CHAPTER

3

Configuring VLAN Trunks
This chapter describes how to configure Fast Ethernet and Gigabit Ethernet VLAN trunks on a switch. For information on adding and deleting VLANs, refer to Chapter 2, “Configuring VTP and Virtual LANs.” Switches support the following trunking methods for transmitting VLAN traffic over 100BaseT and Gigabit Ethernet ports:

• •

Inter-Switch Link (ISL) IEEE 802.1Q

In addition, you can can enable ATM trunking by installing a Catalyst 2900 series XL ATM module in a Catalyst 2900 series XL switch. ATM connectivity is described in the Catalyst 2900 Series XL ATM Modules Installation and Configuration Guide.
Note For complete syntax and usage information for the commands used in this chapter,

refer to the Cisco IOS Desktop Switching Command Reference.

The trunking described in this chapter is not supported on all switches and modules. See the Cisco IOS Desktop Switching Software Configuration Guide for the list of products that support trunking. This chapter consists of these sections:

• • •

“Understanding How VLAN Trunks Work” section on page 3-2 “Configuring a Trunk Port” section on page 3-4 “Load Sharing Using STP” section on page 3-8

Configuring VLAN Trunks 3-1

Understanding How VLAN Trunks Work

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 (the default protocol) 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.

IEEE 802.1Q Configuration Considerations
IEEE 802.1Q trunks impose some limitations on the trunking strategy for a network. The following restrictions apply when using 802.1Q trunks:

• •

Make sure the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk is different from the native VLAN on the other end, spanning-tree loops might result. Disabling STP on the native VLAN of an 802.1Q trunk without disabling STP on every VLAN in the network can potentially cause STP loops. We recommend that you leave STP enabled on the native VLAN of an 802.1Q trunk or disable STP on every VLAN in the network. Make sure your network is loop-free before disabling STP.

3-2

Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide

IEEE 802.1Q Configuration Considerations Figure 3-1 Catalyst 2900 series XL and Catalyst 3500 series XL Switches in an ISL Trunking Environment Catalyst 5000 series switch ISL trunk Catalyst 2900 XL switch ISL trunk ISL trunk ISL trunk Catalyst 3500 XL switch Catalyst 3500 XL switch VLAN3 Catalyst 2900 XL switch VLAN2 VLAN1 VLAN2 VLAN1 VLAN3 15929 Configuring VLAN Trunks 3-3 .

enter this command: switchport trunk encapsulation dotlq Note The Enterprise Edition Software.2(8)SA6. 1Configure configure terminal interface interface_id switchport mode trunk switchport trunk encapsulation isl end show interface interface-id switchport copy running-config startup-config the port to support ISL trunking. To define a port as an ISL trunk port. Save the configuration. Return to privileged EXEC mode. Enter the interface configuration command mode and the port to be added to the VLAN. See the Cisco IOS Desktop Switching Command Reference for more information on how to use this command. how to define the VLANs that can use a port. To configure IEEE 802.1 Q. 3-4 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . If you are assigning a port on a cluster member switch to a VLAN. perform this task from privileged EXEC mode: Task Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 1 Command Enter global configuration mode. does not support trunk negotiation via the Dynamic Trunk Protocol (DTP).Configuring a Trunk Port Configuring a Trunk Port This section describes how to use the CLI to configure an ISL or IEEE 802. Cisco IOS Release 11. Verify your entries. formerly known as Dynamic ISL (DISL).1Q trunk port. Configure the port with a VLAN membership mode of trunk. use the non-negotiate option on the DTP-capable device to configure the switch port to not generate DTP frames. first log in to the member switch by using the privileged EXEC rcommand command. and how to disable a trunk port. If you are connecting a trunk port to a Catalyst 5000 switch or other DTP device.

verify the trunk configuration. and save the change to the startup configuration file: Switch# configure terminal Enter configuration commands. one per line.. 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. End with CNTL/Z. [OK] Switch# Configuring VLAN Trunks 3-5 .1002-1005 Trunking VLANs Active: 1-3 Pruning VLANs Enabled: NONE Switch# copy running-config startup-config Building configuration..Configuring a Trunk Port This example shows how to configure a port as a trunk.

perform this task from privileged EXEC mode: Task Step 1 Step 2 Command Enter global configuration mode. Return to privileged EXEC. However. Configure the VLAN membership mode for trunks. configure terminal interface interface_id Step 3 Step 4 switchport mode trunk switchport trunk allowed vlan remove vlan-list Step 5 Step 6 Step 7 end show interface interface-id switchport allowed-vlan copy running-config startup-config 3-6 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . Enter the interface configuration command mode and the port to be added to the VLAN. To modify the allowed list of a trunk. Save the configuration. use the remove vlan-list parameter to remove specific VLANs from the allowed list. 1 to 1005. preventing traffic from those VLANs from passing over the trunk. The vlan-list parameter is a range of VLAN IDs separated by a hyphen or specific VLAN IDs separated by commas. are allowed on each trunk. Note VLANs 1 and 1002 to 1005 are reserved and cannot be removed. Verify your entries. you can remove VLANs from the allowed list. Define the VLANs that are not allowed to transmit and receive on the port. All VLANs. To restrict the traffic a trunk carries.Configuring a Trunk Port Defining the Allowed VLANs on a Trunk A trunk port by default sends to and receives traffic from all VLANs in the VLAN database.

250. perform the following tasks from privileged EXEC mode: Task Step 1 Step 2 Step 3 Step 4 Step 5 Command Enter global configuration mode.Disabling a Trunk Port This example shows how to define the allowed VLANs list for trunk port Fa0/1 to allow VLANs 1 to 100. configure terminal interface interface_id no switchport mode end show interface interface-id switchport Configuring VLAN Trunks 3-7 . Return to privileged EXEC. Return the port to its default static-access mode. and VLANs 500 to 1005. and how to verify the allowed VLAN list for the trunk: 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. VLAN 250. Enter the interface configuration command mode and the port to be added to the VLAN. Verify your entries. To disable trunking on a port.500-1005" Switch# Disabling a Trunk Port You can disable trunking on a port by returning it to its default static-access mode.

both load-sharing links must be connected to the same switch. Switch(config)# interface 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 End with CNTL/Z. you divide the traffic between the links according to which VLAN the traffic belongs to. Load Sharing Using STP Load sharing divides the bandwidth supplied by parallel trunks connecting switches. one per line.Load Sharing Using STP This example shows how to disable trunking on a port: Switch# configure terminal Enter configuration commands. There are two ways to configure load sharing by using trunk ports: using STP port priorities or using STP path costs. each load-sharing link can be connected to the same switch or to two different switches. 3-8 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . Spanning-Tree Protocol (STP) normally blocks all but one parallel link between switches. To avoid loops. With load sharing. If you configure load sharing using STP path costs. If you configure load sharing using STP port priorities.

the port priority setting determines which port is enabled and which port is in standby mode. Figure 3-2 shows two trunks connecting supported switches. VLANs 3 through 6 are assigned a port priority of 10 on trunk 2. The trunk port with the higher priority (lower values) for a VLAN is forwarding traffic for that VLAN. Figure 3-2 Load Sharing by Using STP Port Priorities Switch 1 Trunk 1 Ns 8 – 10 (priority 10) Ns 3 – 6 (priority 128) Trunk 2 VLANs 3 – 6 (priority 10) VLANs 8 – 10 (priority 128) 15932 Switch 2 Configuring VLAN Trunks 3-9 . In this way. VLANs 3 through 6 retain the default port priority of 128 on trunk 1. There is no duplication of traffic over any trunk port.Load Sharing Using STP Port Priorities Load Sharing Using STP Port Priorities When two ports on the same switch form a loop. One trunk port transmits or receives all traffic for the VLAN. the trunk with the lower priority takes over and carries the traffic for all of the VLANs. If the active trunk fails. VLANs 8 through 10 retain the default port priority of 128 on trunk 2. trunk 1 carries traffic for VLANs 8 through 10. the switches are configured as follows: • • • • VLANs 8 through 10 are assigned a port priority of 10 on trunk 1. In this example. and trunk 2 carries traffic for VLANs 3 through 6. The trunk port with the lower priority (higher values) for the same VLAN remains in a blocking state for that VLAN. You can set the priorities on a parallel trunk port so that the port carries all the traffic for a given VLAN.

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. Exiting..151 at 3-5-93 01:05:21 3-10 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . Step 2 Verify the VTP information by exiting to privileged EXEC mode and displaying the VTP information for both switches..20.128. 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. and configure Switch 1 as a VTP server..Load Sharing Using STP Follow these steps to configure the network shown in Figure 3-2: Step 1 Configure a VTP domain on Switch 1. Switch_1(vlan)# exit APPLY completed.

-------------------------------. Fa0/4. Repeat this procedure to define the trunk ports on Switch 1 and Switch 2.--------------------------1 default active Fa0/2. Fa0/11. The trunks default to ISL trunking. Fa0/3. verify that the VLANs exist in the database on Switch 1. Fa0/10. Switch_1# show vlan VLAN Name Status Ports ---. 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. Fa0/5. Configuring VLAN Trunks 3-11 . Switch_1# configure terminal Enter configuration commands.Load Sharing Using STP Port Priorities Step 3 From privileged EXEC mode.--------. 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 End with CNTL/Z. configure the trunks on Switch 1 ports. one per line.

Verify that switch 2 has learned the VLAN configuration.--------------------------1 default active Fa0/2. Switch_1# configure terminal Enter configuration commands. Fa0/4. Fa0/3. 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 3-12 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . 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. VTP passes the VTP and VLAN information to Switch 2.--------.-------------------------------. Fa0/10. Switch_2# show vlan VLAN Name Status Ports ---. Fa0/11. Fa0/5. one per line.Load Sharing Using STP Step 5 When the trunk links come up.

Configuring VLAN Trunks 3-13 . STP does not disable a port because there are no loops.Load Sharing Using STP Path Cost Step 7 Verify the entries by entering the privileged EXEC show running-config command: Switch_1# show running-config . . The VLANs keep the traffic separate. 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. and redundancy is maintained in the event of a lost link. 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 .

Switch_1(config)# interface fa0/1 Switch_1(config-if)# switchport mode trunk Switch_1(config-if)# end Switch_1# configure terminal Enter configuration commands. End with CNTL/Z. 3-14 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . VLANs 2 through 4 retain the default 100BaseT path cost on trunk port 2 of 19. Switch_1(config)# interface fa0/2 Switch_1(config-if)# switchport mode trunk Switch_1(config-if)# end End with CNTL/Z. The trunk defaults to ISL trunking. one per line.Load Sharing Using STP Figure 3-3 Load-Sharing Trunks with Traffic Distributed by Path Cost Switch 1 Trunk port 1 VLANs 2 – 4 (path cost 30) LANs 8 – 10 (path cost 19) Trunk port 2 VLANs 8 – 10 (path cost 30) VLANs 2 – 4 (path cost 19) 16591 Switch 2 In this example. Switch_1# configure terminal Enter configuration commands. configure the two ports as trunk ports. Follow these steps to configure two parallel trunks to load share based on the STP path cost parameter: Step 1 From privileged EXEC mode. VLANs 8 through 10 are assigned a path cost of 30 on trunk port 2. The path costs for the VLANs are assigned as follows: • • • • VLANs 2 through 4 are assigned a path cost of 30 on trunk port 1. one per line. VLANs 8 through 10 retain the default 100BaseT path cost on trunk port 1 of 19. trunk ports 1 and 2 are 100BaseT ports.

-------------------------------1 default 2 VLAN0002 3 VLAN0003 4 VLAN0004 5 VLAN0005 6 VLAN0006 7 VLAN0007 8 VLAN0008 9 VLAN0009 10 VLAN0010 Configuring VLAN Trunks 3-15 ..Load Sharing Using STP Path Cost Step 2 Verify the entries by entering the privileged EXEC show running-config command: Switch# show running-config Building configuration. Switch 1 receives the VTP information from the other switches.255.128.255. Verify that Switch 1 has learned the VLAN configuration..0 no ip route-cache ! interface FastEthernet0/1 switchport mode trunk ! interface FastEthernet0/2 switchport mode trunk Step 3 When the trunk links come up. Status Ports --------.--------------------------active active active active active active active active active active Switch_1# show vlan VLAN Name ---.20. Current configuration: ! version 11.2 no service pad no service udp-small-servers no service tcp-small-servers ! hostname Switch ! enable password grandkey ! interface VLAN1 ip address 172.178 255.

Switch_1(config)# interface fa0/2 Switch_1(config-if)# spanning-tree vlan 8 9 10 cost 30 Switch_1(config-if)# end 3-16 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide .Load Sharing Using STP Step 4 Use the spanning-tree command to assign the cost parameter to the VLANs that use the trunk on Switch 1. one per line. one per line. End with CNTL/Z. Switch_1# configure terminal Enter configuration commands. End with CNTL/Z. Switch_1(config)# interface fa0/1 Switch_1(config-if)# spanning-tree vlan 2 3 4 cost 30 Switch_1(config-if)# end Switch_1# configure terminal Enter configuration commands.

2 no service pad no service udp-small-servers no service tcp-small-servers ! hostname Switch ! enable password grandkey ! interface VLAN1 ip address 172.Load Sharing Using STP Path Cost Step 5 Verify the entry by entering the privileged EXEC show running-config command: Switch# show running-config Building configuration.179 255..128.255.. Current configuration: ! version 11.20.0 no ip route-cache ! interface FastEthernet0/1 switchport mode trunk spanning-tree vlan 2 cost 30 spanning-tree vlan 3 cost 30 spanning-tree vlan 4 cost 30 ! interface FastEthernet0/2 spanning-tree vlan 8 cost 30 spanning-tree vlan 9 cost 30 spanning-tree vlan 10 cost 30 ! interface FastEthernet0/3 ! interface FastEthernet0/4 Configuring VLAN Trunks 3-17 .255.

Redundant Links Using STP UplinkFast Redundant Links Using STP UplinkFast Switches in hierarchical networks can be grouped into backbone switches. STP UplinkFast is an enhancement that accelerates the choice of a new root port when a link or switch fails or when STP reconfigures itself. This change reduces the chance that the switch will become the root port. the bridge priority of all VLANs is set to 49152. other ports flood the network with multicast packets. When UplinkFast is disabled. Figure 3-4 shows a complex network where distribution switches and access switches each have at least one redundant link that is blocked by STP to prevent loops. and access switches. The root port transitions to the forwarding state immediately without going through the listening and learning states. Enabling STP UplinkFast When you enable UplinkFast. UplinkFast is most useful in edge or access switches and might not be appropriate for backbone devices. 3-18 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . When STP reconfigures the new root port. one for each address that was learned on the port. as it would do with normal STP procedures. if you enter zero. If a switch looses connectivity. and the path cost of all ports and VLAN trunks is increased by 3000. the switch begins using the alternate paths as soon as STP selects a new root port. so the STP topology converges more slowly after a loss of connectivity. Enter this command in global configuration mode to configure UplinkFast: Switch(config)# spanning-tree uplinkfast [max-update-rate pkts-per-second] Note When UplinkFast is enabled. the bridge priorities of all VLANs and path costs of all ports are set to default values. it is enabled for the entire switch and cannot be enabled for individual VLANs. distribution switches. station-learning frames are not generated. You can limit these bursts of multicast traffic by reducing the max-update-rate parameter (the default for this parameter is 150 packets per second). However.

Enabling STP UplinkFast Figure 3-4 Switches in a Hierarchical Network Backbone switches Root bridge 3500 XL 3500 XL Distribution switches 2900 XL 2900 XL 2900 XL Active link Blocked link Access switches Configuring VLAN Trunks 3-19 22037 2900 XL 2900 XL 2900 XL 2900 XL .

if the trunk is acting as a network port. Network port When configured as a network port.1Q.Trunks Interacting with Other Features Trunks Interacting with Other Features ISL. However. a trunk serves as the network port for all VLANs associated with the port. but all trunks in the group must have the same configuration. Secure ports Blocking unicast and multicast packets on a trunk 3-20 Cisco IOS Desktop Switching Enterprise Edition Software Configuration Guide . When a group is first created. The port block command can be used to block the forwarding of unknown unicast and multicast packets to VLANs on a trunk. ISL and 802. all ports follow the parameters set for the first port to be added to the group. unknown unicast packets cannot be blocked. all port cease to be trunks. A static-access port can monitor the traffic of its VLAN on a trunk port. IEEE 802. If you change the configuration of one of the following parameters. A trunk cannot be a secure port. and ATM trunking interacts with other switch features in the following ways: Port monitoring Port grouping A trunk cannot be a monitor port.1Q trunks can be grouped into EtherChannel port groups. ATM ports are always trunks but cannot be part of an EtherChannel port group. A network port receives all unknown unicast traffic on a VLAN. 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.