Understanding Mstp; Multiple Spanning-Tree Regions; Ist, Cist, And Cst - Cisco 3845 - Security Bundle Router Software Manual

Understanding MSTP

Understanding MSTP
MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped into a spanning-tree
instance, with each instance having a spanning-tree topology independent of other spanning-tree
instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing,
and reduces the number of spanning-tree instances required to support a large number of VLANs.
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For configuration information, see the

Multiple Spanning-Tree Regions

For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure
the switches with the same MST configuration information. A collection of interconnected switches that
have the same MST configuration comprises an MST region as shown in
The MST configuration controls to which MST region each switch belongs. The configuration includes
the name of the region, the revision number, and the MST VLAN-to-instance assignment map. You
configure the switch for a region by using the spanning-tree mst configuration global configuration
command, after which the switch enters the MST configuration mode. From this mode, you can map
VLANs to an MST instance by using the instance MST configuration command, specify the region name
by using the name MST configuration command, and set the revision number by using the revision MST
configuration command.
A region can have one member or multiple members with the same MST configuration; each member
must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number
of MST regions in a network, but each region can support up to 65 spanning-tree instances. You can
assign a VLAN to only one spanning-tree instance at a time.

IST, CIST, and CST

Unlike PVST+ and rapid PVST+ in which all the spanning-tree instances are independent, the MSTP
establishes and maintains two types of spanning trees:
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Cisco ME 3800X and 3600X Switch Software Configuration Guide
14-2
Multiple Spanning-Tree Regions, page 14-2
IST, CIST, and CST, page 14-2
Hop Count, page 14-5
Boundary Ports, page 14-6
IEEE 802.1s Implementation, page 14-6
Interoperability with IEEE 802.1D STP, page 14-8
An internal spanning tree (IST), which is the spanning tree that runs in an MST region.
Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a
special instance for a region, known as the internal spanning tree (IST). All other MST instances are
numbered from 1 to 4094.
The IST is the only spanning-tree instance that sends and receives BPDUs; all of the other
spanning-tree instance information is contained in M-records, which are encapsulated within MSTP
BPDUs. Because the MSTP BPDU carries information for all instances, the number of BPDUs that
need to be processed by a switch to support multiple spanning-tree instances is significantly
reduced.
"Configuring MSTP Features" section on page
Chapter 14 Configuring MSTP
14-14.
Figure 14-1 on page 14-4.
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