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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Bridge Identifiers Work
Each MISTP instance root switch propagates the information that is associated with it to all other
switches in the network. This process maintains the network topology because it ensures that each switch
has the same information about the network.
MISTP builds MISTP instances by exchanging MISTP BPDUs with peer entities in the network. MISTP
uses one BPDU for each MISTP instance, rather than one for each VLAN, as in PVST+. Because there
are fewer BPDUs in an MISTP network, MISTP networks converge faster with less overhead. MISTP
discards PVST+ BPDUs.
An MISTP instance can have any number of VLANs that are mapped to it, but a VLAN can be mapped
only to a single MISTP instance. You can easily move a VLAN (or VLANs) in an MISTP topology to
another MISTP instance if it has converged. (However, if ports are added at the same time that the VLAN
is moved, convergence time is required.)
MISTP-PVST+ Mode
MISTP-PVST+ is a transition spanning-tree mode that allows you to use the MISTP functionality on
Catalyst 6500 series switches while continuing to communicate with the Catalyst 5000 and 6500 series
switches in your network that use PVST+. A switch using PVST+ mode that is connected to a switch
using MISTP mode cannot see the BPDUs of the other switch, which is a condition that can cause loops
in the network. MISTP-PVST+ allows interoperability between PVST+ and pure MISTP because it sees
the BPDUs of both modes. To convert your network to MISTP, use MISTP-PVST+ to transition the
network from PVST+ to MISTP.
Because MISTP-PVST+ conforms to the limits of PVST+, you cannot configure more VLAN ports on
your MISTP-PVST+ switches than on your PVST+ switches.
Understanding How Bridge Identifiers Work
These sections explain how MAC addresses are used in PVST+ and MISTP as unique bridge identifiers:
•
MAC Address Allocation, page 7-14
•
MAC Address Reduction, page 7-15
MAC Address Allocation
Catalyst 6500 series switches have a pool of 1024 MAC addresses that can be used as bridge identifiers
for VLANs running under PVST+ or for MISTP instances. You can use the
show module
command to
view the MAC address range.
MAC addresses are allocated sequentially, with the first MAC address in the range assigned to VLAN 1,
the second MAC address in the range assigned to VLAN 2, and so on. The last MAC address in the range
is assigned to the supervisor engine in-band (sc0) management interface.
For example, if the MAC address range is 00-e0-1e-9b-2e-00 to 00-e0-1e-9b-31-ff, the VLAN 1 bridge
ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is
00-e0-1e-9b-2e-02, and so forth. The in-band (sc0) interface MAC address is 00-e0-1e-9b-31-ff.
