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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IP Unicast Routing
Configuring IP Addressing
On switches running the LAN base image, you can only assign an IP address to an SVI and then
configure a static route for the SVI.The switch supports 16 user-configured static routes. See the
“Configuring Static Unicast Routes” section on page 1-94. No other routing configurations are
supported.
Use of Subnet Zero
Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise
if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as
255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.
You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the
use of subnet zero if you need the entire subnet space for your IP address.
Beginning in privileged EXEC mode, follow these steps to enable subnet zero:
Use the no ip subnet-zero global configuration command to restore the default and disable the use of
subnet zero.
Classless Routing
By default, classless routing behavior is enabled on the switch when it is configured to route. With
classless routing, if a router receives packets for a subnet of a network with no default route, the router
forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C
address spaces used to simulate a single, larger address space and is designed to relieve the pressure on
the rapidly depleting Class B address space.
In Figure 1-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
discarding the packet, the router forwards it to the best supernet route. If you disable classless routing
and a router receives packets destined for a subnet of a network with no network default route, the router
discards the packet.
Step 6
end Return to privileged EXEC mode.
Step 7
show ip route
show ip interface [interface-id]
show running-config interface [interface-id]
Verify your entries.
Step 8
copy running-config startup-config (Optional) Save your entries in the configuration file.
Command Purpose
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip subnet-zero Enable the use of subnet zero for interface addresses and routing updates.
Step 3
end Return to privileged EXEC mode.
Step 4
show running-config Verify your entry.
Step 5
copy running-config startup-config (Optional) Save your entry in the configuration file.
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