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VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority
Document ID: 10555
Introduction Before You Begin Conventions Prerequisites Components Used Introduction to VLAN Load Balancing Between Trunks How STP Decides Which Port to Block Configuring VLAN Load Balancing on Catalyst Switches Running CatOS Details on the portvlanpri Command Configuring VLAN Load Balancing onCatalyst Switches Running Integrated IOS Details on the port−priority and vlan port−priority Commands Conclusion Related Information

Introduction
This document provides the theory behind VLAN load balancing between trunks, and also provides configuration examples for switches running CatOS and Integrated IOS.

Before You Begin
Conventions
For more information on document conventions, seethe Cisco Technical Tips Conventions.

Prerequisites
There are no specific prerequisites for this document.

Components Used
This document is not restricted to specific software and hardware versions.

Introduction to VLAN Load Balancing Between Trunks
The commands used in this document are available on the Catalyst 4000, 5000, and 6000 families running CatOS and Integreated IOS. Thetheoretical sections of this document are related to spanning−tree protocol (STP) and are platform independent. The configuration shown in figure 1 below, in which two switches are directly connected through more than one trunk, is very common for redundancy purposes. If one of the two links fail, the second soon becomes available to transmit traffic. When both links are up, the spanning−tree algorithm(STA) disables one of them in order to avoid a bridging loop between the two switches.

In the figure 1 configuration above, with two FastEthernet trunks linking Catalyst R and Catalyst D, the STP elects the same blocking port for all the VLANs configured. In this case, Catalyst R is the Root bridge and Catalyst D decides to block port D2 for both VLAN 1 and VLAN 2. The major issue with thisdesign is that link R2−D2 is simply sacrificed and there is only 100 Mb/s available between the two switches. To take advantage of both links, you can change the configuration and allow VLAN 1 only on link R1−D1, and VLAN 2 only on link R2−D2.

The resulting network, shown in figure 2, lost its redundancy. Nowyou have both links forwarding at the same time, and you are practically able to use 200Mb/s between the two switches. However, if one link fails, you completely loose connectivity for one VLAN. The ideal solution is the one described in figure 3:

In figure 3, you keep the trunks between the two switches, but port D1 is blocking VLAN 1 and forwarding VLAN 2; port D2 is blocking VLAN 2 and forwarding VLAN 1. This design keeps the best features of figure 1 and figure 2: • Bothlinks are forwarding, providing 200 Mb/s aggregate connectivity between the two switches. • If one link fails, the remaining one unblocks the corresponding VLAN and maintains connectivity for both VLANs between the switches. This document explains how to achieve such a configuration, after a short explanation of the STP operations.

How STP Decides Which Port to Block
The detailed description ofhow the STA works is beyond the scope of this document. However, it briefly summarizes how the algorithm decides, in this case, whether a port blocks or forwards. It focuses on the most simple configuration possible with only one VLAN; Catalyst R is the Root bridge in this VLAN and Catalyst D has multiple redundant connections to Catalyst R. Catalyst D blocks all of its ports to Catalyst R but itsRoot port. How does Catalyst D select its Root port? Bridges running the STA exchange Bridge Protocol Data Units (BPDUs) across the links, and these BPDUs can be strictly classified depending on their content. A BPDU is superior than another if it has:

1. A lower Root Bridge ID. 2. A lower path cost to the Root. 3. A lower Sending Bridge ID. 4. A lower Sending Port ID. These four parameters...
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