Understanding How Multiple Spanning Tree Works - Cisco WS-C6506 Software Manual

Understanding How Multiple Spanning Tree Works

If you have a Catalyst switch in your network with MAC address reduction enabled, you should also
enable MAC address reduction on all other Layer-2 connected switches to avoid undesirable root
election and spanning-tree topology issues.
When MAC address reduction is enabled, the root bridge priority becomes a multiple of 4096 plus the
VLAN ID. With MAC address reduction enabled, a switch bridge ID (used by the spanning-tree
algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified
as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480,
24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.
If another bridge in the same spanning-tree domain does not run the MAC address reduction feature, it
could claim and win root bridge ownership because of the finer granularity in the selection of its bridge
ID.
MAC address reduction is enabled by default on Cisco switches that have 64 MAC addresses (to find the
Note
number of MAC addresses supported on a switch, refer to the Catalyst 6500 Series Switch Release Notes
for Software Release 8.x publication).
Understanding How Multiple Spanning Tree Works
The Multiple Spanning Tree (MST) feature is the IEEE 802.1s and is an amendment to 802.1Q. MST extends
the 802.1w Rapid Spanning Tree (RST) algorithm to multiple spanning trees. This extension provides for both
rapid convergence and load balancing in a VLAN environment. In software release 8.3(1), the MST protocol
is compliant with IEEE 802.1s and is backward compatible with 802.1D STP, 802.1w, the Rapid Spanning
Tree Protocol (RSTP), and the Cisco PVST+ architecture that was implemented in previous software releases.
The MST protocol in software release 8.3(1) will interoperate with MST in earlier software releases.
MST allows you to build multiple spanning trees over VLAN trunks. You can group and associate
VLANs to spanning-tree instances. Each instance can have a topology that is independent of other
spanning-tree instances, and each instance can have a different port instance cost and port instance
priority. This architecture provides multiple forwarding paths for data traffic and enables load balancing.
Network fault tolerance is improved because a failure in one instance (forwarding path) does not affect
other instances (forwarding paths).
In large networks, having different VLAN spanning-tree instance assignments that are located in
different parts of the network makes it easier to administrate and optimally utilize redundant paths.
However, a spanning-tree instance can exist only on bridges that have compatible VLAN instance
assignments. MST requires that you configure a set of bridges with the same MST configuration
information, which allows them to participate in a given set of spanning-tree instances. Interconnected
bridges that have the same MST configuration are referred to as an MST region.
MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP). The MST
feature has these characteristics:
•
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
7-16
MST runs a variant of spanning tree that is called Internal Spanning Tree (IST). IST augments the
Common Spanning Tree (CST) information with internal information about the MST region. The
MST region appears as a single bridge to adjacent Single Spanning Tree (SST) and MST regions.
Chapter 7 Configuring Spanning Tree
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