COMPUTER COMMUNICATION NETWORKS. Department of ECE, ACE. Page 2. Every layer clubs together all procedures, protocols, and methods which it. “No man but a blockhead ever wrote, except for money.” - Samuel Johnson. The textbook world is changing. On the one hand, open source. Communication and Computer Networks. Prerequisites. Before proceeding with this tutorial, you need a basic understanding of Computer. You should know the.
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PDF | On Jan 1, , D B Hoang and others published Computer Communication Networks—Lecture Notes. This book is concerned with post-computer communication networks and two of increasingly relying on digital computer technology, and data communication. Computer Communication Networks Lecture Notes Lecture 0, Course Overview, ps · pdf · ps · pdf · ps · pdf · ps · pdf, -. Lecture 1, Network Foundation, ps · pdf.
Serial ports are pre-connected to the serial patch panel. The ports are labeled on their left. You will find the console cable as a UTP cable with one of its ends connected through a small devices to a serial port on the PC. Cisco routers support different modes of operation. Cisco routers use many other modes, but let us keep it simple for now. Connect the PC to R1. Question mark lists commands that can be used in a certain context.
You only need to type enough of a command to differentiate it from all other commands. Type the following commands: The prompt ends with router-config? Go into configuration mode and type the following commands: Type these commands: Now you can configure interface Ethernet0. This command will bring them up. Now type the following command: Again type this command: Cisco commands are not case-sensitive.
Determine which mode you operate in when you first access the router. The command used to save changes made in the running configuration to start-up configuration is: List the interfaces on three routers of your choice. Be sure to indicate the router number. Which of the condition s are possible for an interface: Figure 5. In this kind of protocols we require an addressing scheme and sub netting.
Addressing scheme will be used to determine the network to which a host belongs and to identifying that host on that particular network. All hosts on an internetwork use the services of a routed protocol.
It makes possible for routers to build and maintain routing tables. Although dynamic routing protocols are flexible and adjust to network changes, they do have associated network traffic which competes for network bandwidth with the user data traffic.
Configuring Static Routes Static routes specify a fixed route for a certain destination network. They need to be configured on any router that needs to reach a network that it is not directly connected to.
The IOS command used to configure static routes is ip route. The syntax is: Multiple networks may be combined such that the destination- address and subnet-mask combination matches all hosts on those networks. Adding a static route to an Ethernet or other broadcast interface for example, ip route 0. This configuration is not generally recommended. When the next hop of a static route points to an interface, the router considers each of the hosts within the range of the route to be directly connected through that interface, and therefore it will send ARP requests to any destination addresses that route through the static route.
If unspecified the default value is 1. Connect the network as shown in the network diagram. Configure appropriate ip addresses and clock rates if needed on the router interfaces as specified in the network diagram. For R1, enter the following static routes ip route On R2 enter: On R3 enter: After that verify the static routes by entering the following commands in the privilege mode: Run the command show IP route and write its output.
What is the default administrative distance of static route? Write the IP route command to modify the same. Create a loop back interface on R3 and assign an IP address Now add static routes to each of the other routers to reach this interface. Verify your work by pinging the newly created interface from routers R1 and R2 respectively. Figure 6. An autonomous system is a collection of networks under a single administration, sharing a common routing strategy.
Computer Communications and Networks
A distance-vector protocol, RIP was designed to work with small to medium-sized networks. Some advantages of using RIP, especially in small networks, is that there is very little overhead, in terms of bandwidth used and configuration and management time. RIP is also easy to implement, compared to newer IGPs, and has been implemented in networks around the world. All routers that use RIP send an update message to all of their neighbors approximately every 30 seconds; this process is termed advertising.
The Cisco implementation sends updates every 30 seconds minus up to 15 percent, or 4. When the neighbor has not responded for seconds, the route is marked invalid; seconds is long enough that a route won't be invalidated by a single missed update message. The neighbor is shown to be unreachable by sending a normal update message with a metric of "infinity;" in the case of RIP, this number is If an advertisement is received from a neighbor with a metric of infinity, then the route is placed into hold-down state, advertised with a distance of 16, and kept in the routing table.
No updates from other neighbors for the same route are accepted while the route is in hold-down state. If other neighbors are still advertising the same route when the hold-down timer expires, then their updates will then be accepted.
The route will be advertised with infinity metric for a period of time after the hold-down state if no alternate paths are found. The actual timers used to accomplish the above tasks are a routing-update timer, a route- invalid timer, a route-hold-down timer, and a route-flush timer.
The RIP routing-update timer is generally set to 30 seconds, ensuring that each router will send a complete copy of its routing table to all neighbors every 30 seconds. The route-invalid timer determines how much time must expire without a router having heard about a particular route before that route is considered invalid.
When a route is marked invalid or put in hold-down state, neighbors are notified of this fact. This notification must occur prior to expiration of the route-flush timer. When the route flush-timer expires, the route is removed from the routing table. Typical initial values for these timers are seconds for the route-invalid and route-holddown timers and seconds for the route-flush timer. The values for each of these timers can be adjusted with the timers basic router configuration command.
Several Stability Features To adjust for rapid network-topology changes, RIP specifies numerous stability features that are common to many routing protocols. RIP implements split horizon with poison-reverse and hold-down mechanisms to prevent incorrect routing information from being propagated.
Split horizon prevents incorrect messages from being propagated by not advertising routes over an interface that the router is using to reach the route.
Implementing split horizon helps avoid routing loops. Poison reverse operates by advertising routes that are unreachable with a metric of infinity back to the original source of the route. Hold-down is a method of marking routes invalid expired. As discussed above, no updates from other neighbors for the same route are accepted while the route is in hold-down state. Triggered updates are also an included convergence and stability feature. Updates are triggered whenever a metric for a route changes.
Triggered updates may also contain only information regarding routes that have changed, unlike scheduled updates. There is a minimum delay of five seconds between triggered updates to prevent update storms.
Cable up the network as shown in the diagram.
Assign the IP address as shown in the diagram to the appropriate interfaces. For the serial links, has been used to indicate a DCE port.
Issue RIP routing commands on all the routers starting from the global config mode. On R1: Ping the host from R1. Type escape sequence to abort. Configure RIP version 1 on two routers. Run Debug ip rip and note the address on which updates are sent. Write commands to modify the default update and hold-down timers for RIP v1. RipV2 also sends its complete routing table to its active interfaces at periodic time intervals.
The timers, loop avoidance schemes and administrative distance are the same as Rip version 1. It also allows authentication using MD5 encryption scheme.
And it also supports dis-contiguous networks. Configuring router with RIP version 2 is very simple. Just add the command version 2 under the config-router prompt and the router is running RIPv2. Note down the routing table for Router R1. Run the command debug rip and note down the multicast address on which RIPv2 forwards the updates.
Write down the source IP address for the ping packets when you ping H1 from R1. While working on R1, how could you check if H1 can reach the loopback interface?
In other words, how can you verify if a ping from H1 to loopback of R1 is successful? It is an open standards protocol—that is, it isn't proprietary to any vendor or organization. Link-state routing protocols perform the following functions: Like all link state protocols, OSPF's major advantages over distance vector protocols are fast convergence, support for much larger internetworks, and less susceptibility to bad routing information.
Other features of OSPF are: Each router on the network forms an adjacency with the DR which represents the pseudo-node. In other words, the DR is a property of a router's interface, not the entire router. Fig 8. Router A: This is a block size of 8, which is a wildcard of 7. Similarly the other subnet ,mask, and wildcard can be determined by looking at the IP address of an interface.
Principles of Computer Communication Networks
Router B: Scenario for exercise problems Simulate the network shown above on packet tracer. Write down the configuration commands entered on all three routers for configuration of OSPF. Router 1: Router 2: Router 3: It is important to understand EIGRP because it is probably one of the two most popular routing protocols in use today.
EIGRP is sometimes referred to as a hybrid routing protocol because it has characteristics of both distance vector and link state protocols. And EIGRP has link state characteristics as well — it synchronizes routing tables between neighbours at startup and then sends specific updates only when topology changes occur.
EIGRP has a maximum hop count of the default is set to EIGRP metric calculation: Router1 config router eigrp 1 Router1 config-router network Router2 sh ip route Codes: Reply from The procedures differs depending on the platform and the software used, but in all cases, password recovery requires that the router be taken out of operation and powered down.
Please use cisco as the password where necessary. Please be prepared to do password recovery right away. The group before you might have set a password other than cisco. Use show version command to determine the platform before you try the password recovery. You will be working with the configuration register as part of this lab.
The config-register is a 16 bit register.
If there is no boot system command, the router forms a default boot filename for booting from a network server. If there is no network server configured, as is the case in our lab, the standard setup dialogue is started.
You have access only to the user mode. Follow the instructions below from the user mode. Do not get into privileged mode.
Type show version and record the value of the configuration register. Using the power switch, turn off the router and then turn it on. This is where the procedure differs depending on the platform. For 25XX and The router reboots but ignores its saved configuration. For , , , Type no after each setup question or press Ctrl-C to skip the initial setup procedure. You'll be in enable mode and see the Router prompt.
Do not type config term. Type config term and make the changes. The prompt is now hostname config. Type config-register 0x, or the value you recorded in step 1. Type write mem or copy running startup to commit the changes. Type show version and observe the configuration register setting carefully. Explain the setting when the configuration-register is set to 0x There are many different ways to access a router.
Write down these ways. Explain the need for step 7 in password recovery procedure. When you configure enable password and issue the command show running, you can see the password set for the privileged mode. Is there a method to prevent it from being visible? Set the configuration-register to 0x Reload the router. Does the break sequence work? Cross check with configuration-register settings and see if it matches with the settings.
Is there any difference? One of the most common and easiest to understand uses of access lists is filtering unwanted packets when implementing security policies.
Once the packet matches the condition on a line of the access list, the packet is acted upon and no further comparisons take place.
Each of these rules has some powerful implications when filtering IP packets with access lists, so keep in mind that creating effective access lists truly takes some practice. There are two main types of access lists: Standard access lists 2. Extended access lists Standard access lists These use only the source IP address in an IP packet as the condition test.
All decisions are made based on the source IP address.
This means that standard access lists basically permit or deny an entire suite of protocols. Standard Access list to allow my network Commands on router will be R1 config aaccess-list 1 permit They can evaluate source and destination IP addresses, the protocol field in the Network layer header, and the port number at the Transport layer header.
This gives extended access lists the ability to make much more granular decisions when controlling traffic. Address learning A new switch has empty MAC address table.
As each frame transits switch, it learns source MAC address against the source port. As the switch does not know to which port the destination is attached, it initially transmits the frame to all ports.
This process is called flooding. As the responses are received, the MAC address table is further populated. Thus, the basic function of STP is to prevent switching loops and ensuing broadcast radiation. It is standardized as As the name suggests, it creates a spanning tree within a mesh network of connected layer-2 switches typically Ethernet switches , and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes.
Switch loops must be avoided because they result in flooding the local network. Protocol Operation The collection of switches in a LAN can be considered a graph whose nodes are the bridges and the LAN segments or cables , and whose edges are the interfaces connecting the bridges to the segments. To break loops in the LAN while maintaining access to all LAN segments, the bridges collectively compute a spanning tree.
The spanning tree is not necessarily a minimum cost spanning tree. A network administrator can reduce the cost of a spanning tree, if necessary, by altering some of the configuration parameters in such a way as to affect the choice of the root of the spanning tree.
The spanning tree that the bridges compute using the Spanning Tree Protocol can be determined using the following rules. Select a root bridge. The root bridge of the spanning tree is the bridge with the smallest lowest bridge ID. Each bridge has a unique identifier ID and a configurable priority number; the bridge ID contains both numbers. To compare two bridge IDs, the priority is compared first. If two bridges have equal priority, then the MAC addresses are compared.
If the network administrators would like switch B to become the root bridge, they must set its priority to be less than The computed spanning tree has the property that messages from any connected device to the root bridge traverse a least cost path, i.
The cost of traversing a path is the sum of the costs of the segments on the path. Different technologies have different default costs for network segments. An administrator can configure the cost of traversing a particular network segment. The property that messages always traverse least-cost paths to the root is guaranteed by the following two rules. Least cost path from each bridge. After the root bridge has been chosen, each bridge determines the cost of each possible path from itself to the root.
From these, it picks one with the smallest cost a least-cost path. The port connecting to that path becomes the root port RP of the bridge. Least cost path from each network segment. The bridges on a network segment collectively determine which bridge has the least-cost path from the network segment to the root. The port connecting this bridge to the network segment is then the designated port DP for the segment.
Disable all other root paths. Any active port that is not a root port or a designated port is a blocked port BP. Bridge Protocol Data Units BPDUs The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network. The bridges have to determine the root bridge and compute the port roles root, designated, or blocked with only the information that they have. To ensure that each bridge has enough information, the bridges use special data frames called Bridge Protocol Data Units BPDUs to exchange information about bridge IDs and root path costs.
There are three types of BPDUs: When a device is first attached to a switch port, it will not immediately start to forward data. It will instead go through a number of states while it processes BPDUs and determines the topology of the network.
The time spent in the listening and learning states is determined by a value known as the forward delay default 15 seconds and set by the root bridge.
However, if instead another switch is connected, the port may remain in blocking mode if it is determined that it would cause a loop in the network. TCNs are injected into the network by a non-root switch and propagated to the root.
This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entries.
Coden, Electronic Communications. Prentice Hall, India. Kennedy, G.
Principles of Computer Communication Networks
Davis, Electronic Communication Systems. Gitlin, R.
Data Communication Principles. Plenum Press, NY. Bissell, C. Chapman, Digital Signal Transmission. Cambridge University Press, UK.
Tanenbaum, A. Computer Networks. Stallings, W. Data and Computer Communication. Peterson, L.An administrator can configure the cost of traversing a particular network segment. A, B, C, D, and E. Hold the wire in place and squeeze the crimper handles quite firmly. Switch loops must be avoided because they result in flooding the local network.
The receiver detects the total difference between these two pules. Multiple networks may be combined such that the destination- address and subnet-mask combination matches all hosts on those networks.
By Yann Sopheak.
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