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A host route refers to a route
whose associated mask has all 32 bits set to 1 --- 255.255.255.255. For an
address and mask such as this, there can be only one host.
One common cause of host routes is a route
advertisement that has bits set in the range that the receiving router
interprets as outside of the subnet mask. In this case, the router compared
the advertisement received from an adjacent router with the mask of the
interface it was received over and found a mismatch. This is how RIP V1 and
IGRP determine the mask to apply to advertisements they receive.
Another common cause of host routes is a
routing update explicitly advertised with a mask of 255.255.255.255.
Routing protocols such as OSPF and EIGRP include the masks associated with
the routes in their routing updates. Host routes are also often manually
configured to resolve routing problems related to configuring
dial-on-demand routing (DDR).
Sending Routing Updates
The output of debug ip rip
from RouterA shows RouterA advertising the summarized major network of
168.71.0.0. (see RouterA#debug ip rip
command output).
The output of debug ip rip
from RouterC shows RouterC advertising the summarized major network of
168.71.0.0. (see RouterC#debug ip rip
command output).
Although both routers are advertising the
summarized major net of 168.71.0.0 to each other, neither router will
install the route while using RIP because they have local connections to
the 168.71.0.0 major net.
Pinging the Interfaces
In the output, which is from
RouterA pinging 168.71.8.1, you can see that the pings are successful. (see
RouterA#ping 168.71.8.1
command output).
In the output, which is from
RouterC pinging 168.71.5.1, you can see that the pings are successful. (see
RouterC#ping 168.71.5.1
command output)
The fact that the pings are successful is
the final proof that IP unnumbered has been configured correctly. Strictly
speaking, this step is unnecessary as long as the results from the show
ip route command indicate that proper routes have been installed.
However, it is always better to double-check proper configuration by
pinging the addresses.
The next section is a scenario about what
happens when IP unnumbered has been misconfigured.
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