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This document describes the difference of IEEE 802.1Q (dot1q) trunking between Cisco Catalyst switches that run Cisco IOS® Software.
Ensure that you meet these requirements before you attempt this configuration:
Knowledge of IEEE 802.1Q trunking
Knowledge of the configuration of Catalyst 3560 and Catalyst 6500/6000 series switches with use of command-line interface (CLI)
The information in this document is based on these software and hardware versions:
Catalyst 3560 switch that runs Cisco IOS Software Release 12.2(25)SEA
Catalyst 6509 switch that runs Cisco IOS Software Release 12.1(26)E1
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
This document provides a sample configuration of IEEE 802.1Q (dot1q) trunking between a Cisco Catalyst 3550/3560/3750 switch that runs Cisco IOS® Software and a Catalyst 6500/6000 series switch or Catalyst 4500/4000 series switch that runs Cisco IOS Software. Trunking is a way to carry traffic from several VLANs over a point-to-point link between the two devices.
Two ways in which you can implement Ethernet trunking are:
Inter-Switch Link Protocol (ISL)—A Cisco proprietary protocol
802.1Q—An IEEE standard
The Catalyst 3560 configuration in this document is also applicable to Catalyst 3550/3750 switches that run Cisco IOS Software. The Catalyst 6500/6000 configuration in this document is also applicable to Catalyst 4500/4000 series switches that run Cisco IOS Software.
Note: Refer to this next document to learn about the trunking methods that are supported by various Catalyst switches:
System Requirements to Implement Trunkingon Catalyst switches
Note: This document includes only the configuration files from the switches and the output from the related sample show
commands. For details on how to configure an 802.1Q trunk between Catalyst switches, refer to these next documents:
Configuring VLAN Trunks section of Configuring VLANs—Catalyst 3560 series switches
IEEE 802.1Q uses an internal tagging mechanism. The trunking device inserts a 4-byte tag in order to identify the VLAN to which a frame belongs and then recomputes the frame check sequence (FCS). For more information, refer to these documents:
Basic Characteristics of 802.1Q Trunking section of Trunking Between Catalyst 4500/4000, 5500/5000, and 6500/6000 Series Switches Using 802.1Q Encapsulation with Cisco CatOS System Software
Note: Next are important notes to remember for this configuration:
Any Ethernet interface on a Catalyst 3550/3560/3750 series switch can support 802.1Q and ISL encapsulation. The Ethernet interface on a Catalyst 3550 switch is a Layer 2 (L2) port, by default.
Any Ethernet port on a Catalyst 6500/6000 series switch can support 802.1Q and ISL encapsulation.
By default, the Catalyst 4500 series switch that runs Cisco IOS Software supports both ISL and 802.1Q trunking modes. The support is on all interfaces except blocking Gigabit ports on the WS-X4418-GB and WS-X4412-2GB-T modules. These ports do not support ISL and only support 802.1Q trunking. Ports 3 through 18 are blocking Gigabit ports on the WS-X4418-GB module. Ports 1 through 12 are blocking Gigabit ports on the WS-X4412-2GB-T module.
Note: A port is a blocking port if its connection to the backplane is oversubscribed.
The main difference between the Catalyst 6500/6000 and the Catalyst 4500 platforms is the default interface configuration. The Catalyst 6500/6000 switch that runs Cisco IOS Software has interfaces in shutdown mode that are Layer 3 (L3) routed ports by default. The Catalyst 4500/4000 switch that runs Cisco IOS Software has all the interfaces enabled. The interfaces are L2 switch ports by default.
When 802.1Q encapsulation is used on a trunk interface on the Catalyst 3750 Switches, runts can be seen on show interface
output because valid 802.1Q encapsulated packets which are 61-64 bytes that include the q-tag are counted by the Catalyst 3750 Switch as undersized frames, even though these packets are forwarded correctly. For more details, refer to Cisco bug ID CSCec14238 .
Note: Only registered Cisco users have access to internal Cisco tools and information.
In this section, you are presented with the information to configure the features described in this document.
Note: Use the Command Lookup Tool to obtain more information on the commands used in this section.
Note: Only registered Cisco users have access to internal Cisco tools and information.
This document uses this network setup:
Note: The Gigabit Ethernet interface on the Catalyst 3560 is a 10/100/1000 Mbps negotiated Ethernet interface. Therefore, the Gigabit port on the Catalyst 3560 is connected to a Fast Ethernet (100 Mbps) port on the Catalyst 6500 in this network diagram.
This document uses these configurations:
Catalyst 3560 Switch |
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!--- Notice: This example creates VLAN 1 and VLAN 2 |
Catalyst 6500 Switch |
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!--- Notice: This example creates VLAN 1 and VLAN 2 and sets |
Note: If you assign an interface to a VLAN that does not exist, the interface shuts down until you create the VLAN in the VLAN database. For details, refer to the Creating or Modifying an Ethernet VLAN section of Configuring VLANs.
Use this section to confirm that your configuration works properly.
The Output Interpreter Tool (OIT) supports certain show
commands. Use the OIT to view an analysis of show
command output.
Note: Only registered Cisco users have access to internal Cisco tools and information.
On Catalyst 3550/3560/3750/6500/4500 switches, use these commands:
show interfacesinterface_typemodule/porttrunk
show interfacesinterface_typemodule/portswitchport
show vlan
show vtp status
show
Command Outputshow interfaces interface_type module/por trunk —This command displays the trunk configuration of the interface along with the VLAN numbers for which traffic is able to be carried over the trunk.
3560# show interface gigabitethernet 0/1 trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1 4094 Port Vlans allowed and active in management domain Gi0/1 1-2 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 1-2
show interfaces interface_type module/port switchport —This command displays the switch port configuration of the interface.
In the display, check the Operational Mode and the Operational Trunking Encapsulation fields.
3560# show interface gigabitethernet 0/1 switchport Name: Gi0/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Capture Mode Disabled Capture VLANs Allowed: ALL Protected: false Unknown unicast blocked: disabled Unknown multicast blocked: disabled Appliance trust : none
show vlan—This command gives information about the VLANs and the ports that belong to a particular VLAN.
3560# show vlan VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Gi0/2, Gi0/3, Gi0/4, Gi0/5 2 VLAN0002 active Gi0/6, Gi0/7, Gi0/8, Gi0/9 Gi0/10, Gi0/11, Gi0/12 1002 fddi-default act/unsup 1003 token-ring-default act/unsup 1004 fddinet-default act/unsup 1005 trnet-default act/unsup !--- Output suppressed.
Note: The ports that are displayed in the output are only access ports. But the ports that are configured to be trunk and that are in the 'notconnected' status also show up in the show vlan output.
show vtp status—This command displays general information about the VTP management domain, status, and counters.
3560# show vtp status VTP Version : 2 Configuration Revision : 0 Maximum VLANs supported locally : 1005 Number of existing VLANs : 6 VTP Operating Mode : Transparent VTP Domain Name : VTP Pruning Mode : Disabled VTP V2 Mode : Disabled VTP Traps Generation : Disabled MD5 digest : 0x4A 0x55 0x17 0x84 0xDB 0x99 0x3F 0xD1 Configuration last modified by 10.1.1.1 at 0-0-00 00:00:00 3560# ping 10.1.1.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms 3560#
show interfacesinterface_type module/porttrunk—This command displays the trunk configuration of the interface along with the VLAN numbers for which traffic is able to be carried over the trunk.
Cat6500# show interfaces fastethernet 3/1 trunk Port Mode Encapsulation Status Native vlan Fa3/1 on 802.1q trunking 1 Port Vlans allowed on trunk Fa3/1 1 4094 Port Vlans allowed and active in management domain Fa3/1 1-2 Port Vlans in spanning tree forwarding state and not pruned Fa3/1 1-2
show interfacesinterface_typemodule/portswitchport —This command displays the switch port configuration of the interface. In the display, check the Operational Mode and the Operational Trunking Encapsulation fields.
cat6500# show interface fastethernet 3/1 switchport Name: Fa3/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Capture Mode Disabled Capture VLANs Allowed: ALL
show vlan—This command gives information about the VLANs and the ports that belong to a particular VLAN.
Cat6500# show vlan VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Fa3/2, Fa3/3, Fa3/4, Fa3/5 Fa3/6, Fa3/7, Fa3/8, Fa3/9 Fa3/10, Fa3/11, Fa3/12, Fa3/13 Fa3/14, Fa3/15, Fa3/16, Fa3/17 Fa3/18, Fa3/19, Fa3/20, Fa3/21 Fa3/22, Fa3/23, Fa3/24 2 VLAN0002 active Fa3/25, Fa3/26, Fa3/27, Fa3/28 Fa3/29, Fa3/30, Fa3/31, Fa3/32 Fa3/33, Fa3/34, Fa3/35, Fa3/36 Fa3/37, Fa3/38, Fa3/39, Fa3/40 Fa3/41, Fa3/42, Fa3/43, Fa3/44 Fa3/45, Fa3/46, Fa3/47, Fa3/48 1002 fddi-default act/unsup 1003 token-ring-default act/unsup 1004 fddinet-default act/unsup 1005 trnet-default act/unsup !--- Output suppressed.
Note: The ports that display are only those ports that you have configured as Layer 2 nontrunk (access) ports. The ports that are configured to be trunk and that are in the 'notconnected' status also show up in the show vlan output. For details, refer to the Configuring LAN Interfaces for Layer 2 Switching section of Configuring LAN Ports for Layer 2 Switching.
show vtp status—This command displays general information about the VTP management domain, status, and counters.
Cat6500# show vtp status VTP Version : 2 Configuration Revision : 0 Maximum VLANs supported locally : 1005 Number of existing VLANs : 6 VTP Operating Mode : Transparent VTP Domain Name : VTP Pruning Mode : Disabled VTP V2 Mode : Disabled VTP Traps Generation : Disabled MD5 digest : 0xBF 0x86 0x94 0x45 0xFC 0xDF 0xB5 0x70 Configuration last modified by 10.1.1.2 at 0-0-00 00:00:00
ping
Cat6500# ping 10.1.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
There is currently no specific troubleshoot information available for this configuration. For common problems that relate to trunking and 802.1Q configuration, refer to the Common Errors section of the document Trunking Between Catalyst 4500/4000, 5500/5000, and 6500/6000 Series Switches Using 802.1Q Encapsulation with Cisco CatOS System Software.
Revision | Publish Date | Comments |
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2.0 |
19-Jan-2023 |
Update formatting. Fix CCW Alerts. Recertification. |
1.0 |
29-Nov-2001 |
Initial Release |