Understanding the network.pdf - Back to Home

When the route table is flushed, the router will recreate the table starting with its
directly attached interfaces. If a dynamic protocol is in use, the flush will trigger an
update immediately. Static route entries that are loaded from the configuration file
at boot will also be restored shortly after the table has been flushed. Be aware that
while the router rebuilds its route table, connectivity will be lost. The connectivity
outage will vary in length from a few seconds to several minutes, depending on the
size of network and the routing table.
The privileged EXEC command clears the router's ARP table. If a
situation requires the entire IP route table to be cleared, it is not a bad idea to clear
the ARP table as well, preferably before you flush the IP route table. This way, you
will ensure that the router has accurate ARP entries for its adjacent neighbors before
the route table is cleared. The ARP table will reconstruct itself quickly, since it is only
dependent on connectivity with nodes that are directly accessible to its connected
interfaces.
Because dynamic protocols refresh route entries automatically, only the most
drastic situations will require the entire route table to be flushed. In most cases,
only a single failed route is the problem, such as a dynamic route in a hold-down
state (waiting to expire) or a static route that points to a nonexistent gateway. To
remove an invalid dynamic route from the routing table, use . It is also possible to clear a single ARP table entry with . Removal of an invalid static route must be done from global
configuration EXEC mode. After you're in configuration mode, place a in front
of the same command string you would use to create the route:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#no ip route 192.160.0.4 255.255.255.252 172.16.0.6 20
Router(config)#^Z
Router#
After the static route is removed from the configuration, the IOS will purge it from
the IP routing table.
NOTE