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Troubleshooting Routing with SVIs
There are two big topics to investigate when troubleshooting routing over LANs with SVIs. 
First, you have to make sure the switch has been enabled to support IP routing. Second, the 
VLAN associated with each VLAN interface must be known and active on the local switch; 
otherwise, the VLAN interfaces do not come up.
First, about enabling IP routing, note that some models of Cisco switches default to enable 
Layer 3 switching, and some do not. So, to make sure your switch supports Layer 3 routing, 
look to those first few configuration commands listed in the configuration checklist found in 
the earlier section “Configuring Routing Using Switch SVIs.” Those commands are 
sdm prefer
(followed by a 
reload
) and then 
ip routing
(after the 
reload
).
The 
sdm prefer
command changes how the switch forwarding chips allocate memory for 
different forwarding tables, and changes to those tables require a reload of the switch. By 
default, many access switches that support Layer 3 switching still have an SDM default that 
does not allocate space for an IP routing table. Once changed and reloaded, the 
ip routing
command then enables IPv4 routing in IOS software. Both are necessary before some Cisco 
switches will act as a Layer 3 switch.
Example 17-8 shows some symptoms on a router for which Layer 3 switching had not yet 
been enabled by the 
sdm prefer
command. As you can see, both the 
show ip route
EXEC 
command and the 
ip routing
config command are rejected because they do not exist to IOS 
until the 
sdm prefer
command has been used (followed by a 
reload
of the switch).
Example 17-8
Evidence That a Switch Has Not Yet Enabled IPv4 Routing
SW1#