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The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
Information About VLAN Trunks
A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device such as a router or a switch. Ethernet trunks carry the traffic of multiple VLANs over a single link, and you can extend the VLANs across an entire network.
The following trunking encapsulations are available on all Ethernet interfaces:
Ethernet trunk interfaces support different trunking modes. You can set an interface as trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate trunking, the interfaces must be in the same VTP domain.
Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point Protocol (PPP). However, some internetworking devices might forward DTP frames improperly, which could cause misconfigurations.
Mode |
Function |
---|---|
switchport mode access |
Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface regardless of whether or not the neighboring interface is a trunk interface. |
switchport mode dynamic auto |
Makes the interface able to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. The default switchport mode for all Ethernet interfaces is dynamic auto. |
switchport mode dynamic desirable |
Makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. |
switchport mode trunk |
Puts the interface into permanent trunking mode and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface. |
switchport nonegotiate |
Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link. |
By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk.
To reduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual VLAN trunk port by removing VLAN 1 from the allowed list. When you remove VLAN 1 from a trunk port, the interface continues to send and receive management traffic, for example, Cisco Discovery Protocol (CDP), Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), DTP, and VTP in VLAN 1.
If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If the access VLAN is set to 1, the port will be added to VLAN 1, regardless of the switchport trunk allowed setting. The same is true for any VLAN that has been disabled on the port.
A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not become a member of the new VLAN.
Load sharing divides the bandwidth supplied by parallel trunks connecting switches. To avoid loops, STP normally blocks all but one parallel link between switches. Using load sharing, you divide the traffic between the links according to which VLAN the traffic belongs.
You configure load sharing on trunk ports by using STP port priorities or STP path costs. For load sharing using STP port priorities, both load-sharing links must be connected to the same switch. For load sharing using STP path costs, each load-sharing link can be connected to the same switch or to two different switches.
When two ports on the same switch form a loop, the switch uses the STP port priority to decide which port is enabled and which port is in a blocking state. You can set the priorities on a parallel trunk port so that the port carries all the traffic for a given VLAN. The trunk port with the higher priority (lower values) for a VLAN is forwarding traffic for that VLAN. The trunk port with the lower priority (higher values) for the same VLAN remains in a blocking state for that VLAN. One trunk port sends or receives all traffic for the VLAN.
This figure shows two trunks connecting supported switches.
Trunk 1 carries traffic for VLANs 8 through 10, and Trunk 2 carries traffic for VLANs 3 through 6. If the active trunk fails, the trunk with the lower priority takes over and carries the traffic for all of the VLANs. No duplication of traffic occurs over any trunk port.
You can configure parallel trunks to share VLAN traffic by setting different path costs on a trunk and associating the path costs with different sets of VLANs, blocking different ports for different VLANs. The VLANs keep the traffic separate and maintain redundancy in the event of a lost link.
Trunk ports 1 and 2 are configured as 100BASE-T ports. These VLAN path costs are assigned:
Trunking interacts with other features in these ways:
To avoid trunking misconfigurations, configure interfaces connected to devices that do not support DTP to not forward DTP frames, that is, to turn off DTP.
Configuring an Ethernet Interface as a Trunk Port
Because trunk ports send and receive VTP advertisements, to use VTP you must ensure that at least one trunk port is configured on the switch and that this trunk port is connected to the trunk port of a second switch. Otherwise, the switch cannot receive any VTP advertisements.
By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode dynamic auto. If the neighboring interface supports trunking and is configured to allow trunking, the link is a Layer 2 trunk or, if the interface is in Layer 3 mode, it becomes a Layer 2 trunk when you enter the switchport interface configuration command.
2. interface interface-id
3. switchport mode {dynamic {auto | desirable} | trunk}
4. switchport access vlan vlan-id
5. switchport trunk native vlan vlan-id
7. show interfaces interface-id switchport
8. show interfaces interface-id trunk
9. copy running-config startup-config
VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, and it has previously been a requirement that VLAN 1 always be enabled on every trunk link. You can use the VLAN 1 minimization feature to disable VLAN 1 on any individual VLAN trunk link so that no user traffic (including spanning-tree advertisements) is sent or received on VLAN 1.
2. interface interface-id
3. switchport mode trunk
4. switchport trunk allowed vlan { word | add | all | except | none | remove} vlan-list
6. show interfaces interface-id switchport
7. copy running-config startup-config
The pruning-eligible list applies only to trunk ports. Each trunk port has its own eligibility list. VTP pruning must be enabled for this procedure to take effect.
2. interface interface-id
3. switchport trunk pruning vlan {add | except | none | remove} vlan-list [,vlan [,vlan [,,,]]
5. show interfaces interface-id switchport
6. copy running-config startup-config
A trunk port configured with IEEE 802.1Q tagging can receive both tagged and untagged traffic. By default, the switch forwards untagged traffic in the native VLAN configured for the port. The native VLAN is VLAN 1 by default.
The native VLAN can be assigned any VLAN ID.
If a packet has a VLAN ID that is the same as the outgoing port native VLAN ID, the packet is sent untagged; otherwise, the switch sends the packet with a tag.
2. interface interface-id
3. switchport trunk native vlan vlan-id
4. end
5. show interfaces interface-id switchport
6. copy running-config startup-config
Configuring Trunk Ports for Load Sharing
If your switch is a member of a switch stack, you must use the spanning-tree [vlan vlan-id] cost cost interface configuration command instead of the spanning-tree [vlan vlan-id] port-priority priority interface configuration command to select an interface to put in the forwarding state. Assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last.
These steps describe how to configure a network with load sharing using STP port priorities.
1. configure terminal
2. vtp domain domain-name
3. vtp mode server
5. show vtp status
6. show vlan
7. configure terminal
8. interface interface-id
9. switchport mode trunk
10. end
11. show interfaces interface-id switchport
12. Repeat the above steps on Switch A for a second port in the switch or switch stack.
13. Repeat the above steps on Switch B to configure the trunk ports that connect to the trunk ports configured on Switch A.
14. show vlan
15. configure terminal
16. interface interface-id
17. spanning-tree vlan vlan-range port-priority priority-value
18. exit
19. interface interface-id
20. spanning-tree vlan vlan-range port-priority priority-value
21. end
22. show running-config
23. copy running-config startup-config
These steps describe how to configure a network with load sharing using STP path costs.
1. configure terminal
2. interface interface-id
3. switchport mode trunk
4. exit
5. Repeat Steps 2 through 4 on a second interface in Switch A or in Switch A stack.
7. show running-config
8. show vlan
9. configure terminal
10. interface interface-id
11. spanning-tree vlan vlan-range cost cost-value
12. end
13. Repeat Steps 9 through 13 on the other configured trunk interface on Switch A, and set the spanning-tree path cost to 30 for VLANs 8, 9, and 10.
14. exit
15. show running-config
16. copy running-config startup-config
After configuring VLAN trunks, you can configure the following:
Description | Link |
---|---|
To help you research and resolve system error messages in this release, use the Error Message Decoder tool. |
https://www.cisco.com/cgi-bin/Support/Errordecoder/index.cgi |
Standard/RFC | Title |
---|---|
RFC 1573 |
Evolution of the Interfaces Group of MIB-II |
RFC 1757 |
Remote Network Monitoring Management |
RFC 2021 |
SNMPv2 Management Information Base for the Transmission Control Protocol using SMIv2 |
MIB | MIBs Link |
---|---|
All supported MIBs for this release. |
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
Description | Link |
---|---|
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Release |
Modification |
---|---|
Cisco IOS XE 3.2SE |
This feature was introduced. |