NET-130 (NetAcad Chapter 16)

Ping the Remote LAN Again

Next, a ping from R1 sourced from G0/0/0 is used to verify that R1 can now reach the LAN interface of R3. As a last step in confirmation, the user on PC1 should also test connectivity to the 192.168.2.0/24 LAN. R1# ping 192.168.2.1 source g0/0/0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds: Packet sent with a source address of 172.16.3.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms

Ping the Next-Hop Router

Next, a ping to the S0/1/0 interface on R2 is successful. This ping is sourced from the S0/1/0 interface of R1. Therefore, the issue is not loss of connectivity between R1 and R2. R1# ping 172.16.2.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

Correct the R2 Static Route Configuration

Next, the running configuration does, in fact, reveal the incorrect ip route statement. The incorrect route is removed, and the correct route is then entered. R2# show running-config | include ip route ip route 172.16.3.0 255.255.255.0 192.168.1.1 ip route 192.168.2.0 255.255.255.0 192.168.1.1 R2# R2# configure terminal Enter configuration commands, one per line. End with CNTL/Z. R2(config)# no ip route 172.16.3.0 255.255.255.0 192.168.1.1 R2(config)# ip route 172.16.3.0 255.255.255.0 172.16.2.1 R2(config)#

16.2.1 Network Changes

No matter how well you set up your network, you will have to be ready to troubleshoot some problem. Networks are frequently subject to events that can cause their status to change. For example, an interface can fail, or a service provider drops a connection. Links can become oversaturated, or an administrator may enter a wrong configuration. When there is a change in the network, connectivity may be lost. Network administrators are responsible for pinpointing and solving the problem. To find and solve these issues, a network administrator must be familiar with tools to help isolate routing problems quickly.

3. Refer to the exhibit. What action will R3 take to forward a frame if it does not have an entry in the ARP table to resolve a destination MAC address?

Sends an ARP request

ping

The example displays the result of an extended ping from the source interface of R1 to the LAN interface of R3. An extended ping is an enhanced version of the ping utility. Extended ping enables you to specify the source IP address for the ping packets. R1# ping 192.168.2.1 source 172.16.3.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds: Packet sent with a source address of 172.16.3.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/5 ms R1#

Ping the Remote LAN

The network administrator can test connectivity between the two LANs from R1 instead of PC1. This can be done by sourcing the ping from the G0/0/0 interface on R1 to the G0/0/0 interface on R3, as shown in the example. The ping results show that there is no connectivity between these LANs. R1# ping 192.168.2.1 source g0/0/0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds: Packet sent with a source address of 172.16.3.1 ..... Success rate is 0 percent (0/5)

6. A network engineer issues the show cdp neighbor command on several network devices during the process of network documentation. What is the purpose of performing this command?

To obtain information about directly connected Cisco devices.

16.2.4 Syntax Checker - Troubleshoot IPv4 Static and Default Routes

Troubleshoot IPv4 Static and Default Routes based on the specified requirements

1. Refer to the exhibit. True or False? R1 must encapsulate received packets into new frames before forwarding them to R2.

True

2. Refer to the exhibit. True or False? R2 will forward frames to R3 with an all 1s Layer 2 address.

True

8. A network administrator has entered the following command: ip route 192.168.10.64 255.255.255.192 serial0/0/1 When the network administrator enters the command show ip route, the route is not in the routing table. What should the administrator do next?

Verify that the serial 0/0/1 interface is active and available.

7. A network administrator notices that a correctly entered static route is not in the routing table. What two router commands would an administrator use to determine if the exit interface was up and the next hop address is available? (Choose two.)

ping show ip interface brief

1. Which sequence correctly identifies the order of the steps that a router will perform when it receives a packet on an Ethernet interface?

1. The router examines the destination MAC address. 2. The router identifies the Ethernet Type field. 3. The router de-encapsulates the Ethernet frame. 4. The router examines the destination IP address.

Ping R3 LAN from S0/1/0

A ping from R1 to the R3 interface 192.168.2.1 is successful as well. This ping is sourced from the S0/1/0 interface on R1. R3 has a route back to the network between R1 and R2, 172.16.2.0/24. This confirms that R1 can reach the remote LAN on R3. However, packets sourced from the LAN on R1 cannot. This indicates that either R2 or R3 may have an incorrect or missing route to the LAN on R1. R1# ping 192.168.2.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

show ip interface brief

A quick status of all interfaces on the router is shown using the show ip interface brief command in this example. R1# show ip interface brief Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0/0 172.16.3.1 YES manual up up GigabitEthernet0/0/1 unassigned YES unset up up Serial0/1/0 172.16.2.1 YES manual up up Serial0/1/1 unassigned YES unset up up R1#

16.1.1 Static Routes and Packet Forwarding

Before diving into the troubleshooting portion of this module, this topic provides a brief review of how packets are forwarded in static routes. In the figure, click Play to see the animation where PC1 is sending a packet to PC3. The following describes the packet forwarding process with static routes, as demonstrated in the animation: 1. The packet arrives on the GigabitEthernet 0/0/0 interface of R1. 2. R1 does not have a specific route to the destination network, 192.168.2.0/24. Therefore, R1 uses the default static route. 3. R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an "all 1s" address for the Layer 2 destination address. 4. The frame is forwarded out of the Serial 0/1/0 interface. The packet arrives on the Serial 0/1/0 interface on R2. 5. R2 de-encapsulates the frame and looks for a route to the destination. R2 has a static route to 192.168.2.0/24 out of the Serial 0/1/1 interface. 6. R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an "all 1s" address for the Layer 2 destination address. 7. The frame is forwarded out of the Serial 0/1/1 interface. The packet arrives on the Serial 0/1/1 interface on R3. 8. R3 de-encapsulates the frame and looks for a route to the destination. R3 has a connected route to 192.168.2.0/24 out of the GigabitEthernet 0/0/0 interface. 9. R3 looks up the ARP table entry for 192.168.2.10 to find the Layer 2 Media Access Control (MAC) address for PC3. If no entry exists, R3 sends an Address Resolution Protocol (ARP) request out of the GigabitEthernet 0/0/0 interface, and PC3 responds with an ARP reply, which includes the PC3 MAC address. 10. R3 encapsulates the packet in a new frame with the MAC address of the GigabitEthernet 0/0/0 interface as the source Layer 2 address, and the MAC address of PC3 as the destination MAC address. 11. The frame is forwarded out of GigabitEthernet 0/0/0 interface. The packet arrives on the network interface card (NIC) interface of PC3.

16.1.2 Check Your Understanding - Packet Processing with Static Routes

Check your understanding of packet processing with static routes by choosing the BEST answer to the following questions.

16.2.2 Common Troubleshooting Commands

Common IOS troubleshooting commands include the following: ping traceroute show ip route show ip interface brief show cdp neighbors detail The figure shows the topology used to demonstrate these commands. Click each button for an example and explanation of these common troubleshooting commands.

16.2.3 Solve a Connectivity Problem

Finding a missing (or misconfigured) route is a relatively straightforward process if the right tools are used in a methodical manner. For instance, the user at PC1 reports that he cannot access resources on the R3 LAN. This can be confirmed by pinging the LAN interface of R3 using the LAN interface of R1 as the source. Again, we will use the topology in the figure to demonstrate how to troubleshoot this connectivity problem. Click each button to see how troubleshooting commands are used to solve a connectivity problem.

5. Which statement describes the sequence of processes executed by a router when it receives a packet from a host to be delivered to a host on another network?

It de-encapsulates the packet, selects the appropriate path, and encapsulates the packet to forward it toward the destination host.

10. What does the letter C mean next to an entry in the output of the show ip route command?

It identifies a network that is directly connected to the router.

16.3.3 What did I learn in this module?

Packet Processing with Static Routes 1. The packet arrives on the interface of R1. 2. R1 does not have a specific route to the destination network; therefore, R1 uses the default static route. 3. R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an "all 1s" address for the Layer 2 destination address. 4. The frame is forwarded out of the appropriate interface. The packet arrives on the interface on R2. 5. R2 de-encapsulates the frame and looks for a route to the destination. R2 has a static route to the destination network out of one of its interfaces. 6. R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an "all 1s" address for the Layer 2 destination address. 7. The frame is forwarded out of the appropriate interface. The packet arrives on the interface on R3. 8. R3 de-encapsulates the frame and looks for a route to the destination. R3 has a connected route to the destination network out of one of its interfaces. 9. R3 looks up the ARP table entry for the destination network to find the Layer 2 MAC address for PC3. If no entry exists, R3 sends an ARP request out of one of its interfaces, and PC3 responds with an ARP reply, which includes the PC3 MAC address. 10. R3 encapsulates the packet in a new frame with the MAC address of the appropriate interface as the source Layer 2 address and the MAC address of PC3 as the destination MAC address. 11. The frame is forwarded out of the appropriate interface. The packet arrives on the network interface card (NIC) interface of PC3. Troubleshoot IPv4 Static and Default Route Configuration Networks are frequently subject to events that can cause their status to change. An interface can fail, or a service provider drops a connection. Links can become oversaturated, or an administrator may enter a wrong configuration. Common IOS troubleshooting commands include the following: ping traceroute show ip route show ip interface brief show cdp neighbors detail

4. A static route has been configured on a router. However, the destination network no longer exists. What should an administrator do to remove the static route from the routing table?

Remove the route using the no ip route command

Verify the R2 Routing Table

The next step is to investigate the routing tables of R2 and R3. The routing table for R2 is shown in the example. Notice that the 172.16.3.0/24 network is configured incorrectly. The static route to the 172.16.3.0/24 network has been configured using the next-hop address 192.168.1.1. Therefore, packets destined for the 172.16.3.0/24 network are sent back to R3 instead of to R1. R2# show ip route | begin Gateway Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks C 172.16.1.0/24 is directly connected, GigabitEthernet0/0/0 L 172.16.1.1/32 is directly connected, GigabitEthernet0/0/0 C 172.16.2.0/24 is directly connected, Serial0/1/0 L 172.16.2.2/32 is directly connected, Serial0/1/0 S 172.16.3.0/24 [1/0] via 192.168.1.1 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.1.0/24 is directly connected, Serial0/1/1 L 192.168.1.2/32 is directly connected, Serial0/1/1 S 192.168.2.0/24 [1/0] via 192.168.1.1 R2#

Verify New Static Route is Installed

The routing table on R2 is checked once again to confirm the route entry to the LAN on R1, 172.16.3.0, is correct and pointing toward R1. R2(config) # exit R2# *Sep 20 02:21:51.812: %SYS-5-CONFIG_I: Configured from console by console R2# show ip route | begin Gateway Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks C 172.16.1.0/24 is directly connected, GigabitEthernet0/0/0 L 172.16.1.1/32 is directly connected, GigabitEthernet0/0/0 C 172.16.2.0/24 is directly connected, Serial0/1/0 L 172.16.2.2/32 is directly connected, Serial0/1/0 S 172.16.3.0/24 [1/0] via 172.16.2.1 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.1.0/24 is directly connected, Serial0/1/1 L 192.168.1.2/32 is directly connected, Serial0/1/1 S 192.168.2.0/24 [1/0] via 192.168.1.1 R2#

show cdp neighbors

The show cdp neighbors command provides a list of directly connected Cisco devices. This command validates Layer 2 (and therefore Layer 1) connectivity. For example, if a neighbor device is listed in the command output, but it cannot be pinged, then Layer 3 addressing should be investigated. R1# show cdp neighbors Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, D - Remote, C - CVTA, M - Two-port Mac Relay Device ID Local Intrfce Holdtme Capability Platform Port ID Switch Gig 0/0/1 129 S I WS-C3560- Fas 0/5 R2 Ser 0/1/0 156 R S I ISR4221/K Ser 0/1/0 Total cdp entries displayed : 3 R1#

show ip route

The show ip route command in this example displays the routing table of R1. R1# show ip route | begin Gateway Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks S 172.16.1.0/24 [1/0] via 172.16.2.2 C 172.16.2.0/24 is directly connected, Serial0/1/0 L 172.16.2.1/32 is directly connected, Serial0/1/0 C 172.16.3.0/24 is directly connected, GigabitEthernet0/0/0 L 172.16.3.1/32 is directly connected, GigabitEthernet0/0/0 S 192.168.1.0/24 [1/0] via 172.16.2.2 S 192.168.2.0/24 [1/0] via 172.16.2.2 R1#

traceroute

This example displays the result of a traceroute from R1 to the R3 LAN. Note that each hop route returns an ICMP reply. R1# traceroute 192.168.2.1 Type escape sequence to abort. Tracing the route to 192.168.2.1 VRF info: (vrf in name/id, vrf out name/id) 1 172.16.2.2 1 msec 2 msec 1 msec 2 192.168.1.1 2 msec 3 msec * R1#

9. What will a router do if it does not have a default route configured and a packet needs to be forwarded to a destination network that is not listed in the routing table?

drop it

3. A network administrator has entered a static route to an Ethernet LAN that is connected to an adjacent router. However, the route is not shown in the routing table. Which command would the administrator use to verify that the exit interface is up?

show ip interface brief

2. Which three IOS troubleshooting commands can help to isolate problems with a static route? (Choose three.)

show ip interface brief ping show ip route