAC address of the router it wants to use to perform indirect routing services?
the source issues an ARP request.
The ARP request will be picked up by a router connected to the same network as the source.
This router will issue an ARP reply to the device that originated the ARP request.
The reply will contain the MAC address of the non-local router.
Thus, 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.
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41. What is the difference between routed protocols and routing protocols?
One protocol that you have been learning about is IP, or the Internet protocol. You know that IP is a network layer protocol. Thus far, lessons have focused on IP's features for addressing, however, it also provides features for type-of-service specification, and fragmentation and reassembly. Because IP is routed over an internetwork, it is what is called a routed protocol.
Routers use routing protocols to exchange routing tables and share routing information. In other words, routing protocols are protocols that determine how routed protocols get routed.
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42. Routing Protocols
Routing Information Protocol or RIP the Interior Gateway Routing Protocol or IGRP, the Enhanced Interior Gateway Routing Protocol or EIGRP, and Open Shortest Path First or OSPF.
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43. What is the difference between IGPs and EGPs?
Two types of routing protocols are the Interior Gateway Protocols or IGPs and the Exterior Gateway Protocols or EGPs. RIP, IGRP, EIGRP, and OSPF are all examples of IGPs.
Interior gateway protocols are used to route data into an autonomous system.
Exterior gateway protocols are used to route data between autonomous systems.
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44. What is RIP and how does it work?
Within a network, the most common method used to transfer routing information between routers located on the same network is RIP. This interior gateway protocol calculates distances to a destination. RIP allows routers using this protocol to update their routing tables at a programmable interval, typically every thirty seconds. Because RIP is constantly connecting between neighboring routers, this can cause network traffic to build.
RIP allows routers to determine what path will be used to send data based on a concept known as vector-distance.
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45. What are IGRP and EIGRP?
IGRP and EIGRP are routing protocols developed by Cisco. Therefore, they may be considered to be proprietary routing protocols.
IGRP was developed specifically to address problems associated with routing in large multi-vendor networks that posed problems beyond the scope of protocols such as RIP. Like RIP, IGRP is a distance vector protocol, however it also takes into consideration such things as bandwidth, load, delay, and reliability when determining the best path.
EIGRP is an advanced version of IGRP.
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46. What is OSPF?
Although OSPF means open shortest path first, a better description for this acronym 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.
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47. How do routers learn about networks?
Basically, there are two ways that routers can learn about route information. These are called static and dynamic routing. Manual entries placed in a routing table are examples of static routing. When routes are learned automatically, this is called dynamic routing.
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48. What are some examples of when static routing would be used?
If routers can learn routing information automatically, it might seem pointless to enter information into a router's routing tables manually.
However, such manual routing table entries can be useful anytime a network administrator wants to control which path a router will choose.
For example, routing tables based on static information could be used to test a particular link in the network or conserve wide area bandwidth.
Static routing is also the preferred method for maintaining routing tables whenever there is only one path to a destination as would be the case with a stub network.
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49. What is an example of when dynamic routing would be used?
Adaptive or dynamic routing occurs when routers send periodic routing update messages to each other. Each time such a message is received containing new information, the router recalculates the new best route and sends new update information out to other routers. By using dynamic routing, routers can adjust to changing network conditions.
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50. How do routers use routing protocols such as RIP to route data through networks?
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