Software Configuration Guide (5.5) - Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports [Cisco Catalyst 5000 Series Switches]

Table Of Contents

Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports

Understanding How VLAN Trunks Work

Trunking Overview

Trunking Modes and Encapsulation Types

Trunking Support

802.1Q Trunk Restrictions

Default Trunk Configuration

Configuring a Trunk Link

Configuring an ISL Trunk

Configuring an IEEE 802.1Q Trunk

Configuring an ISL/802.1Q Negotiating Trunk Port

Defining the Allowed VLANs on a Trunk

Disabling a Trunk Port

Example VLAN Trunk Configurations

ISL Trunk Configuration Example

ISL Trunk Over Fast EtherChannel Link Example

IEEE 802.1Q Trunk Over Gigabit EtherChannel Link Example

Load-Sharing VLAN Traffic Over Parallel Trunks Example

IEEE 802.1Q nonegotiate Trunk Configuration Example

Disabling VLAN 1 on Trunks

Disabling VLAN 1 on a Trunk Link

Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports

This chapter describes how to configure Fast Ethernet and Gigabit Ethernet virtual LAN (VLAN) trunks.

Note For complete information on configuring VLANs, refer to "Configuring VLANs."

Note For complete syntax and usage information for the commands used in this chapter, refer to the Command Reference publication for your switch.

This chapter consists of these sections:

•Understanding How VLAN Trunks Work

•Default Trunk Configuration

•Configuring a Trunk Link

•Example VLAN Trunk Configurations

•Disabling VLAN 1 on Trunks

Understanding How VLAN Trunks Work

These sections describe how VLAN trunks work on the Catalyst enterprise LAN switches:

•Trunking Overview

•Trunking Modes and Encapsulation Types

•Trunking Support

•802.1Q Trunk Restrictions

Trunking Overview

A trunk is a point-to-point link between one or more switch ports and another networking device such as a router or a switch. Trunks carry the traffic of multiple VLANs over a single link and allow you to extend VLANs across an entire network.

Two trunking encapsulations are available on Fast Ethernet and Gigabit Ethernet ports:

•Inter-Switch Link (ISL)—ISL is a Cisco-proprietary trunking encapsulation

•IEEE 802.1Q—802.1Q is an industry-standard trunking encapsulation

Note Trunking capabilities are hardware-dependent. For example, the Catalyst 4000 family switch modules support only 802.1Q encapsulation. To determine whether your hardware supports trunking, and to determine which trunking encapsulations are supported, see your hardware documentation or use the show port capabilities command.

You can configure a trunk on a single Fast or Gigabit Ethernet port or on a Fast or Gigabit EtherChannel bundle. For more information about Fast and Gigabit EtherChannel, see "Configuring Fast EtherChannel and Gigabit EtherChannel."

Fast Ethernet and Gigabit Ethernet trunk ports support five different trunking modes (see Table 12-1). In addition, on certain Fast Ethernet and Gigabit Ethernet ports you can specify whether the trunk will use ISL encapsulation, 802.1Q encapsulation, or whether the encapsulation type will be autonegotiated.

For trunking to be autonegotiated on Fast Ethernet and Gigabit Ethernet ports, the ports must be in the same VTP domain. However, you can use the on or nonegotiate mode to force a port to become a trunk, even if it is in a different domain. For more information on VTP domains, see "Configuring VTP."

Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP) in supervisor engine software release 4.2 and later. DTP supports autonegotiation of both ISL and IEEE 802.1Q trunks. In prior releases, trunk negotiation is managed by the Dynamic Inter-Switch Link (DISL) protocol. DISL supports autonegotiation of ISL trunks only. In supervisor engine software release 4.1, you must manually configure IEEE 802.1Q trunks on both ends of the link. IEEE 802.1Q trunks are not supported prior to software release 4.1.

Trunking Modes and Encapsulation Types

Table 12-1 lists the trunking modes used with the set trunk command and describes how they function on Fast Ethernet and Gigabit Ethernet ports.

Table 12-1 Fast Ethernet and Gigabit Ethernet Trunking Modes

Mode

Function

on

Puts the port into permanent trunking mode and negotiates to convert the link into a trunk link. The port becomes a trunk port even if the neighboring port does not agree to the change.

off

Puts the port into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The port becomes a nontrunk port even if the neighboring port does not agree to the change.

desirable

Makes the port actively attempt to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to on, desirable, or auto mode.

auto

Enables the port to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to on or desirable mode. This is the default mode for Fast and Gigabit Ethernet ports.

nonegotiate

Puts the port into permanent trunking mode but prevents the port from generating DTP frames. You must configure the neighboring port manually as a trunk port to establish a trunk link.

Table 12-2 lists the encapsulation types used with the set trunk command and describes how they function on Fast Ethernet and Gigabit Ethernet ports. You can use the show port capabilities command to determine which encapsulation types a particular port supports.

Table 12-2 Fast Ethernet and Gigabit Ethernet Trunk Encapsulation Types

Encapsulation

Function

isl

Specifies ISL encapsulation on the trunk link.

dot1q

Specifies IEEE 802.1Q encapsulation on the trunk link. IEEE 802.1Q trunks are supported in Catalyst 5000 family software release 4.1 and later with 802.1Q-capable hardware. Automatic negotiation of 802.1Q trunks is supported in software release 4.2 and later.

negotiate

Specifies that the port negotiate with the neighboring port to become an ISL (preferred) or 802.1Q trunk, depending on the configuration and capabilities of the neighboring port. This keyword is available in software release 4.2 and later.

The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected ports determine whether a trunk link comes up and the type of trunk the link becomes. Table 12-3 shows the result of the possible trunking configurations.

Table 12-3 Results of Possible Fast Ethernet and Gigabit Ethernet Trunk Configurations

Neighbor Port Trunk Mode and Trunk Encapsulation

Local Port Trunk Mode and Trunk Encapsulation

off
isl or dot1q

on
isl

desirable
isl

auto
isl

on
dot1q

desirable
dot1q

auto
dot1q

desirable
negotiate

auto
negotiate

off
isl or dot1q

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
1Q trunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

on
isl

Local:
Nontrunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
1Q trunk¹

Neighbor:
ISL trunk1

Local:
Nontrunk

Neighbor:
ISL trunk

Local:
Nontrunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

desirable
isl

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
1Q trunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

auto
isl

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
1Q trunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
Nontrunk

Neighbor:
Nontrunk

on
dot1q

Local:
Nontrunk

Neighbor:
1Q trunk

Local:
ISL trunk1

Neighbor:
1Q trunk1

Local:
Nontrunk

Neighbor:
1Q trunk

Local:
Nontrunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

desirable
dot1q

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

auto
dot1q

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
Nontrunk

Neighbor:
Nontrunk

desirable negotiate

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

auto
negotiate

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
1Q trunk

Neighbor:
1Q trunk

Local:
Nontrunk

Neighbor:
Nontrunk

Local:
ISL trunk

Neighbor:
ISL trunk

Local:
Nontrunk

Neighbor:
Nontrunk

¹Using this configuration can result in spanning-tree loops and is not recommended.

Note DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames improperly. To avoid this problem, ensure that trunking is turned off on ports connected to non-switch devices if you do not intend to trunk across those links. When manually enabling trunking on a link to a Cisco router, use the nonegotiate keyword to cause the port to become a trunk but not generate DTP frames. The nonegotiate keyword is available in supervisor engine software release 2.4(3) and later.

Trunking Support

Trunking capabilities are hardware-dependent. Table 12-4 shows which switches have available hardware that supports the two trunking encapsulations. To determine whether a specific piece of hardware supports trunking, and to determine which trunking encapsulations are supported, see your hardware documentation or use the show port capabilities command.

Table 12-4 Trunking Encapsulation Support

Trunking Method

Catalyst 5000 Family

Catalyst 4000 Family

Catalyst 2926G Series

Catalyst 2948G
Catalyst 2980G

ISL

Yes

No

Yes

No

IEEE 802.1Q

Yes

Yes

Yes

Yes

802.1Q Trunk Restrictions

The following configuration guidelines and restrictions apply when using 802.1Q trunks impose some limitations on the trunking strategy for a network. Note these restrictions when using 802.1Q trunks:

•When connecting Cisco switches through an 802.1q trunk, 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 spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on every VLAN in the network can cause spanning-tree loops. We recommend that you leave spanning tree enabled on the native VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree on every VLAN in the network. Make sure your network is free of physical loops before disabling spanning tree.

•When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning-tree BPDUs on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent untagged to the reserved IEEE 802.1d spanning-tree multicast MAC address (01-80-C2-00-00-00). The BPDUs on all other VLANs on the trunk are sent tagged to the reserved Cisco Shared Spanning Tree (SSTP) multicast MAC address (01-00-0c-cc-cc-cd).

•Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (the Mono Spanning Tree, or MST) that defines the spanning-tree topology for all VLANs. When you connect a Cisco switch to a non-Cisco switch through an 802.1Q trunk, the MST of the non-Cisco switch and the native VLAN spanning-tree of the Cisco switch combine to form a single spanning-tree topology known as the Common Spanning Tree (CST).

•Because Cisco switches transmit BPDUs to the SSTP multicast MAC address on VLANs other than the native VLAN of the trunk, non-Cisco switches do not recognize these frames as BPDUs and flood them on all ports in the corresponding VLAN. Other Cisco switches connected to the non-Cisco 802.1q cloud receive these flooded BPDUs. This allows Cisco switches to maintain a per-VLAN spanning-tree topology across a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single broadcast segment between all switches connected to the non-Cisco 802.1q cloud through 802.1q trunks.

•Make certain that the native VLAN is the same on ALL of the 802.1q trunks connecting the Cisco switches to the non-Cisco 802.1q cloud.

•If you are connecting multiple Cisco switches to a non-Cisco 802.1q cloud, all of the connections MUST be through 802.1q trunks. You CANNOT connect Cisco switches to a non-Cisco 802.1q cloud through ISL trunks or through access ports. Doing so will cause the switch to place the ISL trunk port or access port into the spanning-tree "port inconsistent" state and no traffic will pass through the port.

Default Trunk Configuration

Table 12-5 shows the default Fast Ethernet and Gigabit Ethernet trunk configuration.

Table 12-5 Default Fast Ethernet and Gigabit Ethernet Trunk Configuration

Feature

Default Configuration

Trunk mode

auto

Trunk encapsulation

•negotiate (on hardware supporting both ISL and 802.1Q)

•isl (on hardware supporting ISL only)

•dot1q (on hardware supporting 802.1Q only)

Allowed VLAN range

VLANs 1-1005

Configuring a Trunk Link

These sections describe how to configure a trunk link on Fast Ethernet and Gigabit Ethernet ports and how to define the allowed VLAN range on a trunk:

•Configuring an ISL Trunk

•Configuring an IEEE 802.1Q Trunk

•Configuring an ISL/802.1Q Negotiating Trunk Port

•Defining the Allowed VLANs on a Trunk

•Disabling a Trunk Port

Configuring an ISL Trunk

Note Some hardware does not support ISL encapsulation. To determine whether your hardware supports ISL, see your hardware documentation or use the show port capabilities command.

To configure an ISL trunk, perform this task in privileged mode:

Task

Command

Step 1

Configure an ISL trunk.

set trunk mod_num/port_num [on | desirable | auto | nonegotiate] isl

Step 2

Verify the trunking configuration.

show trunk [mod_num/port_num]

This example shows how to configure a port as a trunk and how to verify the trunk configuration. This example assumes that the neighbor port is in auto mode.
Console> (enable) set trunk 1/1 on
Port(s) 1/1 trunk mode set to on.
Console> (enable) 06/16/1998,22:16:39:DTP-5:Port 1/1 has become isl trunk
06/16/1998,22:16:40:PAGP-5:Port 1/1 left bridge port 1/1.
06/16/1998,22:16:40:PAGP-5:Port 1/1 joined bridge port 1/1.
Console> (enable) show trunk
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 1/1      on           isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 1/1      1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 1/1      1,521-524
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 1/1      
Console> (enable)
This example shows how to place a port in desirable mode and how to verify the trunk configuration. This example assumes that the neighbor port is in auto mode.
Console> (enable) set trunk 1/2 desirable
Port(s) 1/2 trunk mode set to desirable.
Console> (enable) 06/16/1998,22:20:16:DTP-5:Port 1/2 has become isl trunk
06/16/1998,22:20:16:PAGP-5:Port 1/2 left bridge port 1/2.
06/16/1998,22:20:16:PAGP-5:Port 1/2 joined bridge port 1/2.
Console> (enable) show trunk 1/2
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 1/2      desirable    isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 1/2      1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 1/2      1,521-524
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 1/2      
Console> (enable)
Configuring an IEEE 802.1Q Trunk

Note Some hardware does not support 802.1Q encapsulation. To determine whether your hardware supports 802.1Q, see your hardware documentation or use the show port capabilities command.

Note IEEE 802.1Q trunks require supervisor engine software release 4.1 or later and 802.1Q-capable hardware. In release 4.1, only the nonegotiate and off modes function with 802.1Q trunks. Autonegotiation of 802.1Q trunks is supported in software release 4.2 and later.

Caution DTP (formerly known as DISL) negotiation does not occur on IEEE 802.1Q trunks in software releases prior to release 4.2. You must configure the ports on both ends of the trunk link as 802.1Q trunks using the set trunk command with the nonegotiate and dot1q keywords. Expect Spanning-Tree Protocol (STP) to block the port on the other end of the trunk link until you configure that end of the link as an 802.1Q trunk as well. Do not configure one end of a trunk as an 802.1Q trunk and the other end as an ISL trunk or a nontrunk port. Errors will occur and no traffic can pass over the link. For more information, see the "Trunking Modes and Encapsulation Types" section.

To configure an IEEE 802.1Q trunk, perform this task in privileged mode:

Task

Command

Step 1

Configure an 802.1Q trunk.

set trunk mod_num/port_num [on | desirable | auto | nonegotiate] dot1q

Step 2

Verify the trunking configuration.

show trunk [mod_num/port_num]

This example shows how to configure an IEEE 802.1Q trunk and how to verify the trunk configuration in software release 4.2 and later:
Console> (enable) set trunk 2/9 desirable dot1q
Port(s) 2/9 trunk mode set to desirable.
Port(s) 2/9 trunk type set to dot1q.
Console> (enable) 07/02/1998,18:22:25:DTP-5:Port 2/9 has become dot1q trunk
Console> (enable) show trunk 
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 2/9      desirable    dot1q          trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 2/9      1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 2/9      1,5,10-32,101-120,150,200,250,300,400,500,600,700,800,900,1000
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 2/9      5,10-32,101-120,150,200,250,300,400,500,600,700,800,900,1000
Console> (enable) 
This example shows how to configure an IEEE 802.1Q trunk in software release 4.1:
Console> (enable) set trunk 4/5 nonegotiate dot1q
Port(s) 4/5 trunk mode set to nonegotiate.
Port(s) 4/5 trunk type set to dot1q.
Console> (enable) 2/20/1998,23:38:35:DISL-5:Port 1/1 has become dot1q trunk
Configuring an ISL/802.1Q Negotiating Trunk Port

To configure a trunk port to negotiate the trunk encapsulation type (either ISL or IEEE 802.1Q), perform this task in privileged mode:

Task

Command

Step 1

Configure a port to negotiate the trunk encapsulation type.

set trunk mod_num/port_num [on | desirable | auto | nonegotiate] negotiate

Step 2

Verify the trunking configuration.

show trunk [mod_num/port_num]

This example shows how to configure a port to negotiate the encapsulation type and how to verify the trunk configuration. This example assumes that the neighbor port is in auto mode with encapsulation set to isl or negotiate.
Console> (enable) set trunk 4/11 desirable negotiate
Port(s) 4/11 trunk mode set to desirable.
Port(s) 4/11 trunk type set to negotiate.
Console> (enable) show trunk 4/11
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 4/11     desirable    n-isl          trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 4/11     1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 4/11     1,5,10-32,55,101-120,998-1000
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 4/11     1,5,10-32,55,101-120,998-1000
Console> (enable)
Defining the Allowed VLANs on a Trunk

When you configure a trunk port, all VLANs are added to the allowed VLANs list for that trunk. However, you can remove VLANs from the allowed list to prevent traffic for those VLANs from passing over the trunk. You cannot remove VLAN 1, the default VLAN, from the allowed list.

Note When you first configure a port as a trunk, the set trunk command always adds all VLANs to the allowed VLAN list for the trunk, even if you specify a VLAN range (any specified VLAN range is ignored). To modify the allowed VLANs list, use a combination of the clear trunk and set trunk commands to specify the allowed VLANs.

To define the allowed VLAN list for a trunk port, perform this task in privileged mode:

Task

Command

Step 1

Remove VLANs from the allowed VLANs list for a trunk.

clear trunk mod_num/port_num vlans

Step 2

(Optional) Add specific VLANs to the allowed VLANs list for a trunk.

set trunk mod_num/port_num vlans

Step 3

Verify the allowed VLAN list for the trunk.

show trunk [mod_num/port_num]

This example shows how to define the allowed VLANs list for trunk port 1/1 to allow VLANs 1-100, VLAN 250, and VLANs 500-1005, and how to verify the allowed VLAN list for the trunk:
Console> (enable) clear trunk 1/1 101-499
Removing Vlan(s) 101-499 from allowed list.
Port 1/1 allowed vlans modified to 1-100,500-1005.
Console> (enable) set trunk 1/1 250
Adding vlans 250 to allowed list.
Port(s) 1/1 allowed vlans modified to 1-100,250,500-1005.
Console> (enable) show trunk 1/1
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 1/1      desirable    isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 1/1      1-100,250,500-1005
Port      Vlans allowed and active in management domain
--------  ---------------------------------------------------------------------
 1/1      1,521-524
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 1/1      1,521-524
Console> (enable)
Disabling a Trunk Port

To explicitly turn off trunking on a port, perform this task in privileged mode:

Task

Command

Step 1

Turn off trunking on a port.

set trunk mod_num/port_num off

Step 2

Verify the trunking configuration.

show trunk [mod_num/port_num]

To return a port to the default trunk type and mode for that port type, perform this task in privileged mode:

Task

Command

Step 1

Return the port to the default trunking type and mode for that port type.

clear trunk mod_num/port_num

Step 2

Verify the trunking configuration.

show trunk [mod_num/port_num]

Example VLAN Trunk Configurations

This section contains example VLAN trunk configurations:

•ISL Trunk Configuration Example

•ISL Trunk Over Fast EtherChannel Link Example

•IEEE 802.1Q Trunk Over Gigabit EtherChannel Link Example

•Load-Sharing VLAN Traffic Over Parallel Trunks Example

•IEEE 802.1Q nonegotiate Trunk Configuration Example

Note For examples of configuring trunk links between switches and routers, refer to the Layer 3 Switching Software Configuration Guide—Catalyst 5000 Family, 4000 Family, 2926G Series, 2926 Series, 2948G, and 2980G Switches publication.

ISL Trunk Configuration Example

This example configuration shows how to configure an ISL trunk between two switches and how to limit the allowed VLANs on the trunk to VLAN 1 and VLANs 520-530.

In this example, port 1/1 (a Catalyst 5000 family supervisor engine Fast Ethernet uplink port) on Switch 1 is connected to a Fast Ethernet port on another switch. Both ports are in their default state, with the trunk mode set to auto (for more information, see the "Default Trunk Configuration" section).

Step 1 Enter the set trunk command to configure port 1/1 on Switch 1 as an ISL trunk port. By specifying the desirable keyword, the trunk is automatically negotiated with the neighboring port (port 1/2 on Switch 2). ISL encapsulation is assumed based on the hardware type.
Switch1> (enable) set trunk 1/1 desirable
Port(s) 1/1 trunk mode set to desirable.
Switch1> (enable) 06/18/1998,12:20:23:DTP-5:Port 1/1 has become isl trunk
06/18/1998,12:20:23:PAGP-5:Port 1/1 left bridge port 1/1.
06/18/1998,12:20:23:PAGP-5:Port 1/1 joined bridge port 1/1.
Switch1> (enable)
Step 2 Enter the show trunk command to check the configuration. The Status field in the screen output indicates that port 1/1 is trunking.
Switch1> (enable) show trunk 1/1
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 1/1      desirable    isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 1/1      1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 1/1      1,521-524
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 1/1      
Switch1> (enable)
Step 3 To define the allowed VLAN list for the trunk, use the clear trunk command to remove the VLANs that should not pass traffic over the trunk link.
Switch1> (enable) clear trunk 1/1 2-519
Removing Vlan(s) 2-519 from allowed list.
Port 1/1 allowed vlans modified to 1,520-1005.
Switch1> (enable) clear trunk 1/1 531-1005
Removing Vlan(s) 531-1005 from allowed list.
Port 1/1 allowed vlans modified to 1,520-530.
Switch1> (enable) show trunk 1/1
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 1/1      desirable    isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 1/1      1,520-530
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 1/1      1,521-524
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 1/1      1,521-524
Switch1> (enable)
Step 4 Verify connectivity across the trunk using the ping command:
Switch1> (enable) ping switch2
switch2 is alive
Switch1> (enable)
ISL Trunk Over Fast EtherChannel Link Example

This example configuration shows how to configure an ISL trunk over a Fast EtherChannel link between two switches.

Figure 12-1 shows two switches connected through two 100BaseTX Fast Ethernet ports.

Figure 12-1 ISL Trunk Over Fast EtherChannel Link

This example shows how to configure the switches to form a two-port Fast EtherChannel bundle, and then configure the EtherChannel bundle as an ISL trunk link.

Note There are a variety of configuration guidelines and restrictions for configuring Fast EtherChannel port bundles. For complete information on configuring Fast EtherChannel, see "Configuring Fast EtherChannel and Gigabit EtherChannel."

Step 1 You can confirm the channeling and trunking status of the switches using the show port channel and show trunk commands.
Switch_A> (enable) show port channel
No ports channelling
Switch_A> (enable) show trunk
No ports trunking.
Switch_A> (enable)
Switch_B> (enable) show port channel
No ports channelling
Switch_B> (enable) show trunk
No ports trunking.
Switch_B> (enable)
Step 2 Configure the ports on Switch A to negotiate a Fast EtherChannel bundle with the neighboring switch. This example assumes that the neighboring ports on Switch B are in EtherChannel auto mode. The system logging messages provide information about the formation of the EtherChannel bundle.
Switch_A> (enable) set port channel 1/1-2 desirable
Port(s) 1/1-2 channel mode set to desirable.
Switch_A> (enable) %PAGP-5-PORTFROMSTP:Port 1/1 left bridge port 1/1
%PAGP-5-PORTFROMSTP:Port 1/2 left bridge port 1/2
%PAGP-5-PORTFROMSTP:Port 1/2 left bridge port 1/2
%PAGP-5-PORTTOSTP:Port 1/1 joined bridge port 1/1-2
%PAGP-5-PORTTOSTP:Port 1/2 joined bridge port 1/1-2
Switch_B> (enable) %PAGP-5-PORTFROMSTP:Port 3/1 left bridge port 3/1
%PAGP-5-PORTFROMSTP:Port 3/2 left bridge port 3/2
%PAGP-5-PORTFROMSTP:Port 3/2 left bridge port 3/2
%PAGP-5-PORTTOSTP:Port 3/1 joined bridge port 3/1-2
%PAGP-5-PORTTOSTP:Port 3/2 joined bridge port 3/1-2
Step 3 After the EtherChannel bundle is negotiated, use the show port channel command to verify the configuration.
Switch_A> (enable) show port channel
Port  Status     Channel   Channel     Neighbor                  Neighbor
                 mode      status      device                    port
----- ---------- --------- ----------- ------------------------- ---------- 
 1/1  connected  desirable channel     WS-C5000    009979082(Sw  3/1       
 1/2  connected  desirable channel     WS-C5000    009979082(Sw  3/2       
----- ---------- --------- ----------- ------------------------- ---------- 
Switch_A> (enable) 
Switch_B> (enable) show port channel
Port  Status     Channel   Channel     Neighbor                  Neighbor
                 mode      status      device                    port
----- ---------- --------- ----------- ------------------------- ---------- 
 3/1  connected  auto      channel     WS-C5500    069003103(Sw  1/1       
 3/2  connected  auto      channel     WS-C5500    069003103(Sw  1/2       
----- ---------- --------- ----------- ------------------------- ---------- 
Switch_B> (enable)
Step 4 Configure one of the ports in the EtherChannel bundle to negotiate an ISL trunk. The configuration is applied to all of the ports in the bundle. This example assumes that the neighboring ports on Switch B are configured to use isl or negotiate encapsulation and are in auto trunk mode. The system logging messages provide information about the formation of the ISL trunk.
Switch_A> (enable) set trunk 1/1 desirable isl
Port(s) 1/1-2 trunk mode set to desirable.
Port(s) 1/1-2 trunk type set to isl.
Switch_A> (enable) %DTP-5-TRUNKPORTON:Port 1/1 has become isl trunk
%DTP-5-TRUNKPORTON:Port 1/2 has become isl trunk
%PAGP-5-PORTFROMSTP:Port 1/1 left bridge port 1/1-2
%PAGP-5-PORTFROMSTP:Port 1/2 left bridge port 1/1-2
%PAGP-5-PORTTOSTP:Port 1/1 joined bridge port 1/1-2
%PAGP-5-PORTTOSTP:Port 1/2 joined bridge port 1/1-2
Switch_B> (enable) %DTP-5-TRUNKPORTON:Port 3/1 has become isl trunk
%DTP-5-TRUNKPORTON:Port 3/2 has become isl trunk
%PAGP-5-PORTFROMSTP:Port 3/1 left bridge port 3/1-2
%PAGP-5-PORTFROMSTP:Port 3/2 left bridge port 3/1-2
%PAGP-5-PORTTOSTP:Port 3/1 joined bridge port 3/1-2
%PAGP-5-PORTTOSTP:Port 3/2 joined bridge port 3/1-2
Step 5 After the ISL trunk link is negotiated, use the show trunk command to verify the configuration.
Switch_A> (enable) show trunk
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 1/1      desirable    isl            trunking      1
 1/2      desirable    isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 1/1      1-1005
 1/2      1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 1/1      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999
 1/2      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 1/1      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999
 1/2      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999
Switch_A> (enable) 
Switch_B> (enable) show trunk
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
 3/1      auto         isl            trunking      1
 3/2      auto         isl            trunking      1
Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
 3/1      1-1005
 3/2      1-1005
Port      Vlans allowed and active in management domain 
--------  ---------------------------------------------------------------------
 3/1      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999
 3/2      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999
Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
 3/1      1-5,10,20,50,152,200,300,400,500,521-524,570,801,850,917,999
 3/2      1-5,10,20,50,152,200,300,400,500,521-524,570,801,850,917,999
Switch_B> (enable)
IEEE 802.1Q Trunk Over Gigabit EtherChannel Link Example

This example configuration shows how to configure an IEEE 802.1Q trunk over a Gigabit EtherChannel link between two switches with 802.1Q-capable hardware. (Use the show port capabilities command to see if your hardware is 802.1Q-capable.)

Figure 12-2 shows two switches connected through four 1000BaseSX Gigabit Ethernet ports.

Figure 12-2 IEEE 802.1Q Trunk Over Gigabit EtherChannel Link

This example shows how to configure the switches to form a four-port Gigabit EtherChannel bundle, and then configure the EtherChannel bundle as an IEEE 802.1Q trunk link.

Note There are a variety of configuration guidelines and restrictions for configuring Gigabit EtherChannel port bundles. For complete information on configuring Gigabit EtherChannel, see "Configuring Fast EtherChannel and Gigabit EtherChannel."

Step 1 Make sure all ports on both Switch A and Switch B are assigned to the same VLAN. This VLAN is used as the IEEE 802.1Q native VLAN for the trunk. In this example, all ports are configured as members of VLAN 1.
Switch_A> (enable) set vlan 1 2/3-6
VLAN  Mod/Ports
---- -----------------------
1     2/1-6
Switch_A> (enable) 
Switch_B> (enable) set vlan 1 3/3-6
VLAN  Mod/Ports
---- -----------------------
1     3/1-6
Switch_B> (enable) 
Step 2 You can confirm the channeling and trunking status of the switches using the show port channel and show trunk commands.
Switch_A> (enable) show port channel
No ports channelling
Switch_A> (enable) show trunk
No ports trunking.
Switch_A> (enable)
Switch_B> (enable) show port channel
No ports channelling
Switch_B> (enable) show trunk
No ports trunking.
Switch_B> (enable)
Step 3 Configure the ports on Switch A to negotiate a Gigabit EtherChannel bundle with the neighboring switch. This example assumes that the neighboring ports on Switch B are in EtherChannel auto mode. The system logging messages provide information about the formation of the EtherChannel bundle.
Switch_A> (enable) set port channel 2/3-6 desirable
Port(s) 2/3-6 channel mode set to desirable.
Switch_A> (enable) %PAGP-5-PORTFROMSTP:Port 2/3 left bridge port 2/3
%PAGP-5-PORTFROMSTP:Port 2/4 left bridge port 2/4
%PAGP-5-PORTFROMSTP:Port 2/5 left bridge port 2/5
%PAGP-5-PORTFROMSTP:Port 2/6 left bridge port 2/6
%PAGP-5-PORTFROMSTP:Port 2/4 left bridge port 2/4
%PAGP-5-PORTFROMSTP:Port 2/5 left bridge port 2/5
%PAGP-5-PORTFROMSTP:Port 2/6 left bridge port 2/6
%PAGP-5-PORTFROMSTP:Port 2/3 left bridge port 2/3
%PAGP-5-PORTTOSTP:Port 2/3 joined bridge port 2/3-6
%PAGP-5-PORTTOSTP:Port 2/4 joined bridge port 2/3-6
%PAGP-5-PORTTOSTP:Port 2/5 joined bridge port 2/3-6
%PAGP-5-PORTTOSTP:Port 2/6 joined bridge port 2/3-6
Switch_B> (enable) %PAGP-5-PORTFROMSTP:Port 3/3 left bridge port 3/3
%PAGP-5-PORTFROMSTP:Port 3/4 left bridge port 3/4
%PAGP-5-PORTFROMSTP:Port 3/5 left bridge port 3/5
%PAGP-5-PORTFROMSTP:Port 3/6 left bridge port 3/6
%PAGP-5-PORTFROMSTP:Port 3/4 left bridge port 3/4
%PAGP-5-PORTFROMSTP:Port 3/5 left bridge port 3/5
%PAGP-5-PORTFROMSTP:Port 3/6 left bridge port 3/6
%PAGP-5-PORTFROMSTP:Port 3/3 left bridge port 3/3
%PAGP-5-PORTTOSTP:Port 3/3 joined bridge port 3/3-6
%PAGP-5-PORTTOSTP:Port 3/4 joined bridge port 3/3-6
%PAGP-5-PORTTOSTP:Port 3/5 joined bridge port 3/3-6
%PAGP-5-PORTTOSTP:Port 3/6 joined bridge port 3/3-6
Step 4 After the EtherChannel bundle is negotiated, use the show port channel command to verify the configuration.
Switch_A> (enable) show port channel
Port  Status     Channel   Channel     Neighbor                  Neighbor
                 mode      status      device                    port
----- ---------- --------- ----------- ------------------------- ---------- 
 2/3  connected  desirable channel     WS-C4003    JAB023806(Sw  2/3       
 2/4  connected  desirable channel     WS-C4003    JAB023806(Sw  2/4       
 2/5  connected  desirable channel     WS-C4003    JAB023806(Sw  2/5       
 2/6  connected  desirable channel     WS-C4003    JAB023806(Sw  2/6       
----- ---------- --------- ----------- ------------------------- ---------- 
Switch_A> (enable)
Switch_B> (enable) show port channel
Port  Status     Channel   Channel     Neighbor                  Neighbor
                 mode      status      device                    port
----- ---------- --------- ----------- ------------------------- ---------- 
 3/3  connected  auto      channel     WS-C4003    JAB023806(Sw  2/3       
 3/4  connected  auto      channel     WS-C4003    JAB023806(Sw  2/4       
 3/5  connected  auto      channel     WS-C4003    JAB023806(Sw  2/5       
 3/6  connected  auto      channel     WS-C4003    JAB023806(Sw  2/6       
----- ---------- --------- ----------- ------------------------- ---------- 
Switch_B> (enable)
Step 5 Configure one of the ports in the EtherChannel bundle to negotiate an IEEE 802.1Q trunk. The configuration is applied to all of the ports in the bundle. This example assumes that the neighboring ports on Switch B are configured to use dot1q or negotiate encapsulation and are in auto trunk mode. The system logging messages provide information about the formation of the 802.1Q trunk.
Switch_A> (enable) set trunk 2/3 desirable dot1q
Port(s) 2/3-6 trunk mode set to desirable.
Port(s) 2/3-6 trunk type set to dot1q.
Switch_A> (enable) %DTP-5-TRUNKPORTON:Port 2/3 has become dot1q trunk
%DTP-5-TRUNKPORTON:Port 2/4 has become dot1q trunk
%PAGP-5-PORTFROMSTP:Port 2/3 left bridge port 2/3-6
%DTP-5-TRUNKPORTON:Port 2/5 has become dot1q trunk
%PAGP-5-PORTFROMSTP:Port 2/4 left bridge port 2/3-6
%PAGP-5-PORTFROMSTP:Port 2/5 left bridge port 2/3-6
%DTP-5-TRUNKPORTON:Port 2/6 has become dot1q trunk
%P