the Routing Table Must Be Reviewed by Using the Command Show Ip Route

April 18, 2022 Post a Comment

Verify the IP routing table

Verifying the Cisco IP routing tabular array is a primal part of troubleshooting Cisco Express Forwarding's network switching. Learn the necessary steps for identifying and remediating problems.

The case in the previous section was based on a directly connected device. To illustrate verifying the IP routing table and subsequent CEF troubleshooting, Example 4-five is based on a router with several routes, every bit illustrated in Figure 4-3. Devices connected to Router 2 in the x.ane.1.0/24 subnet are unable to reach a Secure File Transfer Protocol (SFTP) server with the address x.18.118.184, as shown in the first steps of Example 4-5; specifically, the host ten.1.i.100 is unable to ping 172.18.118.184. For the purpose of this example, assume that all the IP routing configuration and host configurations, such as IP address and default gateways, are configured correctly. In add-on, assume that our effect is strictly limited to Router 2 and not Router one, the Layer 3 switch.

Figure 4-iii Troubleshooting IP Connectivity and Verifying the IP Routing Table

Instance 4-5 IP Connectivity Issue from the Host, x.1.1.100, to the Target, 172.18.118.184, Based on Figure 4-3

The first step in troubleshooting is to access the showtime-hop router, Router two, and verify IP connectivity to the SFTP server, ten.eighteen.118.184. Example four-6 also confirms the road to the host, 10.xviii.118.184, and verifies the ARP entry for the next hop for x.xviii.118.184, which is x.18.114.one. The host entry, 10.xviii.118.184, is known through a static route with a metric of 0 according to the show ip route command. This is the gateway of final resort.

Example iv-half dozen Verifying IP Routing Table and Next-Hop Data

Although the ping from the router was successful, the next stride is to verify the MAC (Hardware Addr) of the next-hop router. This step verifies that the right ARP entry exists for the next-hop router. Example 4-7 illustrates obtaining Router 1'south MAC address for the interface configured with the IP accost 172.18.114.ane.

Example iv-seven Verifying the MAC Address of an Interface in Cisco IOS

The next-hop router's (Router 1's) MAC address is indeed 0008.a37f.cb7c. Therefore, at this betoken in troubleshooting, both the routing entry and ARP entry for the next-hop router are correct.

The next step is to verify IP connectivity to the next-hop router, Router 1, from both the host, 10.1.ane.100, and Router 2, as shown in Instance iv-8.

Example 4-8 Verifying IP Connectivity to the Next Hop from Router ii and the Host, 10.i.1.100

Case 4-8 also issued an ICMP ping of a larger packet size for continued verification of accessibility of the side by side-hop router. Y'all must verify various bundle sizes because defaultsized packets of ICMP echoes tin be routed and switched correctly in the network, whereas larger packets can be dropped for a multitude of reasons. These reasons include encoding errors in WAN circuits, fragmentation configuration issues, hardware issues, Virtual Individual Network (VPN) misconfiguration, software features such as encryption and compression, and then on. Some network routers' security policies tin as well forestall the routers from responding to very high rates of ICMP packets. Therefore, you lot should empathise your network policies when troubleshooting or installing systems. In Example 4-8, the router is indeed reachable through ICMP.

Case 4-8 proved that the next hop is reachable from both Router 2 and the host, 10.1.i.100. Therefore, at this point, the ARP and IP routing tables appear to be right on Router 2. The next footstep is troubleshooting CEF (Footstep half dozen), which is discussed in the side by side section using the same example as that in Figure 4-3.

Sending ICMP echoes, requests, or responses from the CLI of the router is software switched, while ICMP echoes from end devices through many Cisco IOS routers and all current-generation Catalyst switches are hardware switched. The software-switching path might exist correct, only the hardware-switching path might not exist correct. In this example, the software-switching path on Router 2 might be correct, but the hardware-switching path might not exist correct. Troubleshooting the hardware-switching path is outside the telescopic of this affiliate. Affiliate v discusses troubleshooting hardware-switching paths on a Cisco Catalyst 6500 platform. However, you must troubleshoot CEF from the software-switching perspective first because the hardware-switching tables are built from the softwareswitching tables.

The section "Troubleshooting the CEF FIB Table," afterward in this affiliate, continues the investigation of the software-switching path by troubleshooting the Cisco IOS CEF table on Router 2 for Figure 4-3. Every bit noted previously, because the hardware-switching path gets built from the software-switching CEF and adjacency table, you should investigate the software-switching path first.

Using IOS Ping with the Record Choice to Rule Out CEF

CEF does not support all IP package types and must process-switch specific types of packets. One such parcel includes the ICMP echo with the record pick. As such, you tin dominion out CEF as a cause of an IP connectivity problem with some certainty using the ICMP echo with record option.

For example, in Figure 4-3, sending an ICMP echo with the tape option forces all routers along the path to use the process-switching method of forwarding a frame. If an ICMP echo with the tape choice is successful and a standard ICMP echo is not, you can assume with some certainty that CEF is indeed a crusade of your IP connectivity effect somewhere forth the path.

In Cisco IOS, apply the ping command with extended commands option to send ICMP echoes with the record option. Example 4-9 illustrates the use of the ICMP echo with tape option.

Example 4-9 Sending ICMP Packets Using the Record Option

NOTE When troubleshooting CEF, always note that certain packets, mainly those requiring special handling, are not CEF switched.

Learn how to troubleshoot Cisco's Express Forwarding network switching engineering science in "Basic IP Connectivity and CEF Troubleshooting," Chapter 4 from the volume Cisco Express Forwarding by Nakia Stringfield, Russ White and Stacia McKee.

Bones IP Connectivity and CEF Troubleshooting

Introduction
Accurately describe the trouble
Scope the network topology
Review the OSI model for troubleshooting
Verify the ARP table
Verify the IP routing table
Verify the CEF FIB table
Verify the adjacency table
Conduct hardware-specific troubleshooting

Reproduced from the book Cisco Express Forwarding. Copyright 2007, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 Eastward 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses.