| In the scenario shown
in the main figure, the link between router A and router B is now
part of the 168.71.0.0 domain that all other networks are using.
However, the mask has been changed to 255.255.255.252. This was done
to prevent router A and router B from advertising the 168.71.6.0
subnet.
In the
routing table from router C, you can see that router C does not have
a route to 168.71.6.0 or a summarized route to 168.71.0.0. (see RouterC#show ip route
168.71.6.0 and RouterC#show ip route
168.71.0.0
command outputs).
Note:
An access list has been applied to the end of this debug command,
which limits the output of the command to packets that match the
access list. Adding the access list affects only the actual debug
messages. It is a good idea to add an access list whenever a router
with lots of IP traffic is being used to debug a problem related to
the forwarding of IP packets. Remember that debug commands
work only for IP packets that are being process switched. Ping
packets generated by a router are always process switched, so adding
the access list to the debug command will always work in
situations similar to this one.
Note:
To determine whether access-list debugging is available for a
certain debug command, type a question mark (?) at the
end of the command. If access-list debugging is available, the
prompt indicates that you can enter an access-list number.
In the
output of the debug ip packet command from router C, you can
see that router C doesn't have a route to use for packets destined
to 168.71.6.1 (see RouterC#debug ip
packet 103, RouterC#ping
168.71.6. and RouterC#show
access-lists 103
command output).
In the following
routing table from router C, you can see that router C has a gateway
of last resort installed. It was ignored because a router does not
use a gateway of last resort for addresses that are part of its
local domain --- a major network it has connections to. (see RouterC#show ip
route command
output).
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