Configuring VLAN Trunks

Allowed VLANs on a Trunk

Allowed VLANs on a Trunk

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 on Trunk Ports

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.

Network Load Sharing Using STP Priorities

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.

•VLANs 8 through 10 are assigned a port priority of 16 on Trunk 1.

•VLANs 3 through 6 retain the default port priority of 128 on Trunk 1.

•VLANs 3 through 6 are assigned a port priority of 16 on Trunk 2.

•VLANs 8 through 10 retain the default port priority of 128 on Trunk 2.

 

Catalyst 2960-XR Switch VLAN Configuration Guide, Cisco IOS Release 15.0(2)EX1

64

OL-29440-01

Page 77 Page 79
Page 78
Image 78
Cisco Systems WSC2960XR48FPSI manual Allowed VLANs on a Trunk, Load Sharing on Trunk Ports
Page 77 Page 79

WSC2960XR48FPSI specifications

The Cisco WSC2960XR48FPSI is an advanced, high-performance switch that plays a vital role in today's enterprise networking environments. Designed for reliability and efficiency, it serves as a foundational component for organizations concentrating on network agility and scalability.

One of the standout features of the WSC2960XR48FPSI is its ability to support 48 Gigabit Ethernet ports, facilitating high-speed connectivity across multiple devices. This capacity makes it an ideal choice for enterprises that require robust network infrastructure to handle large volumes of traffic effortlessly. Additionally, it includes two 10-Gigabyte SFP+ uplink ports, providing enhanced bandwidth for backbone connections, enabling seamless integration with data center environments.

The switch supports Cisco's StackPower technology, which allows multiple switches to share power resources. This capability not only provides redundancy but also ensures that organizational networks can be efficiently managed, reducing operational costs by utilizing power resources wisely.

In terms of reliability, the WSC2960XR48FPSI offers a fanless design, making it an optimal choice for deployments in noise-sensitive environments such as classrooms or offices. Its features include support for Cisco's FlexStack-Plus technology, which allows for easy stacking of up to eight units. This results in simplified management and increased bandwidth as switches in the stack operate as a single entity.

The switch is powered by Cisco IOS Software, providing robust features, including advanced security protocols, quality of service (QoS) capabilities, and comprehensive network management tools. With support for multicast routing and enhanced security, organizations can safeguard their data while ensuring smooth and reliable communication across applications.

Moreover, the Cisco WSC2960XR48FPSI is designed to be energy-efficient, compliant with IEEE 802.3az Energy Efficient Ethernet standards, which aids in reducing power consumption without sacrificing performance. This commitment to sustainability makes it a favorable option for organizations striving for greener operations.

To summarize, the Cisco WSC2960XR48FPSI is an exemplary model of reliability, scalability, and performance. Its sophisticated features, including high port density, advanced security measures, and energy efficiency, make it a cornerstone for modern enterprise networks, enabling organizations to adapt effectively in an ever-evolving digital landscape.
Top Page Image Contents