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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:
•
•
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 an upcoming IEEE standard: 802.1s for MST 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 and 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 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.
Therefore, 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 2980G Switches Software Configuration Guide
7-14
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 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 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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Release 8.1
78-15486-01
- Quick Links:
- Configuration Guide
- Catalyst 2980G Switch
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