MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO
INTRODUCTION
This paper describes the Multiple Spanning Tree Protocol (MSTP) support for Dell PowerConnect 62xx devices, which include the PC6224, PC6248, PC6224P, PC6248P, PC6224F, and M6220 switches. This document also explains how to configure PowerConnect 62xx switches to interoperate and connect with Cisco IOS and CatOS based switches when using the MSTP industry standards. MSTP is defined in the IEEE 802.1s specification.
This document addresses the following topics:
•MSTP and its support in Dell PowerConnect 62xx devices
•Network operation of MSTP with configuration help for both Dell PowerConnect and Cisco switches (Cisco Cat 3750 is taken as reference)
IEEE 802.1S MULTIPLE SPANNING TREE PROTOCOL OVERVIEW
IEEE 802.1s MSTP supports multiple instances of Spanning Tree Protocol (STP) to efficiently channel VLAN traffic over different interfaces. Each spanning tree instance behaves in the manner specified in IEEE 802.1w (Rapid Spanning Tree) with slight modifications in the operation but not the end result.
The difference between RSTP and traditional STP (IEEE 802.1d) is that RSTP can configure and recognize full duplex connectivity and ports that are connected to end stations. This allows RSTP to perform a rapid transition of the port to the “Forwarding” state and to suppress Topology Change Notifications. These features are represented by the parameters pointtopoint and edgeport.
MSTP is compatible with both RSTP and STP and behaves appropriately with STP and RSTP bridges. You can configure an MSTP bridge to behave entirely as an RSTP bridge or an STP bridge. This means that an IEEE 802.1s bridge also supports IEEE 802.1w and IEEE 802.1d.
DELL POWERCONNECT 62XX MSTP FUNCTIONAL DESCRIPTION
The MSTP algorithm and protocol provides simple and full connectivity for frames assigned to any given VLAN throughout a bridged LAN comprising arbitrarily interconnected networking devices, each operating MSTP, STP or RSTP. MSTP allows frames assigned to different VLANs to follow separate paths, each based on an independent Multiple Spanning Tree Instance (MSTI), within Multiple Spanning Tree (MST) Regions composed of LANs or MSTP Bridges. These regions and the other bridges and LANs are connected into a single Common Spanning Tree (CST). [IEEE DRAFT P802.1s/D13]
MSTP connects all bridges and LANs with a single Common and Internal Spanning Tree (CIST). The CIST supports the automatic determination of each MST region, choosing its maximum possible extent. The connectivity calculated for the CIST provides the CST for interconnecting these regions, and an Internal Spanning Tree (IST) within each region. MSTP ensures that frames with a given VLAN ID (VID) are assigned to one and only one of the MSTIs or the IST within the region, that that assignment is consistent among all the networking devices in the region, and that the stable connectivity of each MSTI and IST at the boundary of the Region matches that of the CST. The stable active topology of the Bridged LAN with respect to frames consistently classified as belonging to any given VLAN thus simply and fully connects all LANs and networking devices throughout the network, though frames belonging to different VLANs can take different paths within any region. [IEEE DRAFT P802.1s/D13]
All bridges, whether they use STP, RSTP or MSTP, send information in Configuration Messages via BPDUs to assign Port Roles that determine each port’s participation in a fully and simply connected active topology based on one or more spanning trees. The information communicated is known as the spanning tree priority vector. The BPDU structure for each protocol is different. An MSTP bridge transmits the appropriate BPDU depending on the received type of BPDU from a particular port.
An MST region has one or more MSTP bridges with the same MST Configuration Identifier. MSTI regions use the same MST instance, and all bridges in the region must be able to send and receive MSTP BPDUs.
APRIL 2008 | 3 |