CCNP TSHOOT 6.0 - The Cisco Learning Network

CCNPv6 TSHOOT
DLS1#show ip cef exact-route 10.1.10.1 10.1.202.1
10.1.10.1 -> 10.1.202.1 => IP adj out of FastEthernet0/5, addr 10.1.2.2
R2#show ip cef exact-route 10.1.202.1 10.1.50.1
10.1.202.1 -> 10.1.50.1 => IP adj out of Serial0/0/0
When you are tracing the packet flow between two specific hosts and the routing table and the FIB lists multiple
entries (because there are multiple equal-cost paths), you must determine which entry is used to forward the
packets associated with the specific source and destination IP address pair that you are troubleshooting. You can
use the show ip cef exact-route command in these situations to determine the specific egress interface
and next-hop IP address for the specific IP address pair.
On multilayer switches, instead of consulting the FIB that is stored in the main memory of the switch, you must
consult the forwarding information stored in the hardware ternary content addressable memory (TCAM), because
packet forwarding is handled by the TCAM, not the Cisco Express Forwarding FIB.
Although the FIB is used to compile the information that is loaded into the TCAM, the load-balancing algorithms
that are used are different and do not necessarily yield the same result.
To learn more about the commands that can be used to verify the Layer 3 forwarding information contained in the
TCAM, see the multilayer switching sections of the TSHOOT Student Guide and this Lab Guide.
Sample Layer 3 Troubleshooting Flow
No end-toend
Layer 3
connectivity
Track Layer
3 path
between
devices
Investigate
configured egress
interface and nexthop
for static
routes
Investigate
points where
routes are
missing
Investigate
points where
packets are
dropped
Investigate routing
protocol for
dynamic routes
© 2009 Cisco Systems, Inc. All rights reserved. TSHOOT v1.0—52
After you have found a point in the network where no route is present in the routing table for the destination IP
address (or when analyzing the return path for the source IP address) of the session, you need to investigate
what caused that route not to be installed in the routing table.
To correctly diagnose why a particular route is missing from the routing table, you first need to consult your
documentation and baselines to find out what is the expected routing source. Is static routing or a routing protocol
used on this router?
If a static route has been configured but it is not listed in the routing table, verify the status of the associated
egress interface. If the egress interface for a static route is down, the route will not be installed in the routing table.
If the route is not configured with an egress interface but with a next-hop IP address, the same rule applies. The
router executes a recursive routing table lookup on the next hop for the static route. If no matching route and
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