Understanding How Mst Works - Cisco WS-C2948G-GE-TX Configuration Manual

Understanding How MST Works

MAC address reduction is always enabled on the Catalyst 4500 series switches; however, it may or may
not be enabled on a Catalyst 4006 switch; this can affect the selection of the root bridge after you migrate
your supervisor engine. Here are two scenarios to consider:
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For more information on migrating your supervisor engine from a Catalyst 4006 switch to a
Catalyst 4500 series switch, see the
a Catalyst 4500 Series Switch" section on page
Understanding How MST 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. The MST protocol is
currently being further developed; the MST feature for this release is based on a draft version of the IEEE
standard. The protocol, as implemented in this release, is backward compatible with 802.1D STP,
802.1w, the Rapid Spanning Tree Protocol (RSTP), and the Cisco PVST+ architecture.
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 independent of other spanning
tree instances. This new 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 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, allowing 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.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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The Catalyst 4006 switch is not a root switch
In this case, the spanning tree topology does not change. If you add a Catalyst 4500 series switch
with MAC address reduction enabled and its default spanning tree bridge ID priority set to 32,768
to the network, the bridge ID priority of the new switch becomes the bridge ID priority that is added
to the system ID extension. The system ID extension is the VLAN number and can vary from 1 to
4094. If the switch is in VLAN 1, the new bridge ID priority will be 32,769. Because 32,769 is
greater than 32,768, this switch cannot become the root switch.
The Catalyst 4006 is a root switch
In this case, the spanning tree topology might change. If the other switches in the network are not
running MAC address reduction, the topology will change after you replace the chassis with a
Catalyst 4500 series switch. The bridge ID priority of the new Catalyst 4500 series switch
increments in the same manner as in the previous scenario (bridge ID priority + VLAN number). If
the switch is in VLAN 1, the new bridge ID will be 32,769. Because 32,769 is greater than 32,768,
this switch cannot become the root switch. The network designates a new root switch; the spanning
tree topology also changes to reflect the new root switch.
If the bridge priority of the Catalyst 4006 has been lowered administratively and you use the same
configuration in the new Catalyst 4500 series switch, then the switch remains the root switch and
the spanning tree topology does not change.
"Migrating a Supervisor Engine II from a Catalyst 4006 Switch to
28-10.
Chapter 7 Configuring Spanning Tree
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