Catalyst 6500 Series Software Configuration Guide, 5.5
Configuring Ethernet VLAN Trunks
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Table Of Contents

Configuring Ethernet VLAN Trunks

Understanding How VLAN Trunks Work

Trunking Overview

Trunking Modes and Encapsulation Types

802.1Q Trunk Restrictions

Default Trunk Configuration

Configuring a Trunk Link

Configuring an ISL Trunk

Configuring an 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 EtherChannel Link Example

802.1Q Trunk Over EtherChannel Link Example

Load-Sharing VLAN Traffic Over Parallel Trunks Example

Disabling VLAN 1 on Trunks

Disabling VLAN 1 on a Trunk Link


Configuring Ethernet VLAN Trunks

This chapter describes how to configure Ethernet VLAN trunks on the Catalyst 6000 family switches.

Note For complete information on configuring VLANs, see "Configuring VLANs."

Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 6000 Family Command Reference publication.

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 6000 family switches:

•Trunking Overview

•Trunking Modes and Encapsulation Types

•802.1Q Trunk Restrictions

Trunking Overview

A trunk is a point-to-point link between one or more Ethernet 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 all Ethernet ports:

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

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

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

Ethernet trunk ports support five different trunking modes (see Table 10-1). In addition, 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, the ports must be in the same VLAN Trunk Protocol (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). DTP supports autonegotiation of both ISL and 802.1Q trunks.

Trunking Modes and Encapsulation Types

Table 10-1 lists the trunking modes used with the set trunk command and describes how they function on Fast Ethernet and Gigabit Ethernet ports.

Table 10-1 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

Makes the port willing 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 all 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 10-2 lists the encapsulation types used with the set trunk command and describes how they function on Ethernet ports. You can use the show port capabilities command to determine which encapsulation types a particular port supports.

Table 10-2 Ethernet Trunk Encapsulation Types 

Encapsulation
Function
isl

Specifies ISL encapsulation on the trunk link.

dot1q

Specifies 802.1Q encapsulation on the trunk link.

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.


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 10-3 shows the result of the possible trunking configurations.

Table 10-3 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

1 Using 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.

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).

When you connect a Cisco switch to a non-Cisco switch the CST is always on VLAN 1. The Cisco switch sends an untagged IEEE BDPU (01-80-C2-00-00-00) on VLAN 1 for the CST and on the native VLAN the Cisco switch sends an untagged Cisco BPDU (01-00-0C-CC-CC-CC) which the non-Cisco switch forwards but does not act on (the IEEE BPDU is not forwarded on the native VLAN).

•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 10-4 shows the default Ethernet trunk configuration.

Table 10-4 Default Ethernet Trunk Configuration

Feature
Default Configuration

Trunk mode

auto

Trunk encapsulation

negotiate

Allowed VLAN range

VLANs 1-1005


Configuring a Trunk Link

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

•Configuring an ISL Trunk

•Configuring an 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

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 neighboring 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 neighboring 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 802.1Q Trunk

To configure an 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 802.1Q trunk and how to verify the trunk configuration: 

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)

Configuring an ISL/802.1Q Negotiating Trunk Port

To configure a trunk port to negotiate the trunk encapsulation type (either ISL or 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 neighboring 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.

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 EtherChannel Link Example

•802.1Q Trunk Over EtherChannel Link Example

•Load-Sharing VLAN Traffic Over Parallel Trunks Example

Note For examples of configuring trunk links between switches and routers, refer to the Catalyst 6000 Family Multilayer Switch Feature Card and Policy Feature Card Configuration Guide.

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 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 EtherChannel Link Example

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

Figure 10-1 shows two switches connected through two 100BaseTX Fast Ethernet ports.

Figure 10-1 ISL Trunk Over Fast EtherChannel Link

This example shows how to configure the switches to form a two-port 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 EtherChannel port bundles. For complete information on configuring EtherChannel, see "Configuring 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 an 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)

802.1Q Trunk Over EtherChannel Link Example

This example configuration shows how to configure an 802.1Q trunk over an EtherChannel link between two switches.

Figure 10-2 shows two switches connected through four 1000BaseSX Gigabit Ethernet ports.

Figure 10-2 802.1Q Trunk Over EtherChannel Link

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

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

%PAGP-5-PORTFROMSTP:Port 2/6 left bridge port 2/3-6

%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) %DTP-5-TRUNKPORTON:Port 3/3 has become dot1q trunk

%DTP-5-TRUNKPORTON:Port 3/4 has become dot1q trunk

%PAGP-5-PORTFROMSTP:Port 3/3 left bridge port 3/3-6

%PAGP-5-PORTFROMSTP:Port 3/4 left bridge port 3/3-6

%PAGP-5-PORTFROMSTP:Port 3/5 left bridge port 3/3-6

%PAGP-5-PORTFROMSTP:Port 3/6 left bridge port 3/3-6

%DTP-5-TRUNKPORTON:Port 3/5 has become dot1q trunk

%DTP-5-TRUNKPORTON:Port 3/6 has become dot1q trunk

%PAGP-5-PORTFROMSTP:Port 3/5 left bridge port 3/3-6

%PAGP-5-PORTFROMSTP:Port 3/6 left bridge port 3/3-6

%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 6 After the 802.1Q trunk link is negotiated, use the show trunk command to verify the configuration. 

Switch_A> (enable) show trunk

Port      Mode         Encapsulation  Status        Native vlan

--------  -----------  -------------  ------------  -----------

 2/3      desirable    dot1q          trunking      1

 2/4      desirable    dot1q          trunking      1

 2/5      desirable    dot1q          trunking      1

 2/6      desirable    dot1q          trunking      1


Port      Vlans allowed on trunk

--------  ---------------------------------------------------------------------

 2/3      1-1005

 2/4      1-1005

 2/5      1-1005

 2/6      1-1005


Port      Vlans allowed and active in management domain 

--------  ---------------------------------------------------------------------

 2/3      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 2/4      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 2/5      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 2/6      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

--------  ---------------------------------------------------------------------

 2/3      

 2/4      

 2/5      

 2/6      

Switch_A> (enable)


Switch_B> (enable) show trunk

Port      Mode         Encapsulation  Status        Native vlan

--------  -----------  -------------  ------------  -----------

 3/3      auto         dot1q          trunking      1

 3/4      auto         dot1q          trunking      1

 3/5      auto         dot1q          trunking      1

 3/6      auto         dot1q          trunking      1


Port      Vlans allowed on trunk

--------  ---------------------------------------------------------------------

 3/3      1-1005

 3/4      1-1005

 3/5      1-1005

 3/6      1-1005


Port      Vlans allowed and active in management domain 

--------  ---------------------------------------------------------------------

 3/3      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 3/4      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 3/5      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 3/6      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

--------  ---------------------------------------------------------------------

 3/3      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 3/4      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 3/5      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

 3/6      1-5,10,20,50,152,200,300,400,500,521-524,570,850,917,999

Switch_B> (enable)

Load-Sharing VLAN Traffic Over Parallel Trunks Example

Using spanning-tree port-VLAN priorities, you can load-share VLAN traffic over parallel trunk ports so that traffic from some VLANs travels over one trunk, while traffic from other VLANs travels over the other trunk. This configuration allows traffic to be carried over both trunks simultaneously (instead of keeping one trunk in blocking mode), which reduces the total traffic carried over each trunk while still maintaining a fault-tolerant configuration.

Figure 10-3