Table of Contents

Understanding How VLAN Trunks Work
Configuring a Trunk Port

Defining the Allowed VLANs on a Trunk
Disabling a Trunk Port

Load Sharing Using STP

Load Sharing Using STP Port Priorities
Load Sharing Using STP Path Cost

Trunks Interacting with Other Features

Configuring VLAN Trunks

This chapter describes how to configure Fast Ethernet and Gigabit Ethernet VLAN trunks on the Catalyst 2900 series XL switches. For information on adding and deleting VLANs, refer to "Configuring VTP and Virtual LANs."

Catalyst 2900 series XL switches support the following trunking methods for transmitting VLAN traffic over 100BaseT and Gigabit Ethernet ports:

• Inter-Switch Link (ISL)

• IEEE 802.1Q

The trunking described in this chapter is not supported on all switches and modules. See the Catalyst 2900 Series XL Installation and Configuration Guide for the list of products that support trunking.

This chapter consists of these sections:

• "Understanding How VLAN Trunks Work" section

• "Configuring a Trunk Port" section

• "Load Sharing Using STP" section

Understanding How VLAN Trunks Work

A trunk is a point-to-point link that transmits and receives traffic between switches or between switches and routers. Trunks carry the traffic of multiple VLANs and can extend VLANs across an entire network. 100BaseT and Gigabit Ethernet trunks use Cisco ISL or industry-standard IEEE 802.1Q to carry traffic for multiple VLANs over a single link.

Frames received from users in the administratively-defined VLANs are identified or tagged for transmission to other devices. Based on rules you define, a unique identifier (the tag) is inserted in each frame header before it is forwarded. The tag is examined and understood by each device before any broadcasts or transmission to other switches, routers, or end stations. When the frame reaches the last switch or router, the tag is removed before the frame is transmitted to the target end station.

Figure 3-1 shows a network of switches that are connected by ISL trunks.

Configuring a Trunk Port

This section describes how to use the CLI to configure an ISL or IEEE 802.1Q trunk port, how to define the VLANs that can use a port, and how to disable a trunk port.

Switch# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)# interface fa0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk encapsulation isl
Switch(config-if)# end
 
Switch# show interface fa0/1 switchport
Name: Fa0/1
Switchport: Enabled

 Administrative mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl

 Operational Trunking Encapsulation: isl
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: 1-3,1002-1005
Trunking VLANs Active: 1-3
Pruning VLANs Enabled: NONE
 
Switch# copy running-config startup-config
Building configuration...
[OK]
Switch#

Defining the Allowed VLANs on a Trunk

A trunk port by default sends and receives traffic from all VLANs in the VLAN database. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk. By default, all VLANs, 1 to 1005, are allowed on each trunk. To restrict the traffic a trunk carries, use the remove vlan-list parameter to remove specific VLANs from the allowed list. You cannot remove VLAN 1, the default VLAN, from the allowed list.

Switch(config)# interface fa0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan remove 101-499
Switch(config-if)# switchport trunk allowed vlan add 250
Switch(config-if)# end
Switch# show interface fa0/1 switchport allowed-vlan 

 "1-100,250,500-1005"
Switch#

Disabling a Trunk Port

You can disable trunking on a port by returning it to its default static-access mode. To disable trunking on a port, perform the following tasks from privileged EXEC mode:

Task	Command
Step 1 Enter global configuration mode.	configure terminal
Step 2 Enter the interface configuration command mode and the port to be added to the VLAN.	interface interface
Step 3 Return the port to its default static-access mode.	no switchport mode
Step 4 Return to privileged EXEC.	end
Step 5 Verify your entries.	show interface interface-id switchport

This example shows how to disable trunking on a port:

Switch# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)# interf fa0/1
Switch(config-if)# no switchport mode
Switch(config-if)# end
 
Switch# show interface fa0/1 switchport
Name: Fa0/1
Switchport: Enabled
Administrative mode: static access

 Operational Mode: static access
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Disabled
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: NONE
Pruning VLANs Enabled: NONE

Load Sharing Using STP

Load sharing divides the bandwidth supplied by parallel trunks connecting switches. To avoid loops, Spanning-Tree Protocol (STP) normally blocks all but one parallel link between switches. With load sharing, you divide the traffic between the links according to which VLAN the traffic belongs to.

Load Sharing Using STP Port Priorities

When two ports on the same switch form a loop, the port priority setting determines which port is enabled and which port is in standby mode. You can set the priorities on a parallel trunk port so that the port carries all the traffic for a given VLAN. The trunk port with the higher priority (lower values) for a VLAN is forwarding traffic for that VLAN. The trunk port with the lower priority (higher values) for the same VLAN remains in a blocking state for that VLAN. One trunk port transmits or receives all traffic for the VLAN.

Figure 3-2 shows two trunks connecting Catalyst 2900 series XL switches. In this example, the switches are configured as follows:

• VLANs 8 through 10 are assigned a port priority of 10 on trunk 1.

• VLANs 3 through 6 retain the default port priority of 128 on trunk 1.

• VLANs 3 through 6 are assigned a port priority of 10 on trunk 2.

• VLANs 8 through 10 retain the default port priority of 128 on trunk 2.

In this way, trunk 1 carries traffic for VLANs 8 through 10, and trunk 2 carries traffic for VLANs 3 through 6. If the active trunk fails, the trunk with the lower priority takes over and carries the traffic for all of the VLANs. There is no duplication of traffic over any trunk port.

Follow these steps to configure the network shown in Figure 3-2:

Step 1 Configure a VTP domain on Switch  1, and configure Switch  1 as a VTP server.

Switch_1# vlan database
Switch_1(vlan)# vtp domain milano
Changing VTP domain name from test to milano
Switch_1(vlan)# vtp server
Device mode already VTP SERVER.
 

Step 2 Verify the VTP information by exiting to privileged EXEC mode and displaying the VTP information for both switches.

Switch_1(vlan)# exit
APPLY completed.
Exiting....
Switch_1# show vtp status
VTP Version                     : 2
Configuration Revision          : 0
Maximum VLANs supported locally : 68
Number of existing VLANs        : 59
VTP Operating Mode              : Server
VTP Domain Name                 : milano
VTP Pruning Mode                : Disabled
VTP V2 Mode                     : Disabled
VTP Traps Generation            : Disabled
MD5 digest                      : 0x53 0x97 0x06 0x02 0xF8 0x6F 0x45 0x85
Configuration last modified by 172.20.128.151 at 3-5-93 01:05:21
 

Step 3 From privileged EXEC mode, verify that the VLANs exist in the database on Switch 1.

Switch_1# show vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- ---------------------------
1    default                          active    Fa0/2, Fa0/3, Fa0/4, Fa0/5,
                                                Fa0/10, Fa0/11, Fa0/12
2    VLAN0002                         active
3    VLAN0003                         active 
4    VLAN0004                         active
5    VLAN0005                         active
6    VLAN0006                         active
7    VLAN0007                         active
8    VLAN0008                         active
9    VLAN0009                         active
10   VLAN0010                         active

Step 4 Beginning from privileged EXEC mode, configure the trunks on Switch 1. The trunks default to ISL trunking.

Switch_1# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Switch_1(config)# interface fa0/1
Switch_1(config-if)# switchport mode trunk
Switch_1(config-if)# end
 
Switch_1# show interface fa0/1 switchport
Name: Fa0/1
Switchport: Enabled
Administrative mode: trunk

 Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1-55
Pruning VLANs Enabled: NONE
 

Repeat this procedure to define the trunk ports on Switch 1 and Switch 2.

Step 5 When the trunk links come up, VTP passes the VTP and VLAN information to Switch 2. Verify that switch 2 has learned the VLAN configuration.

Switch_2# show vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- ---------------------------
1    default                          active    Fa0/2, Fa0/3, Fa0/4, Fa0/5,
                                                Fa0/10, Fa0/11, Fa0/12
2    VLAN0002                         active
3    VLAN0003                         active 
4    VLAN0004                         active
5    VLAN0005                         active
6    VLAN0006                         active
7    VLAN0007                         active
8    VLAN0008                         active
9    VLAN0009                         active
10   VLAN0010                         active

Step 6 Use the spanning-tree command to assign the different port priorities on the different VLANs.

Switch_1# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z
Switch_1(config-if)# interface fa0/1
Switch_1(config-if)# spanning-tree vlan 8 9 10 port-priority 10
Switch_1(config-if)# end
Switch_1(config)# interface fa0/2
Switch_1(config-if)# spanning-tree vlan 3 4 5 6 port-priority 10
Switch_1(config-if)# end
 

Step 7 Verify the entries by entering the privileged EXEC show running-config command:

Switch_1# show running-config
.
 interface FastEthernet0/1
switchport mode trunk
spanning-tree vlan 8 priority 10
spanning-tree vlan 9 priority 10
spanning-tree vlan 10 priority 10
!
 interface FastEthernet0/2
switchport mode trunk
spanning-tree vlan 3 priority 10
spanning-tree vlan 4 priority 10
spanning-tree vlan 5 priority 10
spanning-tree vlan 6 priority 10
!
interface FastEthernet0/3
!
interface FastEthernet0/4
port group 11
.
.

Load Sharing Using STP Path Cost

You can configure parallel trunks to share VLAN traffic by setting different path costs on a trunk and associating the path costs with different sets of VLANs. The VLANs keep the traffic separate, STP does not disable a port because there are no loops, and redundancy is maintained in the event of a lost link.

Trunks Interacting with Other Features

ISL and IEEE 802.1Q trunking interacts with other switch features in the following ways:

Port monitoring	A trunk cannot be a monitor port. A static-access port can monitor the traffic of its VLAN on a trunk port.
Port grouping	Trunks can be grouped into port groups, but all trunks in the group must have the same configuration. If you change the configuration of one of the following parameters, the switch propagates the setting you entered to all ports in the group: Allowed-VLAN list STP path cost for each VLAN STP port priority for each VLAN STP Port Fast setting Trunk status: if one port in a port group ceases to be a trunk, all port cease to be trunks. When a group is first created, all ports follow the parameters set for the first port to be added to the group.
Network port	When configured as a network port, a trunk serves as the network port for all VLANs associated with the port. A network port is forwarded all unknown unicast traffic on a VLAN.
Secure ports	A trunk cannot be a secure port.
Blocking unicast and multicast packets on a trunk	The port block command can be used to block the forwarding of unknown unicast and multicast packets to VLANs on a trunk. However, if the trunk is acting as a network port, unknown unicast packets cannot be blocked.

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