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Using a different major network with no subnet
masking on both routers gives similar results to a proper configuration
that uses the same major network and the same subnet mask. In the case of
different major networks, the routers simply advertise their summarized
routes to each other. Each router installs the other's summarized route,
and full connectivity is possible.
To avoid repetition, this section begins
with a misconfiguration of IP unnumbered. Recall that a misconfiguration
of a subnet mask using the same major net on both routers causes
connectivity to be broken. In this scenario, connectivity is still
possible but the misconfiguration results in some other problems.
A different major network --- 168.72.0.0
--- has been configured on RouterC's Ethernet
interface; see the main figure. In addition, a subnet mask has been added
--- 255.255.255.0. RouterA still has its original IP unnumbered
address of 168.71.5.1 255.255.255.0. Using a subnet mask on both major
nets is a violation of the IP unnumbered rule that states as follows: When
using different major nets, no subnetting is allowed.
The remainder of this scenario shows how
connectivity is still possible even though IP unnumbered has been
misconfigured. It also further explains the problems caused by violating
the no subnet mask rule.
In the show interface
command from RouterC, you can see that the new major network has been
configured with a subnet mask. (see RouterC#show interface ethernet 0
command output)
In the output of the show
interface command from RouterA, you can see that RouterA has the same
IP unnumbered configuration as before. (see RouterA#show interface serial 1
command output)
In the output of the debug ip
packet commands from routers A and C, you can see the two routers
sending and receiving routing updates. Note that a subnet route and a
summarized route are sent in both cases. (see RouterA#debug ip rip
and RouterC#debug ip rip
commands outputs).
In the routing table from RouterA,
note the mask on the subnet route to 168.72.8.0, which is 255.255.255.255.
(see RouterA#show ip route
command output). This is a host route. If RouterA received a packet destined for 168.71.8.2
on its Ethernet interface, the longest match comparison with the route to
168.71.8.0 would fail. This would occur because the mask associated with
the route (255.255.255.255) indicates that in order to match this route,
the last octet of the destination IP address should be a 0.
Connectivity is still possible because
RouterA has installed a summarized route to 168.72.0.0. RouterA installs
this route because it does not have a local connection to 168.72.0.0.
In the show ip route
command
from RouterA, you can see that RouterA will use the summarized major
network to route packets destined for 168.72.8.1. RouterA uses this
network because the mask on 168.72.8.0 is 255.255.255.255. Even though it
looks like a better longest match, it isn't because it has a host mask of
32 bits. (see RouterA#show ip route 168.72.8.1
command output).
The output from RouterA, which
shows it pinging 168.72.8.1, proves that it has connectivity to RouterC
because the pings are successful. (see RouterA#ping 168.72.8.1
command output)
In the routing table from RouterC,
note the mask on the subnet route to 168.71.5.0. (see RouterC#show ip route
command output). Connectivity is still
possible because RouterC has installed a summarized route to 168.71.0.0.
RouterC does so because it does not have a local connection to 168.71.0.0,
as discussed previously.
In the show ip route
command from RouterC, you can see that RouterC will use the summarized
major network to route packets destined for 168.71.5.1. It will do so
because the mask on 168.71.5.0 is 255.255.255.255. Even though it looks
like a better longest match, it isn't because it has a host mask of 32
bits. (see RouterC#show ip route 168.71.5.1
command output).
The output from RouterC, which
shows it pinging 168.71.5.1, proves that it has connectivity to RouterA
because the pings are successful. (see RouterC#ping 168.71.5.1
command output).
Note that the previous step of pinging
RouterA from RouterC to prove connectivity in this direction was redundant
because the pings from RouterA had the source address 168.71.5.1 and the
destination address 168.72.8.1. RouterC was able to send ping replies to
168.71.5.1, so there was little doubt that RouterC would be able to
originate pings to the same address. However, it never hurts to
double-check when you aren't sure.
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