Friday, September 13, 2013

VLAN Up, Line Protocol Down

Creating a VLAN and a SVI

Today, not for the first time, I was asked to troubleshoot a issue with an SVI (Switch Virtual Interface) not coming up.  The customer had created a SVI like so:

switch# conf t
switch (config)# int vlan 10
switch (config-if)# ip address
switch (config-if)# no shut

The problem was that the SVI stayed down, and the VLAN did not even show up when you do a “sh vlan”.  Why did this happen?  Because the customer created the SVI without first creating the Layer 2 VLAN for it to live in.  Here is what the customer should have done:

switch# conf t
switch (config)# vlan 10 ***this creates the layer 2 VLAN***
switch (config-vlan)# name VLAN_NAME
switch (config-vlan)# exit
switch (config)# int vlan 10 ***this creates the layer 3 SVI for the VLAN***
switch (config-if)# ip address ***assigns the IP and netmask to the SVI***
switch (config-if)# no shut

Now if we do a “show vlan” it should show as active.  Whilst we’re on this topic, let’s have a look at what exactly is required for a SVI to come up and be ping-able on your network.

Requirements to get a SVI to the Up/Up state

An SVI must meet all of the below requirements to transition to the “Up/Up” state

  • Its VLAN must exist and show as active in the VLAN database “sh vlan”
  • One switched port in the VLAN (can be access or trunk) must be up
  • The port must be in a STP forwarding state