ICND2 - 3. Troubleshooting Basic Connectivity
When issuing the show interface command, what does the Output errors counter signify?
Output errors indicate errors such as collisions that occur during the transmission of a frame.
When issuing the show ipv6 neighbors command, you see a host in the INCMP state. What does this mean?
The INCMP, or incomplete, state means that address resolution is being performed on the host. A neighbor solicitation message has been sent to the host, but the corresponding neighbor advertisement message has not yet been received.
When issuing the show interface command, what does the Input queue drops counter signify?
The Input queue drops counter signifies that, at some point, more traffic was sent to the router than it could process. If this number is consistently high, it means that the CPU cannot process the packets in time.
When issuing the show interface command, what does the Output queue drops counter signify?
The Output queue drops counter indicates that packets were dropped because of congestion on the interface. Consistently seeing output drops means that there is too much congestion on the network, and mechanisms such as QoS should be implemented and/or the network bandwidth should be increased.
When issuing the show ipv6 neighbors command, you see a host in the STALE state. What does this mean?
The STALE state means that the more than reachable time milliseconds have elapsed since the last positive confirmation was received. In the STALE state, the device takes no action until a packet is sent.
When issuing the show interface command, what does the Input errors counter signify?
Input errors indicate that errors are experienced when receiving the frame. These errors can be CRC errors and could indicate cabling problems, interface hardware problems, or duplex mismatches.
What are the two ways that the mapping of computer names to an IP address can be accomplished?
Mapping of computer names can be accomplished statically or dynamically. Static entries are done in a text file called the HOSTS file by an administrator. This is done by manually entering the computer's name and IP address in the HOSTS file and storing it on all computers. Dynamic entries are done using the DNS protocol and are entered in DNS servers. Note The HOSTS file is stored in the C:\windows\system32\etc\hosts directory on Windows computers. The ip host IOS command is similar to the HOSTS file and allows you to statically map computer names to IP addresses.
What is the cause of multiple collisions on a port?
Multiple collisions are the number of times the transmitting port had more than one collision before successfully transmitting a frame. If you experience multiple collisions on a port, the problem usually lies with a duplex mismatch or an oversaturated medium.
A hub is connected to a switch. Fifteen users are connected to the hub. All users are trying to connect to a server off of the switch, but are experiencing latency. What type of problem is this, and what are some of the causes for this problem?
Because the users are connected to a hub, they are in the same collision domain and are experiencing collision domain connectivity problems. Causes for the latency problem can include the following: The segment is overloaded or oversubscribed. Bad cabling on the segment. NICs on the segment do not have compatible settings. Faulty NICs.
As a network administrator, you want to block all Telnet traffic originating from the network 192.168.1.0/24 that is connected to your router's Gigabit interface 0/0 and permit all other IP traffic. You create the following access list and apply it to Gigabit interface 0/0: access-list 101 deny tcp 192.168.1.0 0.0.0.255 any eq 23 After you apply the access list, hosts connected to the router's Gigabit Ethernet interface cannot communicate with remote networks. Why?
Hosts attached to network 192.168.1.0/24 cannot communicate with remote networks because the access list is denying all IP traffic. At the end of each access list is a deny all statement. Thus, access list 101 is not only denying Telnet traffic but is also denying all IP traffic as well. To resolve the problem, the access list needs to be configured as follows: access-list 101 deny tcp 192.168.1.0 0.0.0.255 any eq 23 access-list 101 permit ip any any
What utilities can be used to test end-to-end IPv6 connectivity?
Ping, tracert, and traceroute can be used to test end-to-end IPv6 connectivity. Like IPv4, ping, tracert, and traceroute can test end-to-end connectivity for any IPv6 host. Note Troubleshooting connectivity in IPv6 is similar to IPv4; the primary difference is the address differences. Tools such as ping and traceroute operate the same way in the two versions.
You have one computer that is connected to a switch that is having very slow and intermittent connections with the network. You log on to the switch and issue theshow interface command on the port that the user is connected to. You see the following: !output omitted! 5 minute input rate 10000 bits/sec, 8 packets/sec 5 minute output rate 10000 bits/sec, 7 packets/sec 1476671 packets input, 363178961 bytes, 0 no buffer Received 20320 broadcasts (12683 multicast) 0 runts, 325 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored What is most likely the cause of the problem?
The computer likely has a faulty NIC. The switch is receiving a lot of giants. Giants are frames greater than the Ethernet maximum transmission unit (MTU) of 1518 bytes. The cause of giants is usually a faulty NIC on the computer.
When issuing the show ip route command on a Cisco router, what do the following codes mean? L C S R O D
The first parts of the output from the show ip route command are the codes. The codes explain the letter representing associated source entries in the routing table. The code definitions are as follows: L: Local host route C: Directly connected networks S: Static routes R: RIP O: OSPF D: EIGRP
What are the five ways that a routing table can be populated?
The five ways that a routing table can be populated are as follows: Directly connected networks: Come from interfaces directly connected to a network segment. Local host routes: Come from the local IP address on the router interface. A host route for IPv4 has a /32 mask; for IPv6, it is a /128 mask. Static routes: Manually entered routes. Dynamic routes: Routes learned from a configured routing protocol. Default routes: Manually entered or dynamically established by a routing protocol, this is the path a packet will take when there is no specific route to a network in its routing table.
What command can be used to test IP connectivity between hosts?
The ping command can test IP connectivity between hosts. Note Other commands to test connectivity are tracert, traceroute, and application layer programs like Telnet and so on.
When users in VLAN 10 are having difficulty connecting to a server in VLAN 20, the connection is very slow. The users are having no problems communicating with each other, only with the server in VLAN 20. As the network administrator, you issue the show interface command on the switch that the server is connected to and you see the following: !Output omitted! Encapsulation ARPA, loopback not set Keepalive set (10 sec) Full-duplex, 1000Mb/s, link type is auto, media type is 10/100/1000-TX The server has a Gigabit network card that is set to half-duplex. What is the problem?
The problem lies with a duplex mismatch between the server and the switch. The show interface command shows that the port's duplex is set to full, but when you look at the server's NIC, its duplex setting is half-duplex. A duplex mismatch would cause a slow connection to the server.
A router receives a packet and does a route lookup. The router does not have the destination network in its routing table and also does not have a default gateway configured. What will the router do with the packet?
The router will discard or drop the packet. If a router does not have a matching entry for a network in its routing table and a default route is not configured, the router will not know where to send the packet; thus, it will drop the packet.
A router receives several packets from a host and does a route lookup. The router has two equal-cost entries with the same prefix in its routing tables out two different interfaces. Where will the router send the packets?
The router will distribute the packets for the destination between the two different interfaces. If a router has more than one entry for a destination in its routing table with the same prefix out multiple interfaces, the router will load-balance the packets received for the destination out of all the defined interfaces.
A router receives a packet and does a route lookup. The router has two equal-cost entries with different prefixes in its routing table out two different interfaces. Where will the router send the packet?
The router will send the packet to the interface that is associated with the route that has the longest prefix match. The prefix is the network portion of the IP address designated by the subnet mask. If a router has multiple entries in its routing table with different prefixes, the most specific route will be chosen as the preferred route. The most specific route is the route with the longest prefix (subnet mask).
What Cisco IOS commands display the contents of all ACLs configured on the device?
The show ip access-lists and show access-lists commands display the contents of all ACLs configured on the device.
List the six steps that can be taken to troubleshoot end-to-end connectivity.
The six steps that can be taken to verify and troubleshoot end-to-end connectivity are as follows: Step 1. Determine whether end-to-end connectivity is operational. If this fails, go to Step 2. Step 2. Determine whether there is a physical connectivity issue. If there is no physical issue, go to Step 3. Step 3. Determine whether the current path is the desired or proper path. If it is not, go to Step 4. Step 4. Determine whether the default gateway is correct. If it is, go to Step 5. If it is not, apply the correct gateway. Step 5. Determine whether DNS is working correctly. If it is, go to Step 6. If it is not, fix DNS. Step 6. Determine whether any ACLs are blocking traffic. If they are, reconfigure the ACLs.
While troubleshooting a switched network, you see the following on a switch interface that is having connectivity problems: !output omitted! 5 minute input rate 10000 bits/sec, 8 packets/sec 5 minute output rate 10000 bits/sec, 7 packets/sec 1476671 packets input, 363178961 bytes, 0 no buffer Received 20320 broadcasts (12683 multicast) 2345 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored What could be the cause of the problem?
The switch is receiving a lot of runts. Runts are frames that are smaller than 64 bytes with a bad frame check sequence (FCS). Bad cabling or inconsistent duplex settings usually cause runts, as do other physical layer issues.
Traffic between two switches is slow. You issue the show interface command on the uplink between the two switches and you see the following: !output omitted! 0 input packets with dribble condition detected 180749 packets output, 8004302 bytes, 0 underruns 0 output errors, 45345 collisions, 0 interface resets 0 babbles, 45345 late collision, 0 deferred 0 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out What is the problem?
The switch port is receiving a lot of late collisions. The problem can be a duplex mismatch or a faulty port, or the distance between the two switches might exceed the cable specifications. Note Duplex mismatches occur when the connecting ends are set to different duplex modes, or when one end's duplex is configured and the other is set to autonegotiation.
What IOS command can be used to test transport layer connectivity?
The telnet IOS command can be used to test transport layer connectivity and also application layer connectivity.
What Cisco IOS command can you use to see whether an IP access list is applied to an interface?
To determine whether an IP access list is applied to an interface, enter the following command: show ip interface interface-type interface-number For example: RouterA# show ip interface s0
What command can you use on a Cisco device to view interface information, statistics, and errors?
To view interface information, such as interface type, speed, duplex settings, or statistics and errors, use the show interface interface-id privileged EXEC command. The following command shows the information for interface g0/1. You should be familiar with the highlighted areas. core# show interface g0/1
What utilities can show you the path that packets take between two hosts?
Tracert and traceroute can be used to observe the path that packets take between two hosts. It is useful to ensure that the packets are taking the proper path. Tracert is used from a Windows host to observe the path that packets take, and traceroute is used from a Cisco device to trace the path that packets are taking. The command syntax for traceroute is traceroute ip-address of host.
