Spanning Tree Basics
15-16 Configuring Spanning Tree
string corresponding to the bridge MAC address. This guarantees that the default behavior of
a bridge is to not be part of an MST region.
• Revision Level – Two octets in length. The default value of 0 may be administratively changed.
• Configuration Digest – 16-octet HMAC-MD5 signature created from the configured VLAN
Identification (VID)/Filtering Identification (FID) to Multiple Spanning Tree Instances (MSTI)
mappings. All devices must have identical mappings to have identical configuration digests.
By default, each bridge is in its own MST region and has a default configuration name derived
from the bridge MAC address. For example, if the bridge MAC address is 00-1f-45-9a-6c-b7, the
default MSTP configuration name is “00:1f:45:9a:6c:b7”. When grouping two or more bridges into
a single MST region, you must assign the same configuration name to each member of the region.
MD5 digests are derived from a mapping of a Filtering Database ID (FID) to a Spanning Tree ID
(SID), referred to as a FID-to-SID mapping (see “Multiple Spanning Tree Instances (MSTI)” on
page 15-16 for more information). Since there is a small probability of different mappings
resulting in the same digest, the addition of administratively assigned name and version
configuration ID parameters guarantee the uniqueness of the region.
SIDs exist within an MST region, each having a separate topology. Within an MST region there
always exists the Internal Spanning Tree (IST) which is SID 0. There are zero or more Multiple
Spanning Tree Instances (MSTIs). Each MSTI corresponds to a set of VIDs. One or more VIDs may
be mapped to an SID using a FID-to-SID mapping. The IST and each MSTI may have different root
bridges. Port path costs and bridge priorities may be different for each port/instance. Each bridge
port has a unique port state per instance. With proper configuration, redundant links may be
utilized to their maximum extent by each forwarding for one or more instances. See “Configuring
MSTP” on page 15-24 for more detail on how to do this.
Multiple Spanning Tree Instances (MSTI)
Inside the MST region, a wholly contained set of topologies is maintained separate from the
outside world. For example, MSTI 1 in MST region A has no correspondence to MSTI 1 in MST
region B.
The Enterasys switch device by default maps VLAN IDs (VIDs) to Filtering IDs (FIDs) in a
one-to-one correlation for bridges with the VLAN learning mode set to individual VLAN learning
(IVL). The Enterasys fixed switches only support IVL.
For example, in an IVL bridge, FID 3 may contain VID 3 and FID 4 may contain VID 4. Regardless
of the type of VLAN learning taking place, one or more FIDs may be mapped to a Spanning Tree
Instance (SID). The end result is a mapping of VIDs to SIDs. SID topologies may then be
configured to provide a type of load balancing. Note that without further configuration, each SID
will have the same topology as the IST. Typically, load balancing will be achieved by choosing
different root bridges in the core for the different instances.
Determining FID-to-SID Mappings
VLANs are mapped to MSTIs through a FID-to-SID mapping which is the key element in an
MSTP configuration. Each VLAN is associated to a FID and is mapped to Spanning Tree IDs using
their FID association. The mapping is performed by the set spantree mstmap command. This
mapping is represented within the MST configuration digest described in the previous section and
Table 15-4 Multiple Spanning Tree Instance Support
Platform Number of Instances
All stackables and standalones (except for
the C5 stackable)
4
C5 stackable 8
To Next Page
Loading...
Need help?
General