Dell 6200 User Manual

MSTP INTEROPERABILITY

™

OF THE DELL POWERCONNECT

™

6200 SERIES SWITCHES

WITH CISCO IOS OR CISCO CA TOS BASED SWITCHES

THIS WHITE PAPER IS FOR INFORMATIONAL PURPOSES ONLY, AND MAY CONTAIN TYPOGRAPHICAL ERRORS AND TECHNICAL INACCURACIES. THE CONTENT IS PROVIDED AS IS, WITHOUT EXPRESS OR IMPLIED WARRANTIES OF ANY KIND.

Dell and PowerConnect are trademarks of Dell Inc. Other trademarks and trade names may be used in this document to refer to either the entities claiming the marks and names or their products. Dell disclaims proprietary interest in the marks and names of others.

Information in this document is subject to change without notice.

CONTENTS

INTRODUCTION 3

IEEE 802.1S MULTIPLE SPANNING TREE PROTOCOL OVERVIEW 3

DELL POWERCONNECT 62XX MSTP FUNCTIONAL DESCRIPTION 3 ACTIVE TOPOLOGY ENFORCEMENT 5 CONTROL PACKET BEHAVIOR 6 MSTP CLI COMMANDS 6 OPERATION IN THE NETWORK 7 SAMPLE SETUP AND CONFIGURATIONS 9 CONFIGURING BRIDGE BrA (WEB INTERFACE) 10 CONFIGURING BRIDGE BrB 16 CONFIGURING BRIDGE BrC 16 CONFIGURING EDGE DEVICES 17

VIEWING THE MSTP STATUS 18

ADDITIONAL MSTP STATUS INFORMATION 19

ADDITIONAL INFORMATION AND SCALABILITY WITH MORE REGIONS

VOICE VLAN CONFIGURATION 4

CLI CONFIGURATION 4

WEB CONFIGURATION 8

FIGURES

FIGURE 1: VID TO FID ALLOCATION 4

FIGURE 2: EXAMPLE FID TO MSTI ALLOCATION 5

FIGURE 3: EXAMPLE RESULTANT VID TO MSTI ALLOCATION 5

FIGURE 4: SMALL BRIDGED NETWORK 7

FIGURE 5: SINGLE STP TOPOLOGY 7

FIGURE 7: MULTIPLE MSTP REGIONS 24

FIGURE 8: MULTIPLE MSTP REGION INTERACTIONS 25

MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES

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 b ut 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 “Forw ard ing” stat e a nd to su ppr ess Top o logy Cha nge Notifications. These features are repr esented by the para meter s p oint topo int 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.

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MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES

The MST Configuration Identifier consists of the following components:

• Configuration Identifier Format Selector – 1 byte value encoded as zero

• Configuration Name – 32 byte string

• Configuration Revision Level – 2 byte value

• Configuration Digest – 16 byte signature of type HMAC-MD5 created from the MST

Configuration Table (a VID to MSTID mapping)

As there are multiple instances of Spanning Tree, there is an MSTP state maintained on a per-port, per-instance basis (or on a per-port, per-VLAN basis – as any VLAN can be in one and only one MSTI or CIST). For example, port A can be forwarding for instance 1 while discarding for instance 2.

The port states have changed since the publication of the IEEE 802.1d specification. The following table shows the port states for STP (802.1d) vs. MSTP (802.1s):

STP Port State

(IEEE 802.1d)

Disabled Disabled Discarding Excluded (Disabled) Disabled Enabled Discarding Excluded (Disabled)

Blocking Excluded (Alternate, Backup) Listening Included (Root, Designated) Learning Included (Root, Designated)

Forwarding

In order to support multiple spanning trees, an MSTP bridge must be configur ed with an unambiguous assignment of VIDs to spanning trees. This is achieved by:

Admin Port State

Enabled Enabled Enabled

Enabled

1. Ensuring that the allocation of VIDs to filtering IDs (FIDs) is unambiguous. The Dell PowerConnect 62xx series switch implements this with a fixed VID to FID

assignment. Every VID is assigned to one and only one FID.

MSTP Port State

(IEEE 802.1s)

Discarding Discarding

Learning

Forwarding

Active Topology

(Port Role)

Included

(Root, Designated, Master)

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Figure 1: VID to FID Allocation

MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES

2. Ensuring that each FID supported by the Bridge is allocated to exactly one

The Dell PowerConnect 62xx series switch implements this through the FID to MSTI Allocation Table. The following diagram shows an example configuration:

The combination of VID to FID and then FID to MSTI allocation defines a mapping of VIDs to spanning tree instances, represented by the MST Configuration Table. The following diagram shows an example configuration:

Spanning Tree Instance.

Figure 2: Example FID to MSTI Allocation

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Figure 3: Example Resultant VID to MSTI Allocation

With this allocation, we ensure that every VLAN is assigned to one and only one MSTI. The CIST is also an instance of spanning tree with an MSTID of 0. We can have an instance which has no VIDs allocated to it but every VLAN must be allocated to one of the other instances of spanning tree.

The portion of the active topology of the network that connects an y two bridges in the same MST region traverses only MST bridges and LANs in that region and never bridges of an y kind outside the region. In other words, connectivity within the region is independent of external connectivity.

ACTIVE TOPOLOGY ENFORCEMENT

Each received frame is allocated to a spanning tree instance by the forwarding process using the VID. The forwarding process selects each port as a potential transmission port if, and only if all of the following conditions are met:

1. The port on which the frame was received is in forwarding mode for that spanning tree instance.

2. The port considered for transmission is in a forwarding state for that spanning tree instance.

3. The port considered for transmission is not the same port on which the frame was rece ived. For each port not selected as a potential transmission port, the frame is discarded.

MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES

CONTROL PACKET BEHAVIOR BPDU: Always transmitted as untagged. The port receives and transmits BPDUs in all three

MSTP states: Discarding, Learning and Forwarding. If MSTP is disabled for the device (manual forwarding on all ports), BPDUs received are switched.

GVRP: Always transmitted as untagged. GVRP PDUs are received and transmitted only when the port is in Forwarding state.

GMRP: GMRP PDUs are transmitted tagged or untagged as per the port’s tag setting. They follow the ingress and egress rules.

LACPDU: LACP DUs are always transmitted untagged and a re received and transmitted in all three MSTP states. These frames are never switched whether MSTP is enabled or not.

Pause Frames: Pause frames are never tagged or switched. The port receives and transmits Pause frames in all three MSTP states. In other words, the STP state of the port has no bearing on the transmission and reception of Pause Frames.

Other Frames to and from the CPU: All other frames are received and transmitted only if the port is in Forwarding state.

All BPDUs (ST, TCN, RST, MST, etc.) use the unique MAC address of the transmitting port in their Source MAC address field and comply with IEEE Std. 802.1D-2004 sub-clauses 7.12 and 7.13.

The unique MAC address for a stacking switch is the base MAC address of the stack unit plus the port number.

MSTP CLI COMMANDS

You can configure MSTP on the Dell PowerConnect 62xx series switch by using the Web interface or the Command-Line Interface (CLI). The following spanning tree and MSTP CLI commands are available:

spanning-tree spanning-tree mode spanning-tree forward-time spanning-tree hello-time spanning-tree max-age spanning-tree priority spanning-tree disable spanning-tree cost spanning-tree port-priority spanning-tree portfast spanning-tree link-type clear spanning-tree detected-protocols spanning-tree mst priority spanning-tree mst max-hops spanning-tree mst port-priority spanning-tree mst cost spanning-tree mst configuration instance (mst) name (mst) revision (mst) show (mst) exit (mst) abort (mst) show spanning-tree

For more information about each command, including the syntax and variables, see the Dell PowerConnect 62xx System CLI Command Reference.

Configure MSTP by using the Web interface on the pages under the Switching > Spanning Tree menu.

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MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES

OPERATION IN THE NETWORK

In the following diagram of a small, 802.1d bridged network, STP is necessary to create an environment with full connectivity and without loops:

Figure 4: Small Bridged Network

Assume that bridge BrA is elected to be the Root Bridge, and Port Pt1 on bridge BrB and BrC are calculated to be the root ports for those bridges, Port Pt2 on bridge BrB and BrC would be placed into Blocking State. A loop-free topology would then exist. End stations in VLAN 10 could talk to other devices in VLAN 10 and end stations in VLAN 20 would only have a single path to communicate with other VLAN 20 devices. The logical single STP network topology would look something like this:

Figure 5: Single STP Topology

For VLAN 10, this Single STP Topology is fine and presents no limitations or inefficiencies. On the other hand, VLAN 20’s traffic pattern is inefficient. All frames from bridge BrB will have to traverse a path through bridge BrA before arriving at bridge BrC. If the ports Pt2 on bridge BrB and BrC could be used, these inefficiencies could be eliminated. MSTP does just that by allowing the configuration of MSTIs based upon a VLAN or groups of VLANs.

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MSTP INTEROPERABILITY OF THE DELL™ POWERCONNECT™ 6200 SERIES SWITCHES WITH CISCO IOS AND CISCO CATOS-BASED SWITCHES

In this simple case, VLAN 10 could be associated with MSTI 1 with an active topology similar to Figure 5, and VLAN 20 could be associated with MSTI 2 where port Pt1 on both bridge BrA and BrB begin discarding and all others begin forwarding. This simple modification creates an active topology with a better distribution of network traffic and an increase in available bandwidth. The logical representation of the MSTP environment for these 3 bridges is shown in Figure 6.

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Figure 6: Logical MSTP Environment

In order for MSTP to correctly establish the different MSTIs that Figure 6 shows, some additional changes are required. For example, the configuration would have to be the same on each and every bridge. That means that bridge BrB would have to add VLAN 1 0 to its list of supported VLANs (shown in Figure 6 with an “*”). This is necessary with MSTP to allow the formation of regions made up of all bridges that exchange the same MST Configuration Identifier. It is only within these MST regions that multiple instances can exist. It will also allow the election of Regional Root Bridges for each instance. One CIST Regional Root for the CIST and an MSTI Regional Root Bridge per instance will enable the possibility of alternate paths through each Region. Above bridge BrA is elected as both the MSTI 1 Regional Root an d the CIST Regional Root Bridge, and after adjusting the Bridge Priority on bridge Br C in MSTI 2, it would be elected as the MSTI 2 Regional Root.

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