1.2.a Evaluate proposed changes to a network



1.2.a [v] Migrate spanning tree protocol

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of layer-2 loops, or the need for manual enabling/ disabling of these backup links. Spanning Tree Protocol (STP) was originally, standardized as IEEE 802.1D most recently in 802.1d-1998, but deprecated as of 802.1d-2004 in favor of Rapid Spanning Tree Protocol (RSTP). RSTP creates a spanning tree within a network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes. While STP can take up to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 x Hello times (default hello interval is 2 seconds) or even within a few milliseconds of a physical link failure.

In 2001 , the IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w. Cisco’s proprietary versions of Spanning Tree Protocol, Per-VLAN Spanning Tree (PVST) and Per-VLAN Spanning Tree Plus (PVST +), create a separate spanning tree for each VLAN.

Rapid Per-VLAN Spanning Tree (RPVST) creates a spanning tree for each VLAN, just like PVST/ PVST +. Multiple Spanning Tree Protocol (MSTP) is similar to Cisco’s Multiple Instances Spanning Tree Protocol (MISTP), and is an evolution of the Spanning Tree Protocol and the Rapid Spanning Tree Protocol.

PVST + to MST Migration:

It is difficult to convert all the switches in the enterprise network to MST at the same time. Because of the backward compatibility, you can convert it step by step. It is recommended to implement the changes in the scheduled maintenance window because the spanning tree reconfiguration can disrupt the traffic flow.

When you enable MST, it also enables RSTP. The spanning tree uplinkfast and backbonefast features are PVST + features, and it is disabled when you enable MST because those features are built within RSTP, and MST relies on RSTP. When you migrate from PVST to RPVST, port status cycles through block and learning before moving to forwarding.

STP to RSTP (802.1w) or MSTP (802.1s)

The IEEE has pretty much incorporated most of the Cisco’s RSTP and MISTP concepts into two standards, namely 802.1w (RSTP) and 802.1s (MST).

Configuration Steps:

● Identify point-to-point and edge ports, ensuring all switch-to-switch links, on which a rapid transition is desired, are full-duplex.

● Figure out how many instances are needed in the switched network (an instance translates to a logical topology)

● Decide what VLANs to map onto those instances, and carefully select a root and a back-up root for each instance.

● Choose a configuration name and a revision number that will be common to all switches in the network.

● Migrate the core first. Change the STP type to MST, and work your way down to the access switches. MST can interact with legacy bridges running PVST + on a per-port basis, so it is not a problem to mix both types of bridges as long as interactions are clearly understood.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1307-1312).  . Kindle Edition.