Final exam study Guide

Transcription

1 Final exam study Guide K-1A * In relationship to the OSI layer model and encapsulation/decapsulation process, what happen to a packet that travels through multiple hops of routers? - What happen to the layer 2 information? - What happen to the layer 3 information? - Refers to the diagram shown below, what information (field) remain constant and does not get change during the transmission. - Refers to the diagram shown below, what information (field) does not remain constant and does get change all the time during the transmission. - Refers to the diagram shown below, you need to be able to identify all the information about: What is your source MAC address? What is your destination MAC address? What is your source IP address? What is your destination IP address? -The Source and Destination IP addresses NEVER change -The Source & Destination MAC addresses CHANGE as packet is forwarded from one router to the next. -TTL field decrement by one until a value of zero is reached at which point router discards packet (prevents packets from endlessly traversing the network) * You need to know the components of route and its related functions: - RAM: RAM stores the instructions and data needed to be executed by the CPU - ROM: Holds diagnostic software used when router is powered up. Stores the router s bootstrap program - FLASH: Contains the operating system (Cisco IOS) - NVRAM: Stores startup configuration. This may include IP addresses (Routing protocol, Hostname of router) - POST: During this self-test, the router executes diagnostics from ROM on several hardware components including the CPU, RAM, and NVRAM

2 * You need to know all the situations of router loading sequences and backup loading sequences (***). - Default complete (IOS and configuration file) loading sequence - Default and backup IOS loading sequence - Default and backup configuration file loading sequence - What happen when router cannot find the valid IOS file? - What happen when router cannot find the startup configuration file? For example: Major phases to the router boot-up process Test router hardware Power-On Self Test (POST) Execute bootstrap loader Locate & load Cisco IOS software -Locate IOS -Load IOS Locate & load startup configuration file or enter setup mode -Bootstrap program looks for configuration file * Make sure you know the different methods of encrypting password and all the commands to setup all the passwords. enable secret (strong password encryption) service password-encryption - This command causes the encryption of passwords to occur when a password is configured (weak password encryption).

3 * You are troubleshooting the router problem. What do you do when you issue the following command see the following output? R1#show ip interface brief <output omitted> FastEthernet0/ YES manual administratively down down * What devices are used to connect 2 DTE routers?

4 * Static Routes Purpose of a static route A manually configured route used when routing from a network to a stub network It does not exchange or propagate information with other routers. * What is the definition of recursive lookup? What is the process of recursive lookup? (There are multiple questions related to this topic.) - Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet. - Which routes in the output does not need to perform the recursive lookup? - For example, When a packet needs to go to /24 network (see the graphic), it finds the route to the destination network and then it sees the exit address of It than perform the 2 nd step to find which interface has that exist address to forward the address. - Your turn! -What happen when a packet needs to go to network /24? * You are expected to be tested on how to setup static route (and there are multiple questions related to this topics). You need to know the syntax of static route and configure the static route. For example, setup static route from R1 to R2 LAN. - R1(config)#ip route (or) - R1(config)#ip route s 0/0/0 If you need to remove a static route from the routing table, use the no ip route command.

5 * Default Route Default static routes are used: When no other routes in the routing table match the packet's destination IP address. A common use is when connecting a company's edge router to the ISP network. When a router has only one other router to which it is connected. This condition is known as a stub router. S* /0 is directly connected, Serial0/0/0 When no other routes in the routing table match the packet's destination IP address. A common use is when connecting a company's edge router to the ISP network. Note the * or asterisk next to the S. As you can see from the Codes table in the figure, the asterisk indicates that this static route is a candidate default route. The key to this configuration is the /0 mask. A /0 mask indicates that zero or no bits are needed to match. * What is CDP? What are the functions of CDP? Who are your neighbors if you are connected to S3? Which OSI layer is the CDP running?

6 * What would be the reason that tracert command failed on the host A? Hint1: Tracert route is a 2-way communication program. It sends out sets of ICMP messages for each hop and wait for them to comeback. Therefore, if tracert fails, the problem could be either the router is not able to find the destination network, or the return ICMP is not able to find the source network. Hint2: So, each time a new set of ICMP packets are sent, the destination network will be different, but source network is always the same. Now, you should have enough information to answer the question. Hints 3: In this example, packet successfully gets to the (Router2) and then it failed. There are only 2 possibilities. o A) Router2 does not know how to send to router3 (or) o B) Router3 received the packet from router2, but don t know how to send back to router 2. * Routing Protocols Metrics To select the best path, the routing protocol must be able to evaluate and differentiate between the available paths. For this purpose a metric is used. Each routing protocol uses its own metric. RIP uses hop count, The hop count refers to the number of routers a packet must cross to reach the destination network. For R3 in the figure, network is two hops, or two routers away. EIGRP uses a combination of bandwidth and delay, OSPF uses bandwidth (cost).

7 * Routing Protocols Metrics * Administrative distance is an integer value from 0 to 255. Purpose of Administrative Distance It s a numeric value that specifies the preference of a particular route source.

8 The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table. * You need to be able to dissect and understand the show ip route output: For example: - R /24 [120/3] via , 00:00:16, S0/0/0 - What is the significant of the information R? - What is the significant of the information ? - What is the significant of the information /24? - What is the significant of the information 120? - What is the significant of the information 3? - What is the significant of the information ? - What is the significant of the information S0/0/0? - O /27 [110/65] via , 00:00:05, Serial0/0/1 - What is the significant of the information O? - What is the significant of the information ? - What is the significant of the information /27? - What is the significant of the information 110? - What is the significant of the information 65? - What is the significant of the information ? - What is the significant of the information S0/0/1? * Understand the rules of route selection for the routing table. - Longest bit match - AD - Metric

9 - If AD and metric is tie, then you have load balancing routes * RIP Triggered Updates Routing table update that is sent immediately to adjacent routers in response to a routing change The receiving routers, in turn, generate triggered updates that notify their neighbors of the change. * What is the purpose of TTL? How does it work to prevent loop on a network? Purpose of the TTL field The TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network How the TTL field works TTL field contains a numeric value. The numeric value is decreased by one by every router on the route to the destination. If numeric value reaches 0 then Packet is discarded. * Routing Loops Routing loops may be caused by: -Incorrectly configured static routes -Incorrectly configured route redistribution -Slow convergence -Incorrectly configured discard routes * What is passive interface? How does it work? Do you know how to configure the passive interface on the router?

10 Using the passive interface command can prevent routers from sending routing updates through a router interface, but the router continues to listen and use routing updates from that neighbor. * There are several troubleshooting question about the IP address assignment. You need to make sure: 1) Default gateway and the host IP address needs to be in the same subnet. 2) IP addresses between 2 serial interfaces needs to be on the same subnet. 3) FastEthernet IIP address on the LAN will be the default gateway address for the PC. Here is a quick table for using subnet mask to figure out the following: - Which one is the network address? - Which one is the broadcast address? - Which are the usable addresses? (You will use this table for several questions) (or /25) (or /26) * (or /27)

11 Same process for (/28) and (/29) * RIPv2 A classless distance vector routing protocol that is an enhancement of RIPv1 s features. support VLSM RIPv2 will automatically summarize routes at major network boundaries and can also summarize routes with a subnet mask that is smaller than the classful subnet mask To disable automatic summarization issue the no auto-summary command Troubleshooting RIP You need to be able to use the show ip protocol command to verify 1) Whether you have all the network statement configured under the rip routing configuration. 2) Which version of RIP is configured (version 1 or version 2)?

12 * You need to know how to read the show ip route output. - C: AD?, - S: AD?, - O: AD?, How is the metric of O calculated? - D: AD?, How is the metric of D calculated? - R: AD?, How is the metric of R calculated? - Is there a default or no default route?

13 * You need to know the process of ip classless. There are many questions (at least 5) related to this ip classless issue. - The command is enable by default for any IOS version older than 11.3 EIGRP includes several features that are not commonly found in other distance vector routing protocols like RIP (RIPv1 and RIPv2) and IGRP. These features include: Reliable Transport Protocol (RTP) Bounded Updates Diffusing Update Algorithm (DUAL) Establishing Adjacencies Neighbor and Topology Tables It than builds the routing table from the topology table. * You need to make sure you know some of the key variable in the EIGRP topology table? - Route is stable or not stable? - Successor and/or feasible successor - If they don t tell you which router the command is issued at, Can you figure out which router are you looking at? [Hint: connected network.]

14 - Active or Passive? Neighbor table: * In order for EIGRP routers to create adjacency (neighbor relationship): - Neighbor routers need to have the same AS numbers. - Neighbor routers need to be in the same subnet. - You can use the show ip eigrp nei command to verify the connection and number of adjacencies for the neighbor.

15 * The EIGRP Null0 Summary Route By default, EIGRP uses the Null0 interface to discard any packets that match the parent route but do not match any of the child routes * The EIGRP Null0 Summary Route EIGRP automatically includes a null0 summary route as a child route whenever Automatic summarization is enabled If you disable the auto-summary, the EIGRP will remove the null interface

16 * What are the commands needed to configure EIGRP for the following network topology? * Troubleshooting the problem. - This is not really emphasized in the class, but you need to know. For any EIGRP routers to be able to establish neighbor relationship and exchange routing table, they need to be located in the same AS environment. Otherwise, no information will be exchanged. - For example: Two EiGRP routers (R1 and R2) and they are directly connected. However, they cannot establish the neighbor relationship. Can you figure out why (see diagram below)?

17 * Fine-Tuning EIGRP To prevents the EIGRP process from over-utilizing a link (low bandwidth connection or slow link) and not allowing enough bandwidth for the routing of normal traffic. The command to change the percentage of bandwidth used by EIGRP is Router(config-if)#ip bandwidth-percent eigrp as-number percent 2 link state routing protocols used for routing IP -Open Shortest Path First (OSPF) -Intermediate System-Intermediate System (IS-IS) In order for OSPF router to be able to create neighbor adjacency, there are 4 variables that must be the same. Can you list them all? There are only 3 listed below. Can you figure out what is the 4 th variable? and * The required commands to configure an OSPF router? - R1(config)#router ospf process-id - R1(config-router)#network network-address wildcard-mask area area-id

18 ** Double check and make sure the OSPF routers have all the network statement to cover all the directly connected networks. * Make sure you know how to configure the OSPF router to redistribute the gateway of last resort. The default-information originate command is only needed on the router that has the static default route configured. All other routers will automatically received the default route. * Two routers may not form an OSPF adjacency if: The subnet masks do not match, causing the routers to be on separate networks. OSPF Hello or Dead Timers do not match. OSPF Network Types do not match. There is a missing or incorrect OSPF network command. * OSPF Hello packets are used to OSPF neighbors to discovery & establish neighbor adjacencies.