CCNP TSHOOT 6.0 - Cisco Learning Home

CCNPv6 TSHOOT
Sample Layer 3 Troubleshooting Flow
No end-toend
Layer 3
connectivity
Verify routing
tables and FIBs
Track Layer
3 path
between
devices
Investigate
points where
routes are
missing
Determine
downstream
connectivity using
ping and traceroute
Investigate
points where
packets are
dropped
© 2009 Cisco Systems, Inc. All rights reserved. TSHOOT v1.0—48
If you have determined that there is a problem with the end-to-end IP connectivity between the affected hosts, you
want to reduce the scope of the problem and isolate the points in the path between the hosts where the
connectivity is lost.
A commonly used method is to track the path of the packets. You can use the following method to diagnose endto-end
IP connectivity problems:
• Determine the Layer 3 path. Based on documentation, baselines, and knowledge of your network in
general, the first step is to determine the path that you would expect packets to follow between the
affected hosts. Determining the expected traffic path beforehand helps in two ways. It provides a
starting point for gathering information about what is happening on the network, and it makes it easier
to spot abnormal behavior. The second step in determining the Layer 3 path is to follow the expected
path and verify that the links on the path are up and forwarding traffic. If the actual traffic path is
different from your expected path, this step might provide clues about which links or protocols are
failing and the cause of these failures.
• To track the path of the packets between the hosts, first track the path that is being used according to
the control plane information. Start at the client and verify the IP address, subnet mask, and default
gateway. Then go to the router that is listed as the default gateway and check which route is used for
the destination IP address. Determine the next-hop router based on the information in the routing
table. Connect to the next-hop router and repeat this procedure until you arrive at the router that is
directly connected to the destination host. Then repeat the process for the route back from the
destination to the source.
• If the router has no route in the table for the destination network, you must diagnose the process that
is the source of the routing information on this router, such as the routing protocol or static routes.
• If you have verified that the routing information is present on the complete path from the source to the
destination and from the destination back to the source but connectivity is failing, you must track the
path again, but now determine at which point packets are being dropped. The likely causes for
dropped packets are Layer 1 problems, Layer 2 problems, or Layer 3 to Layer 2 mapping problems.
When you have determined the point where the packets are dropped, use the specific troubleshooting
methods appropriate for the Layer 2 technology that is used on the egress interface.
These steps do not necessarily have to be taken in the order presented here. Often different aspects of this
generic procedure are combined, and shortcuts can be taken based on the result. For instance, determining
proper packet forwarding is often done in parallel with determining the routes by using ping to verify the
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