Chapter 1: Data Center Layer 2 Interconnect

Excerpt from nterconnecting Data Centers Using VPLS (Ensure Business Continuance on Virtualized Networks by Implementing Layer 2 Connectivity Across Layer 3)

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When a BPDU propagates from the root bridge toward the leaves of the tree, the age field increments each time the BPDU goes through a bridge. Eventually, the bridge discards the BPDU when the age field goes beyond maximum age. Therefore, convergence of the spanning tree is affected if the root bridge is too far away from some bridges in the network.

An aggressive value for the max-age parameter and the forward delay can lead to an unstable STP topology. In such cases, the loss of some BPDUs can cause a loop to appear. Take special care if you plan to change STP timers from the default value to achieve faster STP convergence.

Unlike legacy STP, Rapid STP (RSTP) converges faster because it does not depend on the timers to make a rapid transition. However, STP does not provide the required robustness for large-scale Layer 2 deployments:

  • Network stability is compromised as a result of slow response to network failures (slow convergence). Even new spanning-tree developments such as RSTP and Multiple Spanning Tree (MST) assume good-quality physical connections such as dark fiber or WDM connections. These STP protocols are not built to accommodate frequent link-flapping conditions, high error rates, unidirectional failures, or nonreport of loss of signal. These typical and frequent behaviors of long- and medium-distance links could lead to STP slow convergence or even instability.

  • The primary reason for multisite data centers is disaster recovery. However, because data centers typically require Layer 2 connectivity, failure in one data center can affect other data centers, which could lead to a blackout of all data centers at the same time.

  • A broadcast storm propagates to every data center, which, if uncontrolled, could result in network-wide outage.

  • STP blocks links, which prevents load balancing of traffic across redundant paths in the core network.

Note - For understanding and tuning STP timers and the rules to tune them when absolutely necessary, refer to the Cisco document, “Understanding and Tuning Spanning Tree Protocol Timers,” available at


This chapter provided an overview of HA clusters, legacy deployment models for interconnecting data centers, and problems related to extending Layer 2 networks. The solutions that this book presents address these issues in more detail and provide guidance for designing and deploying DCI.

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Copyright © 2009 IDG Communications, Inc.

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