624
Appendix B
Configuring the Catalyst 1900 Switch
On The interface becomes a permanent ISL trunk port. It can negotiate
with a connected device to convert the link to trunk mode.
Which VLANs are now on the trunked port? All of them by default. You
cannot configure the trunked port to only allow certain VLANs by default.
In the next section, I will show you how to clear VLANs from a trunked port.
Clearing VLANs from Trunk Links
As previously discussed, all VLANs are configured on a trunked link unless
cleared by an administrator. Use the clear trunk command if you don't
want a trunked link to carry VLAN information for two reasons: because
you want to stop broadcasts on a certain VLAN from traversing the trunk
link, or because you want to stop topology-change information from being
sent across a link where a VLAN is not supported.
To delete VLANs from a trunk port on a 1900, use the interface command
no
trunk-vlan. In the following example, I clear VLAN 5 from being com-
municated across the trunked link.
1900EN(config-if)#no trunk-vlan ?
<1-1005> ISL VLAN index
1900EN(config-if)#no trunk-vlan 5
1900EN(config-if)#
Unfortunately, there is no command to clear more than one VLAN at a
time on the 1900. You would not typically clear more than a few VLANs
anyway because, functionally, it makes no difference if they are turned on.
If you had security, broadcast, or routing update issues, you would need to
consider it.
Verifying Trunk Links
To verify your trunk ports, use the show trunk command. If you have more
than one port trunking and want to see statistics on only one trunk port, you
can use the show trunk port_number command.
For the 1900 switch, the Fast Ethernet port 0/26 is identified by trunk A,
and port 0/27 is identified by trunk B. Below, I demonstrate how to view the
trunk port on interface 26:
1900EN#sh trunk ?
A Trunk A
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