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APRIL 2008
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)
Active Topology
(Port Role)
Blocking Excluded (Alternate, Backup)
Listening Included (Root, Designated)
Learning Included (Root, Designated)
Forwarding
Included
(Root, Designated, Master)
Admin Port State
Enabled
Enabled
Enabled
Enabled
MSTP Port State
(IEEE 802.1s)
Discarding
Discarding
Learning
Forwarding
Disabled Disabled Discarding Excluded (Disabled)
Disabled Enabled Discarding Excluded (Disabled)
In order to support multiple spanning trees, an MSTP bridge must be configured with an
unambiguous assignment of VIDs to spanning trees. This is achieved by:
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.
Figure 1: VID to FID Allocation