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By default, all the ports on a Catalyst switch are in VLAN 1. If you want to use VLANs and
trunks, you need to configure them on the switches throughout the network. The steps you
use to configure and verify VLANs on a switched network include the following:
■ Determine whether to use VTP. If VTP will be used, enable VTP in server, client, or
transparent mode.
■ Enable trunking on the inter-switch connections.
■ Create the VLANs on a VTP server and have those VLANs propagate to other
switches.
■ Assign switch ports to a VLAN using static or dynamic assignment.
■ Save the VLAN configuration.
VTP Configuration
When creating VLANs, you must decide whether to use VTP in your network. With VTP,
you can make configuration changes on one or more switches, and those changes are
automatically communicated to all other switches in the same VTP domain.
Default VTP configuration values depend on the switch model and the software version.
The default values for Cisco Catalyst switches are as follows:
802.1Q
Trunk
802.1Q
Trunk
802.1Q
Trunk
802.1Q
Trunk
VLAN 1
VLAN 2
Host X
VLAN 3 Broadcast Traffic Pruned
VLAN 1 VLAN 2
Switch 5 VLAN 3 Broadcast Traffic
Switch 1 Switch 2 Switch 3 Switch 4
VLAN 3
VLAN 3
Implementing VLANs and Trunks 31
■ VTP domain name: Null
■ VTP mode: Server
■ VTP password: None
■ VTP pruning: Enabled/Disabled (OS version specific)
■ VTP version: Version 1
The VTP domain name can be specified or learned. By default, the domain name is not set.
You can set a password for the VTP management domain. However, if you do not assign
the same password for each switch in the domain, VTP does not function properly.
VTP pruning eligibility is one VLAN parameter that the VTP protocol advertises. Enabling
or disabling VTP pruning on a VTP server propagates the change throughout the
management domain.
Use the vtp global configuration command to modify the VTP configuration, domain name,
interface, and mode:
SwitchX# configure terminal
SwitchX(config)# vtp mode [ server | client | transparent ]
SwitchX(config)# vtp domain domain-name
SwitchX(config)# vtp password password
SwitchX(config)# vtp pruning
SwitchX(config)# end
Use the no form of this command to remove the filename or to return to the default settings.
When the VTP mode is transparent, you can save the VTP configuration in the switch
configuration file by entering the copy running-config startup-config privileged EXEC
command.
Example: VTP Configuration
Example 2-1 demonstrates the commands that you would enter to configure VTP and
display VTP status. The characteristics of the switch in this example are as follows:
■ The switch is transparent in the VTP domain.
■ The VTP domain name is ICND.
NOTE The domain name and password are case sensitive. You cannot remove a domain
name after it is assigned; you can only reassign it.
32 Chapter 2: Medium-Sized Switched Network Construction
■ Pruning is disabled.
■ The configuration revision is 0.
802.1Q Trunking Configuration
The 802.1Q protocol carries traffic for multiple VLANs over a single link on a multivendor
network.
802.1Q trunks impose several limitations on the trunking strategy for a network. You should
consider the following:
■ Ensure that the native VLAN for an 802.1Q trunk is the same on both ends of the trunk
link. If they are different, spanning-tree loops might result.
■ Native VLAN frames are untagged.
NOTE In the output from the show vtp status command, “VTP Version” identifies what
version of VTP the switch is capable of running, and “VTP V2 Mode” indicates whether
VTP Version 2 is being used. If “VTP V2 Mode” shows disabled, VTP Version 1 is being
used.
Example 2-1 Configuring VTP and Displaying VTP Status
SwitchX(config)# vtp domain ICND
Changing VTP domain name to ICND
SwitchX(config)# vtp mode transparent
Setting device to VTP TRANSPARENT mode.
SwitchX(config)# end
SwitchX# show vtp status
VTP Version : 2
Configuration Revision : 0
Maximum VLANs supported locally : 64
Number of existing VLANs : 17
VTP Operating Mode : Transparent
VTP Domain Name : ICND
VTP Pruning Mode : Disabled
VTP V2 Mode : Disabled
VTP Traps Generation : Disabled
MD5 digest : 0x7D 0x6E 0x5E 0x3D 0xAF 0xA0 0x2F 0xAA
Configuration last modified by 10.1.1.4 at 3-3-93 20:08:05
SwitchX#
Implementing VLANs and Trunks 33
Table 2-6 shows how 802.1Q trunking interacts with other switch features.
1 STP = Spanning Tree Protocol
Use the switchport mode interface configuration command to set a FastEthernet or Gigabit
Ethernet port to trunk mode. Many Cisco Catalyst switches support the Dynamic Trunking
Protocol (DTP), which manages automatic trunk negotiation.
Four options for the switchport mode command are listed in Table 2-7.
Table 2-6 Switch Feature Trunk Interaction
Switch Feature Trunk Port Interaction
Secure ports A trunk port cannot be a secure port.
Port grouping You can group 802.1Q trunks into EtherChannel port groups, but all
trunks in the group must have the same configuration.
When you create a group, all ports follow the parameters that are set for
the first port you add to the group. If you change the configuration of
one of these parameters, the switch propagates the setting that you enter
to all ports in the group. The settings include the following:
• Allowed VLAN list
• STP1 path cost for each VLAN
• STP port priority for each VLAN
• STP PortFast setting
• Trunk status; if one port in a port group ceases to be a trunk, all ports
cease to be trunks
Table 2-7 switchport mode Parameters
Parameter Description
trunk Configures the port into permanent 802.1Q trunk mode and negotiates with
the connected device to convert the link to trunk mode.
access Disables port trunk mode and negotiates with the connected device to
convert the link to nontrunk.
dynamic
desirable
Triggers the port to negotiate the link from nontrunk to trunk mode. The port
negotiates to a trunk port if the connected device is in trunk state, desirable
state, or auto state. Otherwise, the port becomes a nontrunk port.
dynamic
auto
Enables a port to become a trunk only if the connected device has the state
set to trunk or desirable. Otherwise, the port becomes a nontrunk port.
34 Chapter 2: Medium-Sized Switched Network Construction
The switchport nonegotiate interface command specifies that DTP negotiation packets are
not sent on the Layer 2 interface. The switch does not engage in DTP negotiation on this
interface. This command is valid only when the interface switchport mode is access or trunk
(configured by using the switchport mode access or the switchport mode trunk interface
configuration command). This command returns an error if you attempt to execute it in
dynamic (auto or desirable) mode. Use the no form of this command to return to the default
setting. When you configure a port with the switchport nonegotiate command, the port
trunks only if the other end of the link is specifically set to trunk. The switchport
nonegotiate command does not form a trunk link with ports in either dynamic desirable or
dynamic auto mode.
Table 2-8 shows the steps to configure a port as an 802.1Q trunk port, beginning in
privileged EXEC mode.
Some Cisco Catalyst switches support only 802.1Q encapsulation, which is configured
automatically when trunking is enabled on the interface by using the switchport mode
trunk command.
To verify a trunk configuration on many Cisco Catalyst switches, use the show interfaces
interface switchport or the show interfaces interface trunk command to display the trunk
parameters and VLAN information of the port, as demonstrated
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