200-301 online test

200-301 online test.Using the ping command with other devices on the LAN can quickly determine if the LAN can transmit packets and frames. In particular, a valid ping command can troubleshoot the root cause of the problem, including the EthernetICMP messages.§iH . LAN feature, Figure 4-12 shows the ping 172.16.1.51 that occurs when R1 issues a ping to Host A on the same VLAN Figure 4-12 The standard ping command can determine that the LAN is in a working state 2.2 Basics 97 If the ping command is in a working state, it determines the following facts and is able to rule out a number of potential topics... The host has been replied to address 172.16.1.51. the LAN is able to transmit a unicast frame from R1 to host 172.16.1.51 and vice versa. It is reasonable to assume that the switch knows the MAC addresses on the router and the host and adds them to the MAC address table. . Host A and router R1 complete the ARP process and add each other to their respective ARP tables. If the pingl72.16.1.51 on R1 fails, the result could be caused by several things. Broad statement. Host A may be statically configured with the wrong IP address. Articulation. If Dynamic Host Configuration Protocol (DHCP) is used, this can cause many problems: host A may be using a different IP address than 172.16.1.51, the The DHCP configuration may be wrong. the router may not have obtained a DHCP reply configuration and therefore Host A never publishes its IPv4 address. etc. . The router may be configured for 802.1 Q trunk configuration, while the switch is not (and vice versa). . Any LAN issues discussed in the first part of this book. Therefore, whether the ping command succeeds or fails, simply executing a ping command from the router to the LAN can help further isolate the problem. Testing LAN neighbors with the extended ping command The standard LAN host ping command on a router will not test the host's default router settings, but the extended ping command can test the host's default router settings. Both tests are very useful, and even more so for problem isolation. The reasons for this are as follows. ,ready&. If the standard ping command for the local LAN host is valid ...... I sprawl point but the extended ping command on the same LAN host fails ...... problem is most likely caused by the host's default router settings. First, it is important to understand why the standard and extended ping command results would have different effects. You need to first consider the standard ping 172.16.1.51 command on R1, as shown earlier in Figure 4-12. As a standard ping command, R1 uses its LAN interface IP address (172.16.1.1) as the source of the ICMP Echo. Therefore, when host (A) sends back its ICMP EchoReply, host A assumes that the destination of 172.16.1.1 is located on the same subnet. The ICMP Echo Reply message sent back to Host A at 172.16.1.1 remains valid even when Host A has no default router settings at all. In contrast, Figure 4-13 shows the difference when the extended ping command is used on router R1. The extended ping from locally on R1 uses R1's S0/0/0 IP address of 172.16.4.1 as the source of the ICMP EchoRequest, which means that Host A's ICMP EchoReply will flow to an address on another subnet, which causes Host A to use its default router settings. Side destination address 172.1641... Another subnet! 172.16.1.0/24 ping 172.16.1.51 T 172.16.1.1 meal ^^172.1641 response request ::172.16.4.0/24 Responding to an Answer : Figure 4-13 The extended ping command can test the default router settings for host A A comparison of the previous two figures shows one of the most common mistakes made when troubleshooting. Sometimes, when connecting to a router and executing the ping command against a host on the LAN, and seeing that this works, the engineer proceeds and assumes that the network layer before the router and host is working properly, but in reality there is still a problem with the host's default router settings. Testing WAN Neighbors with the Standard ping Command Like the standard ping command to test the LAN, the standard ping command can also test two routers on consecutive WAN links to see if the link is capable of carrying IPv4 packets. With the proper IPv4 address arrangement, two routers on the same contiguous link can have IP addresses on the same subnet. A ping performed on an IP address from one router to the other router 98 Chapter 4, Part 1 IPv4 Routing Troubleshooting command can determine that IP packets can be sent to that link and back, as shown in Figure 4-14 for the ping 172.16.4.2 command on R1. If the ping command is working properly. it will determine several specific facts as follows. /gij. both consecutive interfaces of the router are out of the up/up state. [sprawlpoint. The linked Layer 1 and Layer 2 are functioning properly. The router believes that the IP addresses of the adjacent routers are located in the same subnet. The ACLs in both routers are not filtering packets entering them separately. The remote router is configured with the expected IP address (in this case 172.16.4.2). If the ping test fails, using the same list helps to find the root cause. For example, connect to the router's CLI and quickly check the router's interface status and IP address/mask synthesis. Then, for Layer 1 and Layer 2 issues, you can use the details discussed in Chapter 12. Executing the ping command against adjacent routers will not test many other features; for example, executing the ping command against consecutive IP addresses of adjacent routers will only test one route per router: the route linked to the subnet on the consecutive link. This ping command does not test routes to any subnets on the LAN. In addition, neither the source IP address nor the destination IP address will match the two hosts with the original problem, so this test will not provide much help in finding ACL issues. Although the scope of this test is limited, the news can help you troubleshoot some WIN link problems, such as problems at Layer 1 or Layer 2, and some basic Layer 3 address problems. Using the ping command with name and IP address So far, all of the ping command examples presented in this chapter have shown pings with IP addresses, but the ping command can also use host names, and performing a ping command on a host name allows network engineers to further test that the Domain Name System (DNS) is working properly. First, almost all existing TCP/IP applications use hostnames (not IP addresses) to identify other devices. No one would open a web browser and type http://72.1634161/. They would type in a web address, such as http://www.cisco.com,www.cisco.com. Next, before a host can send data to a specific IP address, it must first request that a DNS server assign the host name to the matching IP address. For example, in the smaller network used in several examples in this chapter, the pingB command on host A is used to test A's DNS settings, as shown in Figure 4-15. When Host A sees that hostname (B) is in use, it first looks at its local DNS name cache to determine if it has assigned name B. If not, Host A first requests that DNS provide (assign) the name to its matching IP address (step in the figure). Only then does Host A send the packet to 172.16.2.101, the IP address of Host B (step ). Figure 4-15 Host A resolving DNS names 3.2 Foundation content 99 3.2 Basic Content 100 When troubleshooting, it is useful to test by executing the ping command from a host using the hostname. Of course, the command tests the host's own DNS client settings. For example, a common difference is that the first ping destination host uses a hostname that requires a DNS request. Then the same test is repeated, but using the IP address of the destination host instead of its name, which does not require a DNS request. If the ping command for the host name fails, but the ping for the IP address works fine, then the problem is usually DNS related. 4.2.3 Using the traceroute Command to Isolate Problems As with the ping command, the traceroute command can help network engineers isolate problems. Here is a comparison of the two. /. Both send messages across the network to perform tests of connectivity.1. traceroute. Both rely on other devices to return replies. . Both are capable of supporting many different operating systems. Both are capable of using host names or IP addresses to identify destinations. . On the router. both have standard and extended versions that allow for better testing of reverse routes. The biggest difference is that the output of the traceroute command produces more detailed results, and traceroute takes more time and work to build that output. This subsection will examine how traceroute works, and also provide some suggestions on how to use this more detailed information to quickly isolate IP routing problems. traceroute basics Imagine that some network engineer or CSR starts troubleshooting a problem, and they execute the ping command from the user's host and a nearby router, and after executing a few commands, they think the host is able to send and receive IP packets, and although the problem is still not resolved, they can tell that the problem is not on the network. Now imagine the next problem, and this time the ping command fails. This could indicate that there is indeed some problem in the IP network. What is the problem? How should the engineer discover the problem further? While the ping command can effectively isolate the source of the problem, traceroute is a better option. traceroute shows how far packets travel through the IP network before they are dropped, and can be used to more accurately pinpoint routing problems. The traceroute command identifies the routers in the path from the source host to the destination host. In particular, it can enumerate the next-hop IP addresses of all routers on each single route. For example, in Figure 4-16, the traceroute172.16.2.101 command on a host identifies the IP address on router R1, as well as the IP addresses of router R2 and host B. The next example in 4-4 lists the The output of the command from host A. Example 4-4 Host A traceroute 172.16.2.101 output Wende11-Odoms-iMac:~ wendellodom$traceroute 172.16.2.101 Figure 4-16 identified by the successful traceroute172.16.2.101 command on host A 3.2 Base Content 101 traceroute to 172.16.2.101, 64 hops max, 52 byte packets (172.16.1.1) 0.870 ms 0.520 ms 0.496 ms 2 172.16.4.2 (172.16.4.2) 8.263 ms 7.518 ms 9.319 ms 3 172.16.2.101 (172.16.2.101) 16.770 ms 9.819 ms 9.830 ms 102 Chapter 4, Part 1 Troubleshooting IPv4 Routing How traceroute works traceroute collects information by generating packets that can trigger error messages from a router; these messages identify the router and enable the traceroute command to enumerate the router's IP address in the command output. The error messages are ICMP Time-to-Live Exceeded (TTLExceeded) messages, which are essentially notifications that are sent to hosts when packets are cycled through the network. In a way, IPv4 routers defeat routing loops by dropping IP packets. To do so, the Ipv4 header will have a field called Time to Live (TTL). The original host that created the packet will set an initial TTL value. Each router that forwards the packet will then decrement the TTL value by 1. When a router decrements the TTL value to 0, that router realizes that the packet is in a loop and drops the packet. The router also notifies the host that sent the dropped packet by sending an ICMP TTLExceeded message. Now back to traceroute traceroute sends a message with a lower TTL value in order for the router to return a TTL o Exceeded message. In particular, traceroute starts by sending several packets (usually three), each with a TTL field equal to 1. When the packet arrives at the next router-the default router for host A in the Figure 4-17 example, R1- -that router passes the TTL value. -this router decrements the TTL value to 0 and discards the packet. The router then sends a TTL Exceeded message to Host A to identify the IP address of that router to the traceroute command. Figure 4-17 How traceroute Identifies the First Router in a Route The traceroute command sends several TTL=1 packets and checks them to determine if the TTL Exceeded messages are from the same router and are based on the source IP address of the TTLExceeded messages. Assuming the messages came from the same router, the traceroute command lists that IP address in the next line of the command output. The router can choose the IP address to use, but as you can guess, the router will use the IP address of the output interface. In this example, the output interface for R1's TTLExceeded message is 172.16.1.lo In order to find all the routers in the path and eventually determine that all packets from all directions will flow to the destination host, the traceroute command will send TTL=1, TTL=2, TTL=3, and TTL Diao packets in sequence until the destination host replies. Figure 4-18 shows a packet from the second setting (TTL=2), in which case one router (R1) actually forwards the packet while the other router (R2) decrements the TTL value to 0, which in turn causes the TTLExceeded message to be sent back to host A. The following 4 steps are shown in the figure. Step 1 traceroute sends a packet from the second setting, TTL=2. Step 2 Router R1 processes the packet and decrements the value to 1. R1 forwards the packet. Step 3 Router R2 processes the packet and decrements the TTL R2 drops the packet. Step 4 R2 notifies the host that wants to discard the packet by sending a TTL ExceededICMP message. The source IP address of the message 4.2 Base Content 93 address is R2's message output interface (in this case, 172.16.4.2). Standard and extended traceroute The standard and extended options of the traceroute command provide many of the same options as the ping command; for example, Example 4-5 lists the standard traceroute command output for router R1. As with the standard ping command, the standard traceroute command selects an IP address based on the output interface of the packets sent by the command. Thus, in this example, the packet sent by R1 comes from the source IP address 172.16.4.1, which is in R1's S0/0/0 IP address. Example 4-5 Standard traceroute on R1 Rl# traceroute172.16.2.101 Type escapesequence to abort. Tracing the route to 172.16.2.101 VRF info: (vrf in name/id, vrf out name/id) 1 172.16.4.2I0msec0msec0msec 2 172.16.2.1010msec0msec * The extended traceroute command, shown in Example 4.6, follows the same basic command structure as the extended ping command. The user is able to output all parameters on a single command line, but it is much easier to just enter traceroute and press Enter to have the IOS supply all parameters, including the source IP address of the packet (172.161 in this example). Example 4.6 Extended traceroute on R1 Rl# traceroute Protocol [ip]: Target IP address: 172.16.2.101 Sourceaddress: 172.16.1.1 Numeric display [n]: Timeout in seconds [3]: Probe count [3]: Minimum Time to Live [1]: Maximum Time to Live [30]: Port Number [33434]: Loose, Strict, Record, Timestamp, Verbose [none]: Type escapesequence to abort. Tracing the route to 172.16.2.101 VRF info: (vrf in name/id, vrf out name/id) 1 172.16.4.20 msec 0 msec 0 msec 0 msec 0 msec * The ping command and the traceroute command are present in most operating systems, including Cisco IOS; however, some operating systems have a slightly different syntax for traceroute. For example, most Windows operating systems support tracert and pathping, but not tracerouteo Linux and OS X support the traceroute command. Tip: The host OS traceroute command typically creates ICMP Echo Requestso while the Cisco OS traceroute command creates IP packets to UDP headers. This may not seem important at this point, but it is important to note that ACLs may actually filter out the transmission of host traceroute messages (rather than router traceroute commands), or vice versa. Using traceroute to isolate problems to two routers One of the best features of the traceroute command, compared to the ping command, is that when a task is not completed, it immediately provides a trail for the next step. When using the ping command, when the ping command fails, the next step is usually to use more ping commands.104 Chapter 4, Part 1 Troubleshooting IPv4 Routing And when using traceroute, it tells you which router to try and connect to next and which direction to look at. Tip: As a reminder, the route used to send packets sent by the ping or traceroute command becomes the forward route, while the route used to return packets becomes the reverse route, for the purposes of terminology. . The traceroute command generates an incomplete list of routers when a particular problem exists. The command then either ends with the incomplete list, or it continues to run until the user has to stop the command. In either case, the output does not list all the routers on the end-to-end route because of the underlying problem. Tip: In addition, the traceroute command will not end even if there are no problems on the network. Routers and firewalls may filter out messages sent by the traceroute command or TTL Exceeded messages, which will prevent the display of some or all of the paths. The last router listed in the output tells us which places to look at next, as shown below. . Connect to the CLI of the last router listed to view forwarding routing related topics. . Connect to the CLI of the next router that should be listed and look for reverse routing related topics. To see why, consider the network-based example in Figure 4-19. In this example, R1 is using an extended traceroute to host 5.5.5.5, and the source IP address for the output of this command lists router 2.222, then 33.3.3, and then the command fails to complete. First, Figure 4-19 focuses on the first line of output, which lists the first hop router 2.222. The figure shows the TTL=1 message at the top and the TTLExceeded message at the bottom. The first pair of messages in the figure must work because without them, the traceroute command on R1 cannot be informed of the presence of the router at address 2.222. The first (hop) message requires R1 to have a route to 5.5.5.5 that is capable of sending packets to R2 behind it. The TTL Exceeded message requires R2 to have a route that matches address 11.1 to return the packet to R1's LAN IP address. Next, the message of interest in Figure 4-20 allows the second line of output from the sampling traceroute command to appear on R1: the line can correctly list TTL Exceeded [ Figure 4-19 enables the traceroute command to list the 222.2 message Figure 4-20 enables the traceroute command to list the message for 3.3.3.3 Following the same logic, the traceroute output will list 33.3.3 because the message in Figure 4-20 works properly. in order for these messages to flow, the route listed in Figure 4-19 must exist, and the new route listed in Figure 4-20 is also necessary. of the TTL. In particular, the packet with TTL=20 at the top requires R2 to have a route to 5.5.5.5 that can send the packet to R3 later.TTLExceeded messages require R3 to have a route matching address 1.1.1.1 to return the packet to 4.3 Pre-test Preparation 103 back to Rl's LAN IP address. In this example, the traceroute5.5.5.5 command does not list any routers other than 222.2 and 3.3.33. Nonetheless, it is clear from these diagrams that 4A.4.4 would be the next IP address to be listed. To help isolate the problem further, consider why the next message one - the TTL=3 message and its reply - would fail? Figure 4-21 identifies the routing issues that prevent this command from listing 4.4.4.4 as the next route. First, R3 must have a forwarding route that matches destination 5.5.5.5 and can forward the packet to R4. The return message needs a reverse route that matches the destination 1.1.1.1 and is capable of forwarding the packet to R3. Output q... >.............................. | Use a working route to 5.5.5.5 ~|| A problem with the route to 5S.5.5? ~ v _ L 5.5.5S TTL Exceeded Figure 4-21 Message that might make traceroute list 4.4.4.4 To summarize, in this example, if a routing problem prevents the traceroute command from working properly, the problem should exist in one of two places: a forwarding route on router R3 to 5.5.5.5, or a reverse route on R4 to 1.1.1.1. 4.3 Preparing for the Exam 4.3.1 Reviewing the Exam Essentials Note the Exam Points" symbols in the margins of this chapter; these relevant points are the focus of this chapter. Table 4-1 lists the content of these exam points. And Table M, "Chapter 4 Exam PointsM Table "Chapter 4 Exam PointsList Host Routing LogicList Router routing logicFigure 4.3 Router Routing LogicFigure 4-4 Router End-to-End Decapsulation and Encapsulation through the NetworkFigure 4.5 ARP Examples and ConceptsFigure 4-10 ARP Table on Layer 3 Hosts, MAC Address Table on Layer 2 Switches