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190 Chapter 4: Bridges/Switches and LAN Design
Notice that some configuration seems to be missing. VLAN 1, with name VLAN1, is not
configured because it is configured automatically. In fact, the name cannot be changed. Also,
any ports without a specific static VLAN configuration are considered to be in VLAN1. Also,
the IP address of the switch is considered to be in VLAN1's broadcast domain. Ports 5 through
8 are statically configured for VLAN2; similarly, VLAN3 comprises ports 9 through 12. In
addition, the yet-unexplained VTP is set to transparent mode, with a meaningless domain name
of dummy--this setting is not important (yet); it simply must be set.
After the VLAN is configured, the parameters for that VLAN should be confirmed to assure
validity. To verify the parameters of a VLAN, use the show vlan vlan# privileged EXEC
command to display information about a particular VLAN. Use show vlan to show all
configured VLANs. Example 4-9 demonstrates the show command output, which shows the
switch ports assigned to the VLAN.
Other VLAN parameters shown in Example 4-9 include the type (default is Ethernet), SAID
(used for FDDI trunk), MTU (default is 1500 for Ethernet VLAN), Spanning-Tree Protocol (the
1900 supports only the 802.1D Spanning-Tree Protocol standard), and other parameters used
for Token Ring or FDDI VLANs.
Sample Configuration for Multiple Switches
To allow VLANs to span multiple switches, you must configure trunks to interconnect the
switches. Trunks are simply LANs connecting switches. Cisco calls the use of a trunking
protocol such as ISL over such a link trunking, so the command to enable these protocols is
called trunk.
Use the trunk interface configuration command to set a Fast Ethernet port to trunk mode. On
the Catalyst 1900, the two Fast Ethernet ports are interfaces fa0/26 and fa0/27. Enabling and
defining the type of trunking protocol can be done statically, or dynamically for ISL using the
Example 4-9
show vlan Output
Switch1#sh vlan 3
VLAN Name Status Ports
-------------------------------------------------
3 VLAN3 Enabled 9-12
-------------------------------------------------
VLAN Type SAID MTU Parent RingNo BridgeNo Stp Trans1 Trans2
--------------------------------------------------------------------------------
3 Ethernet 100003 1500 0 1 1 Unkn 0 0
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ch04.fm Page 190 Monday, March 20, 2000 5:02 PM